4 Oilfield Review

Electromagnetic Sounding for Hydrocarbons

Recent advancements in identifying subsurface features by resistivity contrasts

have added a significant tool in the quest to locate hydrocarbon resources.

The electromagnetic sounding technique comprises two related technologies,

magnetotelluric and controlled-source electromagnetic surveys, that provide distinctly

different insights into the subsurface. Their ability to clarify structures and to help

identify possible hydrocarbon deposits before drilling is exciting explorationists.

James BradyTracy Campbell Alastair FenwickMarcus GanzStewart K. SandbergHouston, Texas, USA

Marco Polo Pereira Buonora Luiz Felipe Rodrigues Petrobras E&PRio de Janeiro, Brazil

Chuck CampbellACCEL Services Inc.Houston, Texas

Leendert CombeeOslo, Norway

Arnie FersterKenneth E. UmbachEnCana CorporationCalgary, Alberta, Canada

Tiziano Labruzzo Andrea Zerilli Rio de Janeiro, Brazil

Edward A. NicholsClamart, France

Steve PatmoreCairn Energy Plc Edinburgh, Scotland

Jan StillingNunaoil A/SNuuk, Greenland

Oilfield Review Spring 2009: 21, no. 1. Copyright © 2009 Schlumberger.For help in preparation of this article, thanks to Graeme Cairns, George Jamieson, Jeff Mayville, Fred Snyder and Xianghong Wu, Houston.MMCI and Petrel are marks of Schlumberger.

The Sun provides us with energy in many forms. A surprising connection between exploration for energy resources and the Sun is becoming increas-ingly significant for the E&P industry. Ions emitted by the Sun experience a complex interplay with the Earth’s magnetic field, generating propagat-ing electromagnetic fields that penetrate the Earth and interact with its conductive layers. As the industry’s search for hydrocarbon resources intensifies, more geoscientists are relying on these electromagnetic fields to probe areas that are difficult to image with seismic methods.

The study of electrical currents in the Earth, called tellurics, is not new. Conrad Schlumberger, one of the founders of Schlumberger, used the phenomenon in early surface studies that he directed in the 1920s, prior to his start in wire-line logging.1 Louis Cagniard, a professor at the Sorbonne in Paris, first reported combining a measurement of electric and magnetic fields, termed magnetotellurics (MT), for exploration of the Earth’s subsurface in 1952.2 However, MT has become an important tool for exploration-ists in the E&P industry only within the past few years—thanks to advances in 3D modeling and inversion technology. Now, MT results can be combined more efficiently with seismic and grav-ity surveys, resulting in a more-calibrated model of the earth.

Although Cagniard also discussed a method related to MT that uses an artificially imposed electromagnetic field, techniques for generat-ing and detecting a signal strong enough for use in the E&P industry came decades later, in the 1960s on land and then in the 1980s in the marine environment. This method is now termed controlled-source electromagnetics (CSEM).

The interaction of the earth with imping-ing electric and magnetic fields is complex. Two important factors in MT analysis are the frequency spectrum of the fields and the resis-tivity (or its inverse, the conductivity) of the particular medium through which the field waves propagate. Analyzing data from the frequency spectrum helps obtain an apparent resistivity as a function of frequency.3 This apparent resistiv-ity can be related to the true resistivity of the formation at various depths. If the subsurface is homogeneous, the measured apparent resistiv-ity is the same as the true resistivity, but if the resistivity changes with depth, apparent resistiv-ity is a conflation of measurement effects and some average of the resistivities. Through data analysis, interpreters can determine the depths of bodies with contrasting resistivities, providing a result termed an MT sounding.

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This article discusses the physics of these electromagnetic interactions and how they are interpreted to give information useful in basin and reservoir evaluation. It also describes the equipment used to detect and, in the case of CSEM, to generate relevant electromagnetic fields. Case studies from the Gulf of Mexico, Brazil and Greenland illustrate these technologies for offshore salt mapping and reservoir illumina-tion. A companion article describes near-surface applications of CSEM on land (see “Near-Surface Electromagnetic Surveying,” page 20). The next section focuses on natural electromagnetic fields and their interactions with the Earth.

Blowing in the WindThe solar wind is a stream of positive and nega-tive ions emitted by the Sun. Wind intensity varies, increasing during periods of high sunspot activity. This ionic wind “blows” through space; auroras manifest its interaction with the Earth’s magnetic field in spectacularly colorful ways.4

Although most solar ions are deflected by the magnetic field in a region known as the magnetopause, which is several Earth radii out in space, some ions leak in. Those that reach the upper atmosphere can ionize particles in the ionosphere, which ranges from 75 to 550 km [50 to 340 mi] above the surface of the Earth. In

the ionosphere, the particle velocities are high enough and the particle density low enough that charged ions do not immediately recombine into neutral atoms and molecules: They form a plasma of charged particles. This plasma makes the ionosphere a conducting layer, unlike the nonconducting layers of the lower atmosphere where the particle density is too high to maintain charged ions for a significant period of time.

The motions of charges in the ionosphere are constrained by the Earth’s magnetic field, whose lines of force stretch from pole to pole. When solar ions enter the plasma within this magnetic field, they generate electromagnetic (EM) pulses

1. Leonardon EG: “Some Observations Upon Telluric Currents and Their Applications to Electrical Prospecting,” Terrestrial Magnetism and Atmospheric Electricity 33 (March–December 1928): 91–94.

2. Cagniard L: “Basic Theory of the Magneto-Telluric Method of Geophysical Prospecting,” Geophysics 18 (1953): 605–635.

3. Apparent resistivity is a volume average of the true resistivities of the media within the volume measured by a device, such as a resistivity or induction tool, or a magnetotelluric receiver.

4. For a recent discussion about the origin of the auroras: Brown D and Layton L: “NASA Satellites Discover What Powers Northern Lights,” NASA News & Features, http://www.nasa.gov/home/hqnews/2008/jul/HQ_08185_THEMIS.html (accessed March 2, 2009).

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that resonate in the ionosphere, traveling along the magnetic field lines. The result is analogous to plucking the string of a guitar; just as the string resonates at characteristic frequencies, so too does the ionosphere resonate electromagnet-ically. The complex interaction of magnetic field, atmospheric plasma and solar ions results in a broad spectrum of EM frequencies, including the visible-light phenomena of the aurora borealis and the aurora australis. The spectral range use-ful for E&P-related MT extends from frequencies of about 0.001 Hz to 10 kHz; for studies extend-ing to the Earth’s mantle even lower frequencies

are used (above). Frequencies above 1 Hz are severely attenuated through conductive seawater and thus create no subsea earth response, mak-ing this the effective upper-frequency limit for marine MT.

The amplitude and frequency spectrum of the signal is highly variable.5 The fluctuations in the solar wind reflect the 11- to 14-year cycle of sunspot activity. The spectrum also depends on the season and time of day, since sunlight influences the degree of polarization in the ionosphere. Signal levels in equatorial regions are low, whereas they are high in polar regions.

This stronger signal near the poles or near the peaks of the solar-activity cycle results in higher-quality MT data; conversely, obtaining data from deepwater equatorial areas, especially during low-activity periods, is more challenging (below left).

A part of the frequency spectrum is influenced by lightning. A lightning discharge can generate current in the range of 20 to 50 kA, which initi-ates a strong interaction in the ionosphere. The charge pulse follows the magnetic field lines around the Earth, reflecting near the poles and playing its own resonance notes.6 The EM fields resulting from a lightning strike are global.7

The lower atmosphere is a poor electrical conductor, so the EM waves propagate with vir-tually no attenuation.8 This lack of attenuation allows radio broadcasts to be heard far from the source when atmospheric conditions are right for refracting them to listeners. In contrast, once the waves reach the surface layers of the Earth, they interact with seawater and formations that are electrically conductive to a greater or lesser extent. Conductive bodies attenuate EM waves.

Most of a rock’s solid matrix conducts elec-tricity poorly. However, various saturating fluids have differing conductivities. Brine conducts well, but oil and gas have high resistivities. Adjacent formations with a marked resistivity contrast—such as a hydrocarbon-bearing zone surrounded by brine-saturated strata—affect the propagating EM field in different and potentially measurable ways. The resistivity contrast is also high between brine-filled sedimentary layers and some specific lithologies, such as salt, basalt and resistive carbonates.

The EM waves interact with conductive forma-tions and induce a response wave that propagates back to the surface. Although the geometry of signal and response is sometimes depicted as analogous to that of a seismic reflection, the EM effect has a different physical origin and differ-ent behavior than a reflected seismic wave.9 The time-varying EM signal induces a current loop in the conducting layer. The properties of this induced eddy current depend on the resistivity of the conducting formation and the magnitude and time rate of change—or the frequency—of the source signal. The eddy current, in turn, induces a magnetic field, which propagates from the formation. Sensors on the surface measure this response field.

> Typical magnetic-field amplitude spectrum from the atmosphere. The ionospheric signal originating from interactions of the Earth’s magnetic field decays rapidly with increasing electromagnetic frequency. Lightning generates signals in a region called the Schumann bands in the spectrum between about 7.8 and 60 Hz.

0.000001

0.00001

0.0001

0.001

0.01

0.1

0.001 0.01 0.1 1Frequency, Hz

10 100 1,000 10,000

1

10

Mag

netic

fiel

d sp

ectra

l am

plitu

de

EM_FIGURE 01

> Electromagnetic activity. Planetary electromagnetic activity is estimated from measurements of a geomagnetic index taken by the US National Oceanic and Atmospheric Administration (see reference 5) at several locations. The activity fluctuates both annually and weekly, as shown for 2008 (black). The solar cycle is currently in a quiet period.

12/31/0810/1/087/1/084/1/081/1/08

Annualaverage

Date

Geom

agne

tic in

dex

16

12

8

4

0

2006

2007

20082005

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The eddy current in the conducting formation opposes the change in the source field. The result of the eddy current and the transfer of energy to the response signal is attenuation of the incoming EM wave. Thus, as the wave passes successively deeper into the conductor, the eddy current becomes incrementally weaker, making the response field smaller also. As this process continues, the inci-dent signal decays, while weaker response signals form at each successive increment of depth within the conducting formation. This decay is known as the skin effect (above).

A characteristic distance for penetration of the signal into a conductor, termed the skin depth, is obtained by determining when the field amplitude drops by a factor of 1/e, the inverse of the exponen-tial function. Attenuation is frequency dependent; high frequencies attenuate more rapidly than low frequencies. It is also a function of the formation

conductivity: In more-conductive formations the impinging field induces greater current flow that partially cancels the source field. In a typical geo-logical section, the natural frequencies used in MT have skin depths of a few tens to a few tens of thousands of meters. The high-frequency compo-nents useful for detecting thin, shallow formations are present only for land-based (or extremely shallow-water) surveys because of attenuation by conductive seawater. The deeper a target structure is buried, the larger it must be to enable detection through MT evaluation; this basic MT-resolution problem at depth is more severe than resolving small, deep features using seismic waves.

The response signal contains information in the impedance value about the resistive proper-ties of the formations. Impedance is a complex term—comprising real and imaginary parts—that designates the difficulty of propagating the

EM energy through a medium. It is determined from the amplitude and phase relationship that exists between the measured electric and mag-netic fields.10 It is also a tensor quantity that can be related to the apparent resistivity of the for-mation. Impedance varies with the frequency of the incoming signal.

Because the source is so distant, the MT fields impinging on an E&P survey area can be approximated over a wide bandwidth as verti-cally incident plane waves with the electric field horizontally polarized.11 MT fields are sensitive

ful in studies of large salt, basalt and carbonate bodies due to the contrast of these resistive features with the conductive surroundings. However, the attenuation of the MT fields with depth—the skin effect—makes them insensitive to resistivity contrasts of thin formations such as

5. Data are available from the US National Oceanic and Atmospheric Administration, http://www.swpc.noaa. gov/ftpmenu/indices/old_indices.html (accessed May 5, 2009).

6. This response to lightning is termed a Schumann resonance, after German physicist Winfried Otto Schumann, who predicted the resonances mathematically in 1952.

7. Active storms generating lightning seem to be linked: Synchronized lightning strikes from widely spaced geographic locations have been observed from the

> Skin effect. A downward-moving electromagnetic field (blue curve) leaving a highly resistive medium, such as air, begins to decay when it enters a more-conductive medium, such as rock. Lower-frequency waves (left ) propagate farther than higher-frequency waves (center left and center right ), and waves propagate farther in less-conductive media (right ). The amplitude has an exponential decay (red) that is a function of the conductivity of the medium, σ, and the frequency of the wave, ω. The

skin depth is the distance at which the amplitude has decayed to 1/e of the incident value. The wave in the conductive medium also experiences a gradual delay in the phase. Since the phase change is difficult to see in this example, one illustration (far left ) also shows an attenuated wave without the phase change (violet). Frequency and conductivity values are relative among these examples.

EM_FIGURE 04

Skin depthSkin depth

Skin depth

Skin depth

ω = 2

σ ~ 0

σ ~ 10

σ ~ 0

σ ~ 1

ω = 5 ω = 10 ω = 10

NASA Space Shuttle. For more on synchronized lightning strikes: Yair Y, Aviv R, Ravid G, Yaniv R, Ziv B and Price C: “Evidence for Synchronicity of Lightning Activity in Networks of Spatially Remote Thunderstorms,” Journal of Atmospheric and Solar-Terrestrial Physics 68, no. 12 (August 2006): 1401–1415.

8. Electromagnetic waves propagate through a vacuum with no attenuation.

9. EM energy in a conductive medium has a diffusive nature rather than a wave nature.

10. The phase of a wave describes where it is in its amplitude cycle of maximum to minimum and back to maximum as the phase angle goes from 0° to 360°. The electric and magnetic fields of a propagating wave are not necessarily at 0° phase at the same time, and the difference between the two is also referred to as the phase angle.

11. The waves impinge vertically because air is not conductive. The uniformity of signal for MT surveys is based on the large distance to the ionosphere compared with the length of a survey line. However, if the signal comes from a lightning strike that is close to the survey area, the plane-wave assumption does not hold and the local geometry influences the interpretation.

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to large conductive features, making them use-

8 Oilfield Review

hydrocarbon-bearing sediments. Generally, to be resolved by MT, the layer’s thickness should be at least 5% of its burial depth, and the layer should be more conductive than its surroundings. These limitations led to the development of the CSEM method (above right).

The CSEM method imposes a powerful, arti-ficially generated EM signal. The source is a localized electric dipole with a controlled signal that extends over a narrow bandwidth, often just a few fundamental frequencies and their harmon-ics. The EM fields generated by such a source are not plane waves. The composition and geometry of the signal are chosen to make it more sensitive for detection of a thin formation at a particular

hypothesized location and with a resistivity value that contrasts to that of surrounding formations. This difference between MT and CSEM source signals affects the method of processing the data and impacts the type of structures that can be detected by the two methods, as discussed in the next two sections.

Deep Vision with MTThe atmospheric source for MT signals varies randomly in time, but at any given time the ver-tically incident waves are uniform over a large area. The wavefields are planar and vertically incident on the surface of the Earth; the electric field has only horizontal components, as does the orthogonal magnetic field. As a matter of nomen-clature, the portion of the electric field that can be resolved along the strike of a geologic feature is termed the transverse electric (TE) mode; the portion across the strike is the transverse mag-netic (TM) mode.

Because of the vertical and planar geometry of MT, the impedance of the subsurface can be obtained by taking the ratio of the horizontal electric field in one direction to the horizon-tal magnetic field in the orthogonal direction (above left).12 This calculation removes the tem-poral variability of the incident signal, leaving only the desired formation response.

The complex impedance can be calculated to obtain the apparent resistivity, ρa, of the underly-ing formations and the phase angle, φ, between the electric and magnetic fields. Geoscientists use these results to interpret the subsurface structure through forward modeling or through inversion.13 Forward modeling assumes a struc-ture and certain properties, such as layer depth and resistivity, and predicts the earth’s elec-tromagnetic response to the assumed model. Comparing or normalizing processed data against this model assesses its goodness of fit. Inversion is the reverse of forward modeling, using the data to step backward through the physical process

to obtain a model of the earth. The result is not unique, so the process iterates until the result is acceptable. Many algorithms are in use for con-verging the inversion on a particular model.

A key step of preacquisition planning is to determine if different models will be distinguish-able in the data. This is typically accomplished by first forward modeling the response of various predicted scenarios, then possibly employing inversion on modeled synthetic data. To inves-tigate whether the original model can be recovered, the synthetic data include noise rep-resenting expected background or measurement noise. This step can help justify the usefulness of a proposed survey or, alternatively, advise against its application. Acquisition parameters such as the location of instruments and how long they must remain on the ground are also results of this process. In CSEM surveys, the optimal frequen-cies can also be determined through modeling.

Recent interest in MT measurements has focused on evaluations in marine environments, driven by the increasing costs of drilling in deep water and the complexity of imaging below salt and basalt. As a result, technologies that increase the chance of economic success after locating drilling targets have great value.

As with seismic surveys, EM surveys require deployment of equipment, either on land or at sea. Marine MT surveys are acquired using small vessels and small crews. CSEM surveys need larger vessels to handle the source equipment and larger crews to operate and maintain that equipment. Typically, both MT and CSEM sur-veys are targeted, examining specific ambiguous structures or promising anomalies on a seismic section. Thus, the duration and areal scope of these studies are typically smaller than those for seismic surveys.

Subsea EM measurements—both MT and CSEM—are similar to land measurements aside from the vast difference in the resistivities of sea-water and air. At the air/land interface there can

> Comparison of marine MT and CSEM survey technologies.

EM_FIGURE 05

Marine MT Marine CSEM

Passive (atmospheric) source

Plane waves, vertically incident

Basin scale

Detection of structure and lithology

Wave-field sensitive to conductors

Active controlled source

Localized dipole source

Reservoir scale

Detection of resistivity contrast, such as that caused by a resistive pore fluidagainst a conductive background

Wave-field sensitive to resistors

> Sensing impedance. A vertically incident EM wave interacts with the Earth through the formation impedance, Z. The Z value can be determined by measuring the horizontal electric field, E, and the magnetic field, H, at the surface or on the seabed (tan). The apparent resistivity, ρa, is the aggregate resistivity of the formation layers beneath the electric dipole antenna and the magnetometer coil of a sensor (yellow). In the case shown, E and H are in phase; if the zero crossings of the two fields were out of synchronization, there would be a phase angle between the two fields.

Exiρa µHy

Z = = ω

EM_FIGURE 07

Ex =VL

ωformation resistivityfrequencymagnetic permeabilityformation impedanceelectric fieldmagnetic fieldtimevoltage drop across dipoledipole length –1

ρa ==

µ =Z =E =H =t =

V =L =i =

xxxx

Y

X

Ex (t)

Hy (t)

V

L

H

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be no vertical electric current because the air is not conductive, but on the seabed a vertical elec-tric current can exist in the conductive water. The consequence of this difference is subtle. On land, the electric field responds significantly to changes in resistivity in subsurface layers, but the magnetic field has much less variation. In contrast, in the marine environment it is the magnetic field rather than the electric field that displays the greater variation with change in subsurface structure, although both fields carry information on structure.14

Measuring the Signal The two basic devices for measuring EM fields are a pair of electrodes to sense an electric field potential difference and a magnetometer to sense magnetic field variations. The pair of electrodes forms an electric dipole, allowing measurement of the potential voltage difference between them. A magnetometer is a coil of conducting wire that generates a measurable current based on the changing magnetic flux through the coil.

When only two sensors of one type are used, they are oriented to measure the orthogonal field components in the horizontal plane. The vertical component of the field is measured only if a third sensor is used.

The primary recent interest in the E&P indus-try has been offshore, and considerable effort has been made over the past decade to develop a sensor for marine use. The Scripps Institution of Oceanography in La Jolla, California, USA, devel-oped the basic electric field sensor that is used by WesternGeco today. The magnetometers were developed by Electromagnetic Instruments Inc., which was acquired by Schlumberger in 2001.15

In the WesternGeco device two horizontal electric dipoles are formed by silver–silver chlo-ride electrodes placed at the ends of four long fiberglass tubes, extending from each of the four sides of the receiver frame (above right). The pres-ent configuration includes a vertical dipole with a length of 1.82 m [6 ft]. Its length is restricted by the need to maintain orthogonality and stability—a longer dipole is more susceptible to seabed currents that move the dipole antenna and introduce noise into the measurement within the frequency bandwidth of interest.

The magnetometers, multiturn coils in a non-metal housing, are used to sense the magnetic flux. The magnetometer tubes are secured hori-zontally into holes in the frame. The operating range is from 0.0001 to 100 Hz.

Calibration of both types of sensor is criti-cal. The WesternGeco sensors and amplifiers are individually calibrated far from electromagnetic

noise in a remote part of the Norwegian coun-tryside. In addition, data quality requires strict adherence to deployment procedures on the survey ship.

A concrete block attached to the bottom of the receiver frame provides weight to take it to the ocean floor. This concrete anchor also helps to stabilize the instrument against forces from sea currents; antenna rotation as tiny as 1 μrad can easily be detected by the magnetic induc-tion coil moving in the Earth’s magnetic field. At the conclusion of the survey, an acoustic signal from surface triggers release from the block, and air-filled glass spheres lift the receiver to surface for retrieval.

The cost and logistics of establishing electri-cal connections with multiple receivers placed on the seabed in deep water are prohibitive, so engineers designed the receiver to operate inde-pendently and to be retrieved at the end of the test. Each receiver carries a data logger that controls operation and records the signals on a compact flash card. High-resolution data from the dipoles and magnetometers come from 24-bit analog-to-digital converters, which accurately record time so that the signals can be synchro-

nized later with the source record and with each other.

The unit has several independent battery packs. One provides power to the data-logger electronics. A separate battery powers the anchor-release devices, and another powers an acoustic positioning beacon that indicates the unit’s loca-tion on the seabed. The battery pack that powers the data logger lasts up to 40 days; the long battery life provides time to deploy the sensors and then acquire data. The anchor-release battery pack lasts more than a year, in case circumstances prevent immediate removal of the device after the survey.

12. Cagniard, reference 2.13. For more on inversion: Barclay F, Bruun A, Rasmussen KB,

Camara Alfaro J, Cooke A, Cooke D, Salter D, Godfrey R, Lowden D, McHugo S, Özdemir H, Pickering S, Gonzalez Pineda F, Herwanger J, Volterrrani S, Murineddu A, Rasmussen A and Roberts R: “Seismic Inversion: Reading Between the Lines,” Oilfield Review 20, no. 1 (Spring 2008): 42–63.

14. Constable SC, Orange AS, Hoversten GM and Morrison HF: “Marine Magnetotellurics for Petroleum Exploration, Part I: A Sea-Floor Equipment System,” Geophysics 63, no. 3 (May–June 1998): 816–825.

15. Webb SC, Constable SC, Cox CS and Deaton TK: “A Seafloor Electric Field Instrument,” Journal of Geomagnetism and Geoelectricity 37, no. 12 (1985): 1115–1129.

Constable et al, reference 14.

> CSEM receiver. Orthogonal dipole antennas on the receiver measure Ex and Ey and two induction coil magnetometers measure Hx and Hy . Each tube containing an antenna is 3.6 m [12 ft] long; coupled with the dimension of the frame, the electric dipole length formed by a pair pointing in opposite directions is 10 m [32.8 ft]. A concrete anchor carries the receiver to the seafloor, where it remains throughout the test. The electronic logger records for a set time. At the conclusion of the test, an acoustic signal from the ship triggers a mechanism to burn through the wire holding the device to the anchor. Air-filled glass spheres raise the receiver to the surface, where it is retrieved and the data are captured. In some cases, the receiver includes a vertical dipole to measure the vertical electric field, Ez (not shown). (Image courtesy of Scripps Institution of Oceanography.)

Instrumented strayline float Dipole for

electric field

Induction coilmagnetometers

Burnwire releasemechanisms

Gas flotation

Logger

Acoustics

Concreteanchor

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The seabed orientation of the horizontal sen-sors is random. The measurement directions are resolved to a desired orientation during process-ing. The newest devices have a compass, but in the past the orientation for each receiver was obtained either by comparison with land-based sensors or by orientation based on the direction of a towed source in a CSEM survey.

CSEM: Focusing on Hydrocarbon DetectionMT measurements are not sensitive to thin resis-tive layers, so they are not well suited for evalu-ating potential hydrocarbon reservoirs. Over the course of a few decades starting in the 1980s, several research institutes and companies devel-oped the equipment, modeling and interpretation tools that became the marine CSEM technique (see “Marine CSEM: Evolution of a Technology,” page 1).16 The systems are now widely available.

Since the same receivers function for both CSEM and MT measurement, both responses can be recorded during a survey. The CSEM technique focuses on measuring and interpret-ing the response from the controlled source, while between those measurements, MT data are recorded. The processed and interpreted MT data establish a background model for the CSEM interpretation or inversion.

The typical marine CSEM transmitter source is a long horizontal dipole (above left). The source comprises two neutrally buoyant antenna cables, each terminating in an electrode, thereby forming a dipole. The electrodes are pulled through the water behind a streamlined sen-sor platform, called a towfish, that is towed by the ship at a nominal speed of 2.8 to 3.7 km/h [1.7 to 2.3 mi/h or 1.5 to 2.0 knots] at an altitude of 50 to 100 m [160 to 330 ft] above the seabed. To provide accurate values for processing, the tow-fish measures seawater conductivity, local sound velocity and altitude above the seafloor.

The strength of the dipole source is given by its dipole moment. This value is the product of the magnitude of the electric current flowing through the electrodes—given by the strength of the first harmonic of the output signal—and the distance between the electrodes.

The power to generate a high-current, low-voltage source signal and propagate it along

16. The first development was by Charles Cox of Scripps Institution of Oceanography: Cox CS: “On the Electrical Conductivity of the Oceanic Lithosphere,” Physics of the Earth and Planetary Interiors 25, no. 3 (May 1981): 196–201.

For a recent overview of the history of CSEM: Constable S and Srnka LJ: “An Introduction to Marine Controlled-Source Electromagnetic Methods for Hydrocarbon Exploration,” Geophysics 72, no. 2 (March–April 2007): WA3–WA12.

> CSEM transmitter. The transmitter comprises a towfish—the head section containing power and instrumentation—and a streamer antenna with dipole electrodes at the ends of two cables. The dipole is the source of the CSEM signal. The signal transmission and waveform parameters are set from the survey vessel during operations, and results are telemetered to the operators for real-time quality control of the signal. The photograph (top) shows a towfish being removed from the ocean, with the antenna trailing in the water.

EM_FIGURE 09

Tow-cable terminationCable to

survey vessel

Towfish Streamer Antenna

A

A: Telemetry and signal conditioningB: Transmitter power section

B

Instrumentsuite

2.5 mStrain relief

Neutrallybuoyant cable

Transponder

Transponders

Electrode 1

300-m dipole

Electrode 2

20 m

> Square-wave components. A square wave (magenta) can be broken into an infinite series of sine waves by using the Fourier transform (equation). The fundamental frequency, w 0, has the greatest amplitude; each subsequent odd harmonic has a lower amplitude. Even-numbered harmonics are not included because of the symmetry of the square wave.

1.5

1.0

0.5

0

0 1 2 3 4

–0.5

–1.0

–1.5

Ampl

itude

Time, s

9ω0

Five-term sum

5ω0

3ω0

7ω0

ω0

EM_FIGURE 10

Square wave (ω0) = 4

πsin(ω0t)

sin(3ω0t)

3++

sin(5ω0t)

5+

sin(7ω0t)

7+ +

sin(9ω0t)

9...

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several kilometers of cable is typically provided by a 250-kV.A system on the ship. Transformers convert this to a low-current, high-voltage signal sent along the cable. In the towfish the signal is transformed back to the high-current, low- voltage signal.

The towfish generates a designed waveform based on commands from the ship. The actual current waveform transmitted by the source elec-trodes is measured and recorded by a data logger in the towfish and transmitted to the vessel in real time for quality control via high-speed telem-etry. Because the waveform transmitted by the antenna is affected by antenna impedance and wear and by water salinity, accurate monitoring of the actual waveform is required to correctly resolve the survey data.

Although the power emitted at the source is large—nominally 50 kW—the signal decays rapidly with distance. At a receiver placed 10 km

[6.2 mi] away, the electric-field magnitude is small, less than 1 nV/m. For the typical 10-m span of a seabed receiver dipole, the measured 10 nV is about 80 million times smaller than a AAA battery’s 1.2 V. The response magnetic-field mag-nitude at that distance from the source is about 0.0001 nT, which corresponds to about 2 parts in a billion of the Earth’s DC magnetic field.

The controlled source typically generates square waves or sequences of square waves at user-defined fundamental frequencies. Fourier analysis resolves the square wave into sinusoidal waves of many frequencies (previous page, bot-tom left). The strongest components are the primary frequency w0 and the odd harmon-ics 3w0, 5w0 and 7w0, each with sequentially decreasing magnitudes. The combination of the skin-depth relationship to frequency and use of multiple frequencies means this process samples at several depths and with several resolutions.

The data from the receivers are collected as time-series data, but for the CSEM method, they must be synchronized to the source square-wave signal through accurate time measurement. Thus, in addition to the source GPS synchroni-zation, each receiver has a high-precision clock that is GPS synchronized upon deployment and recovery. The instantaneous dipole-source posi-tion and orientation must also be captured for accurate inversion. Acoustic transponders in several locations along the antenna give this information by transmitting their positions at 1- to 4-s intervals. Accurate measurement of the feathering or tilt of the antenna is important for correct processing.

The measurements of the fields are time-domain data, but these are typically converted to the frequency domain using a Fourier trans-form (below). The data are stacked by overlaying responses from multiple, sequential square-wave

> Converting time-domain measurements to amplitude versus offset. Each receiver records data for two horizontal electric- and magnetic-field measurements (top). A Fourier transform converts these time-domain signals into the frequency domain. Fourier conversions of similar measurements at many receiver locations allow development of a frequency-dependent amplitude versus offset relationship (bottom). This can be developed for each measured component of the electric field (only one is shown) and the magnetic field. The resistivity of the subsurface affects the shape of these curves.

Ex

Ey

Hx

Hy

Time, min 5 10 15 20 25 30 35 40

–10

Scal

ed e

lect

ric a

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itude

, V/(A

.m2 )

Source-receiver offset, km

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–9 –8 –7 –6 –5 –4 –3 –2 –1 0 1 2 3 4 5 6 7 8 9 10

0.06250.18750.250.3150.43750.751.251.75

Frequency, Hz

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12 Oilfield Review

series, called a time gather, to improve the signal/ noise ratio. The window for the time gather must be short enough that the source movement does not significantly alter the sampled volume of the subsurface.

Since the objective of E&P prospecting is to detect hydrocarbons, the signal from the CSEM source is optimized to find thin, noncon-ducting layers (possible hydrocarbon-bearing formations) in a conducting background (water-bearing formations). The discussion on skin depth pointed out that detecting thin formations requires higher-frequency components than available using MT. The typical frequency range of the CSEM signal is between 0.05 and 5 Hz; 1 Hz is the effective upper limit for marine MT studies.

As a first-order approximation, the signal can take three general paths between the source and the receivers (above). When the source-receiver offset distance is short, the direct path through the water dominates the signal. The strength of the signal decreases rapidly with

distance because of its attenuation in conduc-tive water. A second contribution comes from the air wave. The electromagnetic field travels to the water surface, where it encounters highly resistive air. The resistance contrast forces the wave propagation to follow the air/water interface. In deep water, the air-wave signal dominates only at long offsets, normally beyond 10 km, because, unlike the signals following the other two paths, the signal at the air/water inter-face has little attenuation.

The third portion of the signal travels through the subsurface. Under the proper conditions of frequency, water depth and subsurface conduc-tivity, there is a range of offsets for which the third path dominates the signal. For this path, waves propagate into the subsurface, where they interact with resistive formations and generate a response field; some of that energy travels back to the seafloor receivers. This response signal appears at receivers at offset distances that are typically greater than the reservoir depth below the seabed, but at even greater offsets it attenu-

ates so much that the air-wave signal overwhelms it. Since the waves propagate more easily though a resistive than a conductive formation, the presence of a reservoir enhances the received signal compared to a uniform subsurface lack-ing a resistive layer. Geoscientists can identify resistivity anomalies and therefore infer geologic information by analytic means through compar-ing the observed data with predictive models or by numeric means through inversion.

At a certain offset distance, the natural noise limitation of the receiver exceeds the strength of the signal that originated at the source transmit-ter, presenting an effective limit on the depth of investigation in the subsurface. This limitation, or noise floor, varies with frequency and depends on the characteristics of the receiver and its environment—such as mechanical noise gener-ated by water waves moving the antennas. The noise floor can be lowered through improved instrumentation, such as quieter electronics or more-stable mechanics, or through intelligent signal processing to remove motion noise or coherent noise across the survey.

The source, receiver and environmental characteristics can be incorporated into a pre-survey analysis to determine whether a resistive target at a certain depth can be detected (next page). Carbonates, which are resistive, present a problem: A trap with low oil saturation inside a resistive carbonate host may have insufficient detectable contrast.

The receiver data can be presented as electric- or magnetic-field amplitudes and phases that are functions of the offset distance between source and receiver. The effect of a resistive anomaly can be highlighted by several methods: analytic methods using only measured data, model- based methods derived during survey planning, and inversion.

EM_FIGURE 11

Resistor

Conductor

Water

Air-wave signal

Geologic signal

Direct signal

010-12

10-11

10-10

10-9

10-8

10-7

10-6

10-5

10-4

10-3

1 2 3 4 5Source-receiver separation, km

Background(no resistive formation)

ReservoirElec

tric

field

, V/m

6 7 8 9 10

> Paths from marine source to receivers. Signal energy from the marine source reaches the receivers by following three types of paths. A direct signal passes through the water to the receiver; this signal is strongest at the near-offset receivers. Signal energy that enters the subsurface interacts with layers of varying resistivity and generates a response signal containing geologic information that travels up to the receivers. Signal energy that reaches the air interface travels along the interface as an air wave, which also travels to receivers. In shallow water or at long source-receiver offsets in deep water, the air-wave signal is strongest.

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One of the analytic methods normalizes the electric- and magnetic-field amplitudes versus offset response over the anomaly to the response of a distant receiver that does not sense the anomaly. A second analytic method compares the normalized response of the inline measurement with the crossline measurement, essentially comparing the two horizontal components of the electric field, Ex and Ey. The presence of an underlying resistive structure, such as a hydro-carbon-bearing formation, has greater effect on the inline response because of the polarization of the signal.

A third analytic method converts the field data to apparent resistivity in a 2D or 3D pseudo-section plotted as a function of source-receiver offset and signal frequency.17 When the dataset is normalized to a section space that contains no anomaly, the anomalous apparent resistivity values appear as deviations from unity.

Alternatively, presurvey models can be built when seismic data or data from nearby wells are available. Typically, a WesternGeco survey includes at least two 3D models that are based on the target properties and survey geometry. One model incorporates a resistive body; the other uses a uniform earth without a resistive body. Response curves are extracted from the 3D models for each receiver-site and tow-line combi-nation. Once data are acquired, the observations can be normalized to each of the models to deter-mine which provides the best fit.

Beyond these analytic and model-based methods, CSEM inversion is a powerful way to derive the earth’s resistivity profile from observed data. However, like most inversion methods, the solution is not unique. Forward-modeling codes are run iteratively with model parameters per-turbed until the output result matches the data within an acceptable range. Jointly inverting as many significant channels and frequencies as possible constrains the possible solutions, but at a cost of longer processing time. Additional constraints—such as placement of known geo-logic structures—are sometimes introduced. Log and seismic data provide a starting model to help constrain the inversion.

MT data also have limited resolution, so the modeling step benefits from information based on other types of measurement. Seismic inter-pretations often serve as constraints. Gravity surveys provide an independent constraint, as do well logs. The WesternGeco MMCI multi-measurement-constrained imaging technique uses an iterative approach with gravity, MT and seismic data to improve inversion results, leading to a final, more-constrained depth image.

Although marine MT and CSEM receivers have been used in studies since the 1990s, the industry’s interest has risen rapidly in the last few years, resulting in a rapid increase in the total number of sites evaluated. A large, multi-phase study recently performed in the Gulf of Mexico included more marine MT receivers than the total deployed worldwide to that date.

Finding the Base of Salt In 2006, WesternGeco began a test of the MMCI concept in the Garden Banks area of the Gulf of

17. A pseudosection uses approximate or pseudo spatial coordinates. It provides a semiquantitative way to look at spatial data.

> Presurvey modeling. To optimize CSEM acquisition parameters, the subsurface is modeled as a series of resistive layers (left ). Two models having identical geometries are compared. One model incorporates a layer of highly resistive basalt (yellow and brown); the other model assigns that layer a lower resistivity (yellow only). The two models have different phase and amplitude responses to a simulated CSEM pulse. The amplitude ratio between the models (top right ) is maximum (red) at an offset—distance from source to receiver—of about 7,000 m and at a frequency of about 0.7 Hz. The phase difference (bottom right ) has a maximum (red) at about 8,500 m and at a frequency less than 0.1 Hz, and another maximum (violet) at long offset and high frequency. Based on the information in both plots, geoscientists determined that the optimal offset to maximize the chance of detecting this anomaly is about 8,000 m, at frequencies of 0.5 and 0.125 Hz. The contour lines indicate various levels of receiver noise floors (labeled by the power of 10), which depend on the sensors, electronics and the environment. Although the noise floor in some environments may be as poor as 10-14, these plots extend to a noise floor of 10-15, which can usually be achieved.

EM_FIGURE 05a

Resistivity, ohm.m Transmitter-receiver offset, m

Freq

uenc

y, Hz

Basalt

Seawater

1 0

0.2

0.3

0.4

0.5

0.6

0.7

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y, Hz

0.9

1.0

1.1

1.2

1.3

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0.2

0.3

0.4

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0.7

0.8

0.9

1.0

1.1

1.2

1.3

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1.5

2,000 4,000 6,000 8,000 10,000 12,000

Phasedifference, °

Amplituderatio

Transmitter-receiver offset, m0 2,000 4,000 6,000 8,000 10,000 12,000

10 100

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1

2

3

4

5

6

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010203040

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6,000

5,000

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Dept

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500

0

Mexico, offshore Louisiana, USA.18 Exploration companies have had an interest in evaluating hydrocarbon potential in subsalt formations in this area. The seismic data available at the time, a legacy survey called E-Cat, had been reprocessed recently over Garden Banks, but it had insufficient resolution to reliably determine the base of a salt intrusion. The objective of the new study was to integrate marine MT, full- tensor gravity and seismic measurements using an MMCI evaluation to improve the interpreta-tion of the base of salt.

18. Sandberg SK, Roper T and Campbell T: “Marine Magnetotelluric (MMT) Data Interpretation in the Gulf of Mexico for Subsalt Imaging,” paper OTC 19659, presented at the 2008 Offshore Technology Conference, Houston, May 5–8, 2008.

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The Garden Banks study included 171 seabed receivers, more than any previous marine MT survey, although surveys of this density are more common today. The survey utilized five paral-lel north-south lines of receivers, about 2.5 km [1.6 mi] apart, and an east-west cross line (right). Additional receivers placed between these lines provided denser coverage near the cen-ter of the survey area. Bathymetry data indicated seabed expressions of the underlying salt domes.

During the course of the project, two events provided additional data for this investiga-tion. During October and November of 2007, WesternGeco acquired a multiclient wide- azimuth (WAZ) seismic survey over the area, which provided significantly better resolution for base of salt than did the previous E-Cat narrow-azimuth survey. However, even with WAZ illumination, the base of salt was poorly resolved in some areas.19

The second event occurred near the end of 2007, when BP released logging data from its Tamara well in Garden Banks Block 873. This well was drilled through the central portion of the survey area. The gamma ray log indicating the base of salt became available after most of the MT interpretation was completed, providing a base-truth point for comparison.

An approach combining 1D models for each receiver station detected the salt body, but the details of the structure were incorrect because of its complex geometry. Several 2D approaches

19. For more on WAZ surveys: Camara Alfaro J, Corcoran C, Davies K, Gonzalez Pineda F, Hampson G, Hill D, Howard M, Kapoor J, Moldoveanu N and Kragh E: “Reducing Exploration Risk,” Oilfield Review 19, no. 1 (Spring 2007): 26–43.

>MT survey in Garden Banks area. The MT receivers (inset) were placed in five north-south lines and one east-west crossline. Additional receivers were placed in the central area, near the Tamara well. The color-coding indicates seawater depth from bathymetry.

EM_FIGURE 13

750

Seafloordepth, m

975

1,200

1,425

1,650

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2,100

Line

1

Line

2

Line

3

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5

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20. An autochthonous formation is one that was deposited in its current location. This salt would be the source of the shallower salt bodies that moved to their current positions because of density difference and salt plasticity.

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were also used, but the results of all the 2D inversions indicated thinner salt bodies than shown by data from the Tamara well. The three- dimensional nature of the body dictated a 3D approach to modeling.

The first 3D approach taken by the study team was to fit the MT data independent of seis-mic and gravity data. The model started with a homogeneous and isotropic resistivity below the seabed. During iterations, each cell resistivity was allowed to change to match the apparent resistivity and phase measurements. A smooth inversion algorithm ensured that the resistivity changed as smoothly as possible from cell to cell.

Agreement above the main salt body was good among the WAZ seismic result, the MT fit and the gravity model. In addition, the interpreted base of salt is within a few hundred feet of the log-derived base of salt in the Tamara well—a good match. However, the gravity model required adjustments to fit the measured gravity data. Similar gravity measurement results can be obtained for differ-ent configurations (above right). In this case, salt could be added either to the salt layer within the model space or to the autochthonous salt—which was mostly below the maximum depth of the seismic-velocity volume—or the subsalt formation densities could be altered to match the result.20

A second approach used the interpreted seismic survey to provide a starting point for the shape of the salt body. Resistivity for this a priori model was initially set at 50 ohm.m inside the salt body and at 1.2 ohm.m in the surrounding sedi-ments. The inversion changes the values of the resistivity in the grid blocks to fit the measure-ment data while preserving the initial model as much as possible.

The best interpretation used MMCI proce-dures, incorporating all available information, including MT, gravity and WAZ seismic data. Porosities were computed from the WAZ veloc-ity field using local knowledge of the sand/shale ratio of the sedimentary section, and densities were computed from the matrix densities of sand and shale, the density of seawater and the velocity-derived porosity. Density in the salt was assumed constant at 2.16 g/cm3.

The 3D model result in the Garden Banks area had an improved interpretation of the base of salt compared with that based on seismic data alone (right). Resistivity data indicated that a large lobe suggested by the seismic interpreta-tion is not a part of the salt, but belongs to an underlying formation.

The success of this proof-of-concept study was the impetus for a large-scale multisurvey MT project that has been active since May 2007 in other key areas in the Gulf of Mexico. For example, in the Keathley Canyon area, deter-

mining base of salt from seismic data alone was difficult. Gravity data provided improvement, but several alternative interpretations could not be distinguished. By adding MT data and combining all the information through the MMCI approach,

>Nonuniqueness of gravity survey. A gravity survey responds to the mass of an anomaly. A solution can propose one object or many, or have different density and size, as long as the mass and the center-of-mass location for the anomaly are the same. In this example, all three readings measure the same mass.

EM_FIGURE 16

Gravitysurveyresponse

Density2.1 kg/m3

2.7 kg/m3 Volume = 3,600 m3 3,600 m3 2,800 m3

> Confirmation by drilling. MT measurements detected a high-resistivity salt intrusion (pink). The Tamara well, drilled near MT receiver Line 3, provides a point of reference for interpretations of base of salt. The base-salt interpretation (gray) of the best WAZ data available shows a lobe to the southeast that is not supported by the MT resistivity data; the 35- to 50-ohm.m area of resistivity (pink) excludes that lobe from the salt. The 3D MMCI interpretation of seismic, gravity and MT data indicates a base of salt (white) within a few hundred vertical feet of the base determined from the well gamma ray log (turquoise). At the base of salt, the well log resistivity (orange) decreases significantly. MT receiver locations are shown on the seabed (white squares).

2,500

NW SE

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16 Oilfield Review

the analysis team obtained a consistent inter-pretation of the structure, including the base of salt (below). In parts of the survey area, the difference in interpretation of the base of salt was almost 3,000 m [9,700 ft].

EM Studies Offshore Brazil Marine MT surveys have also improved depth imaging in other parts of the world. The Santos basin, offshore Brazil, contains recent subsalt discoveries made by Petrobras. High-resolution

seismic imaging has mapped the stratigraphy of hydrocarbon-producing turbidite reservoirs and the geometries of salt structures, includ-ing a thick sedimentary sequence in a syn-rift structure beneath the salt.21 The lithology of this sequence was defined by the first discovery well of the Tupi area. An MT survey northwest of Tupi confirmed the complex structure and demonstrated the utility of marine MT surveys to Petrobras.22

To the east of the Santos basin MT survey just described, Petrobras and WesternGeco per-formed a marine CSEM survey in the Tambuatá block of the basin as part of a cooperative project (next page, top).23 The survey location was about 170 km [106 mi] south of Rio de Janeiro. The water depth was taken from bathymetry data, and processing also included the variation in seawater resistivity as a function of depth.

The survey used 180 receivers spaced approximately 1 km [0.6 mi] apart and deployed on the seabed over known reservoirs. A vessel towed the source over the receiver lines. Data acquisition used 0.25- and 0.0625-Hz square-wave signals that are also rich in odd harmonics of these frequencies.24

Analysts processed multicomponent electric- and magnetic-field responses for all frequencies in the survey using an advanced workflow based on instantaneous measures of dipole length, dipole moment, dipole altitude, feather angle and dip. The data interpretation proceeded in stages, starting with generating a background model to compare with the processed measurements.

Borehole measurements provided informa-tion on background resistivities, but the log data have more detail than CSEM measurements can discriminate. Thus, analysts reduced the num-ber of layers in the resistivity model to reflect the resolving power of CSEM, but they ensured the resampled well logs retained the same CSEM response as the detailed log-based layering would have. To determine where the boundaries had to be placed, both the cumulative resistance and cumulative conductance were calculated from the well logs and coupled with stratigra-phy. This not only clarified the locations of the layer interfaces but also determined the resis-tivities of the layers and the anisotropy caused by interbedding low- and high-resistivity layers. Analysts conducted detailed 3D modeling based on the blocked well log resistivities and based on model geometries derived from seismic sec-tions without incorporating any reservoirs. The resulting models generated reference background fields, which provided a basis to normalize processed multicomponent field data at each receiver location.

> Keathley Canyon interpretation. The base of salt is difficult to find in the WAZ seismic section (top). The best pick based on the seismic data had a thick section to the right of middle (white outline, bottom). MT resistivity data (colors) add significant new information. Combining seismic, MT and gravity data in the MMCI evaluation improves the previous interpretations of the base of salt and gives interpreters greater confidence in their result (yellow dashed line).

Distance, km

Dept

h, m

10

1

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stiv

ity, o

hm.m

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SE

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Selected tow lines were interpreted using a 2.5D inversion. The 2.5D analysis incorporates a 2D geological model and solves for multiple trans-mitter positions simultaneously, but the sources and receivers are not confined to the plane of the geological model. Thus, realistic acquisition geometries can be simulated (bottom right). The known reservoir underlying the survey area appeared in the EM response as a zone of higher resistivity than the surrounding formations.

As with the MT project farther west in the Santos basin, the CSEM project also shows prom-ise for adding considerable value in upstream applications. Both projects underscore the need for advanced integrated interpretation to improve the result over individual seismic, well log and electromagnetic measurements. They also advance the case for the industry to include these novel integration paradigms in standard applications. Petrobras has a technical collabo-ration agreement with Schlumberger to develop technology that integrates marine EM into other technologies for enhanced depth imaging and reservoir characterization.

Risking Prospects in the Arctic FrontierAs operators move into increasingly difficult environments, the Arctic beckons as one of the last mostly unexploited frontiers. In 2008, the US Geological Survey (USGS) estimated undis-covered resources north of the Arctic Circle at 14 billion m3 [90 billion bbl] of oil and 47.8 tril-lion m3 [1,669 Tcf] of gas—of that total the province west of Greenland and east of Canada had an estimated 1.1 billion m3 [7 billion bbl] of oil and 1.5 trillion m3 [52 Tcf] of gas.25

>Marine MT and CSEM surveys, offshore Brazil. Three lines of receivers for the MT survey (red) extended offshore toward the southeast and into deeper water. The main line was about 148 km [93 mi] long, starting about 42 km [26 mi] offshore, and the two adjacent lines were each about 54 km [34 mi] long. The CSEM survey lines (white) to the east of the MT survey covered the Tambuatá block (red). The map shows the ground elevation and ocean depth.

EM_FIGURE 17

Bacia de Santos

Survey areasSão Paulo

–48° –46° –44° –42°

–48°

–26°

–22°

–24°

–26°

–46° –44° –42°

Rio de Janeiro

MT

Tupi area

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662

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Altitude, m

Oceandepth, m 0 100km

0 100miles

21. Syn-rift refers to events that occur at the same time as the process of rifting. A syn-rift basin is formed along with, and as a consequence of, the rifting process. In the Santos basin, the rifting refers to the early stages of the separation of the South American and African continents.

22. de Lugao PP, Fontes SL, La Terra EF, Zerilli A, Labruzzo T and Buonora MP: “First Application of Marine Magnetotellurics Improves Depth Imaging in the Santos Basin–Brazil,” paper P192, presented at the 70th EAGE Conference and Exhibition, Rome, June 9–12, 2008.

23. Buonora MP, Zerilli A, Labruzzo T and Rodrigues LF: “Advancing Marine Controlled Source Electromagnetics in the Santos Basin, Brazil,” paper G008, presented at the 70th EAGE Conference and Exhibition, Rome, June 9–12, 2008.

24. The strongest harmonics are 0.75, 1.25 and 1.75 Hz for the 0.25-Hz signal, and they are 0.1875, 0.3125 and 0.4375 Hz for the 0.0625-Hz signal.

25. Bird KJ, Charpentier RR, Gautier DL, Houseknecht DW, Klett TR, Pitman JK, Moore TE, Schenk CJ, Tennyson ME and Wandrey CJ: “Circum-Arctic Resource Appraisal: Estimates of Undiscovered Oil and Gas North of the Arctic Circle,” USGS Fact Sheet 2008-3049 (2008), http://pubs.usgs.gov/fs/2008/3049/ (accessed March 31, 2009).

> Combined analysis for the Tambuatá block. Reservoirs (green and pink outlines, top) identified by seismic interpretation were the targets of a CSEM and MT study. Receivers (white triangles) were laid in orthogonal sets, and the CSEM source was towed along the same lines (black). A 2.5D MMCI inversion based on EM and seismic data resulted in a section color-coded for resistivity, with seismic data providing texture (bottom). Along tow line LTAM10 N, a 20-ohm.m resistive anomaly (red) is clearly distinguished from the more-conductive background of about 1.2 ohm.m (green). Seismic results constrained the anomaly shape—by defined control points (white circles, bottom)—for the data inversion.

N

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EnCana Corporation and its joint-venture (JV) partners Nunaoil A/S and Cairn Energy have exploration prospects in two blocks in the frontier basin offshore Greenland, 120 to 200 km [75 to 124 mi] west of the capital city, Nuuk. The ocean depth over the prospects ranges from 250 to 1,800 m [820 to 5,900 ft]. Geologists believe this area’s rifting and sedimentary-fill history is similar to that of the productive North Sea basins. However, the nearest well control is more than 120 km away, and there are no proven petroleum systems in the basins. The JV needed a way to lessen the risk of drilling dry holes, so a CSEM survey was acquired to help identify poten-tial hydrocarbon-bearing features.26

Sedimentary filling of the basin following rift-ing created a fairly simple geology, with the major complication coming from Paleocene volcanic activity. The volcanic flows are easily identifiable geologically, seismically and magnetically. These volcanic rocks are the only known resistive litho-logic units in the survey area above basement, and they are well separated from the Cretaceous

exploration targets. For more information on volcanic formations, see “Evaluating Volcanic Reservoirs,” page 36.

Before conducting the CSEM survey, WesternGeco performed extensive 3D resistivity modeling over each prospect. This step confirmed that the survey could help define the presence of hydrocarbon-bearing reservoirs at up to 3,000 m below the seafloor. Synthetic data were used in forward-modeling and inversion methods. Based on well log data from key distant offset wells, a simplified starting model was created that included a reasonably uniform, 1.5-ohm.m clas-tic sedimentary section from the seafloor to the target depth, a deeper layer with 4-ohm.m resis-tivity extending to the basement, and a 60-ohm.m basement formation.

As part of this presurvey analysis, geoscien-tists optimized the design for target sensitivity, presence of volcanic cover, reservoir proximity to basement and signal waveform, as a few exam-ple parameters. This optimization helped the EnCana JV plan a survey covering the vast area in a cost-efficient way.

The survey layout based on this analysis com-prised 24 transmitter lines and 182 receivers. The tow-line geometry generated data from multiple angles on the receivers. The resulting vertical resolution was designed to be 50 m [164 ft] for the Cretaceous targets at depths of 3,500 m [11,500 ft] below the seafloor.

A high-quality CSEM dataset was obtained in the summer of 2008. Processing the electric- and magnetic-field measurements yielded amplitude and phase responses at each receiver. Starting with electric-field responses, geoscientists analyzed these data using a complex 3D aniso-tropic-resistivity model. The starting geo metry used the JV’s seismic interpretation and well log resistivity information, but no potential reservoirs were included. The 3D inversions required consid-erable computation time and interpreter input.27 The results were numerically stable with electrical models that were geologically consistent.

The inversion process identified resistive anomalies over 8 of 14 prospects. The team used Petrel seismic-to-simulation software to visualize the resistivity volume data for these eight anoma-lies with geologic, seismic, gravity, magnetic and marine MT data (left). The results were insen-sitive to reasonable variations in the starting model, with each variation converging to a simi-lar resistivity solution.

The known Paleocene volcanic rocks pro-vided another indication that the inversions were robust and geologically meaningful. Although the isolated volcanic features were not included in the starting models for the inversions, the inver-sion procedure located them correctly.

The EnCana JV’s objective for obtaining the CSEM study was to improve the assessment of the probability that the structures were charged with hydrocarbons. With firm data lacking prior to the study, the hydrocarbon-charge probability was indeterminate and the JV assigned it an ini-tial value of 50% for each of the eight prospects. The team’s analysis increased the probability of hydrocarbon charging for several features and decreased it for others.

The prospect with the greatest probability for hydrocarbon charging displays many of the characteristics the geoscientists looked for in the analysis. Its resistivity anomaly conforms well with the target interval. The CSEM inversion resistivity within the anomaly increases upward from 10 ohm.m at the base of the structure to 35 ohm.m at the crest. Finally, the anomaly base is flat, which could suggest a hydrocarbon/ water contact.

> Prospects with resistive anomalies. Several prospects in a block west of Greenland were interpreted from seismic data (green outlines). The survey design placed lines of CSEM receivers (white icons) along the source tow lines (white lines) above the seismically determined prospects. The CSEM study distinguished the structures with vertical resistive anomalies (oranges and yellows) from those with no anomaly (representative locations labeled). Volcanic flows above the target formation are also identified along the lines. In this view, resistivities less than 10 ohm.m are not shown. The contour lines indicate depth of the seismic horizon of the target; each contour line represents a 100-m [328-ft] depth difference (also represented as the background color sequence).

EM_FIGURE 26

Volcanic flows

Prospect withoutresistive anomalies

Prospects withresistive anomalies

2468101214161820

Volcanicflows

Resistivity,ohm.m

N

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EnCana and its partners are now prioritizing their prospects to identify the most prospective drilling candidates based on the geology, the geophysical mapping and the CSEM 3D model inversion results. The risk for exploration in this frontier Arctic basin remains great, but CSEM technology offers promising potential to reduce dry holes.

Sounding for the Next GenerationAlthough MT and CSEM surveys have been per-formed for many years, commercial use of the marine technology in the E&P industry is rela-tively new. The industry is still in its infancy in interpreting this electromagnetic survey data and combining the information with that of seis-mic surveys.

The seabed receivers used by WesternGeco follow the basic design developed by Scripps Institution of Oceanography, but the devices and methodologies are continually being upgraded to improve instrument efficiency and reliability. In addition to changes in materials used in the manufacture of the dipoles and magnetometers and their packaging, new equipment has been added to the receiver pack, such as a high- precision compass.

The dipole source for CSEM is also under-going improvement by the industry. Equipment vendors have worked to refine the timing synchronization of the source waveform and the precise positioning of the source antenna.

Major obstacles to marine EM efficiency are the cost and time involved in data collection. Seismic measurements over large 3D areas are efficient because vessels tow multiple receiving streamers and source array guns. In contrast, CSEM surveys cover less area because either sources or receivers, or both, are deployed indi-vidually, with receivers remaining stationary during the survey and then recovered (above). The development of a purely surface-towed, deep-reading EM system is likely at the forefront of R&D activities at many geophysical companies.

The problems are noise inherent in the motion of sensors through the water and signal attenu-ation in seawater, which dramatically reduce the coupling of the source with the seafloor and the amplitude of the response field. The dipole antennas are long, and even with the present seabed configuration, currents can move the antennas and impact data quality.

The National Petroleum Council (NPC), an industry body that advises the US government, studied several advancements related to CSEM, rating them as highly significant for exploration activities.28 To secure energy resources for the future, this expert group identified two improve-ments in CSEM technologies needed over the short term. Development of fast CSEM 3D model-ing and inversion could reduce the number of false positives, or resistive anomalies that currently may be misinterpreted as a commercial petroleum response. These include hydrates, salt bodies and volcanic lithologies. The second short-term goal is integration of CSEM with structural information from seismic surveys to improve the resolution of the EM data. As discussed in the case studies in this article, this work is currently underway through efforts such as the MMCI method.29

Over a longer term, the NPC experts also rated advancing the realm of CSEM studies into shallow water, onshore, and deeper formations as highly significant. The signals in shallow water and onshore are much noisier than in deep water because of the air wave. Signal strength now limits the depth of the CSEM surveys, but the NPC group saw that develop-ments leading to evaluating deeper formations would extend the application to new basins. Alternative acquisition geometries might play a role in ultradeep reservoirs.

The term “electromagnetic sounding” is not yet commonly heard in the E&P industry, but impressive results from this generation of tools and interpretation methods have already sent a clear message. With commercial success will come further advances in technology and a wider variety of applications. —MAA

> Deployment of CSEM receiver. Each receiver is assembled on the deck using defined deployment protocols. Then the receiver is hoisted and dropped at a specified location.

26. Umbach KE, Ferster A, Lovatini A and Watts D: “Hydrocarbon Charge Risk Assessment Using 3D CSEM Inversion Derived Resistivity in a Frontier Basin, Offshore West Greenland,” CSPG CSEG CWLS Convention, Calgary, May 4–8, 2009.

27. Mackie R, Watts D and Rodi W: “Joint 3D Inversion of Marine CSEM and MT Data,” SEG Expanded Abstracts 26, no. 1 (2007): 574–578.

28. National Petroleum Council (ed): Hard Truths: Facing the Hard Truths about Energy. Washington, DC: National Petroleum Council, 2007. Also available online at http://www.npchardtruthsreport.org/ (accessed May 5, 2009).

29. WesternGeco regularly performs 3D modeling studies and offers 3D CSEM inversion including the use of algorithms in which the MT data are jointly inverted to help constrain the CSEM inversion.

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