3. chapter 3 components of seismic acquisition

Chapter 3 Components of Seismic Acquisition

Introduction

In 1849, Robert Mallet gathered all the necessary components to perform his experiment to record ground motion. Nevertheless, he failed to record adequate data (see Chapter 1). Today, the goals of seismic experiments certainly are more ambitious, but possible failures can arise from the very same causes. The causes of Mallet’s failure are obvious:

weak source: Gunpowder is 50% less energetic than dynamite.•low sensitivity of receiver: It takes a fairly large ground motion to create ripples that •can be observed at the surface of a mercury bowl.high instrument noise: Poor firing-time control, limited clock accuracy, and •observer reflex time add up to constitute a large imprecision (noise) in time measurement.lack of understanding of wave propagation: A phenomenon as complex as wave •propagation cannot be understood from one single observation.

In this chapter, I shall review briefly how the first three causes of Mallet’s failure have been addressed in the past and shall discuss the way they are addressed today. I will try to refrain from predicting too much about how they could be addressed tomorrow. In Chapter 4, I will discuss the last cause of Mallet’s failure — lack of under-standing of wave propagation.

Seismic sources

Explosives

Explosives remained the only artificial source for nearly 100 years after Mallet’s experiment. They are also the source with which every other source is compared.

Conventional land and marine charges

Figure 1 represents a seismic source point (SP) in the marine environment in 1961. The charge was a few tens of kilograms. Charges have decreased considerably, but explo-sives still are used extensively in land operations when alternative sources cannot be used or would lead to significantly higher costs. The reason for the popularity of explo-sives is their very high ratio of radiated energy over mass, even though less than 5% of their internal energy is radiated in the seismic bandwidth (Lavergne, 1970).

Distinguished Instructor Short Course • 37

Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Figure 2 (Lavergne, 1970), gives the repartition of energy in an underwater explo-sion. The bubble effect corresponds to the successive expansions and contractions of the gas generated in the explosion and trapped in the water. In Lavergne’s example, it con-sumes almost 50% of the energy. The rest of the lost energy is dissipated in heat or is radiated at frequencies outside the seismic bandwidth.

Sharpe (1942) analyzes production of elastic waves by explosion in land. He starts with empirical observations that “have become a part of the lore of every field operator” (Sharpe, 1942, p. 144):

1. A given amount of explosive detonated in a clay or water-saturated sand formation results in a greater amplitude of reflected motion than an equal charge detonated in a dense, rigid formation, such as limestone.

2. If a hole is sprung by an initial large charge in order to form a sizable cav-ity, later small charges will result in a larger amplitude of reflected motion than would be produced in the absence of springing.

3. The frequency spectrum of reflected motion is a function of the formation in which the charge is fired: shots fired in the low velocity zone result in very low frequency motion, compared to shots fired below the low velocity zone; shots fired in a rigid material, such as a limestone, result in a much higher frequency motion than shots made in, for example, a shale; in a general way, the high frequency content of reflected motion increases with an increase in shooting depth.

4. The frequency spectrum of reflected motion is a function of charge size, a larger charge having a tendency to increase the proportion of low frequen-cies in the reflected motion.

5. The amplitude of reflected motion produced by a given quantity of a high speed explosive, such as a dynamite, is considerably greater than that produced by a quantity of a low speed explo-sive, such as black powder, even when this quantity has been selected such that the maximum pressure attained is the same for both explosives (Sharpe, 1942, p. 144–145).

Sharpe (1942) then develops a theory on the production of seismic energy from an explosion to account for his five pieces of empirical knowledge. That knowledge still applies today. Because of the com-plexity of the medium in which charges are detonated, it essentially remains

Figure 1. A seismic explosion in southern France, 1961. Used by permission of CGGVeritas.

Seismic Acquisition from Yesterday to Tomorrow

38 • SocietyofExplorationGeophysicists/EuropeanAssociationofGeoscientists&Engineers

Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

empirical, despite other theoretical studies. Among those works, at least two are worth mentioning: Cole (1948) and Ziolkowski and Lerwill (1979).

R. H. Cole worked for the United States Army during and after World War II. Cole (1948) extensively covers the subject of underwater explosions, both theoreti-cally and experimentally. In particular, Cole gives an empirical formula (Cole, 1948, p. 239) that somewhat closely matches values obtained theoretically:

P

MR

=ÊËÁ

ˆ¯̃

5231 3 1 13/ .

.

(1)

Here, P is the pressure in bars observed at a distance R (m) of the explosion of a charge M (kg) of TNT. However, all of Cole’s work relates to underwater explo-sions using military-grade explosives, and therefore, it must be extrapolated with care for seismic-grade explosives in consolidated media.

Sharpe’s analysis (Sharpe, 1942) is revisited many years later by Ziolkowski and Lerwill (1979), working under these assumptions: (1) Radiation generated by the explosion is spherically symmetric, and (2) for a given type of explosive in a given medium, the fraction of the total explosive energy that is converted into seismic energy is a constant, independent of the mass of explosives M.

Ziolkowski and Lerwill (1979) demonstrate that both the duration and the ampli-tude of the pulse are proportional to M1/3. The amplitude spectrum is proportional to M2/3. This theoretical result must be understood properly before being compared with actual experiments. In a medium with no absorption, dividing the charge by eight would result in a pulse with maximum amplitude divided by two but also with frequen-cies multiplied by two. If the observation is made in identical (seismic) frequency ranges, reduction in the maximum amplitude of the pulse would be considerably larger. It would be very close to four in the water, but it would be any amount from four to eight or more in a consolidated material.

Reduced-charge explosive marine sources

Before explosives were banned totally offshore, considerable efforts were devoted to limiting their impact on the marine environment. The basic idea was simple: Reduce the charge. In addition, safety was a major concern taken into account by developers. Lugg (1979) cites the following methods:

• Maxipulse(WesternGeophysicalCo.)fires224-gchargessentbywaterpressurethrough a hose, at a depth of 12 m. The depth of the explosion selected to increase

Figure 2. Distribution of the total chemical energy in an underwater explosion of a high explosive, TNT (trinitrotoluene). One-half of the released energy goes into the shock wave, and only one-fifth is radiated in broadband waves, with less than one-twentieth of this energy left in the seismic bandwidth. From Lavergne, 1970, Figure 1.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

the amplitude of the initial pulse and to reduce damage to marine life results in the creation of a bubble, which oscillates until it loses all its energy. It is necessary to record the signature to enable reduc-tion of the oscillations by processing the recorded data.

• Flexotir(InstitutFrançaisduPétrole)uses two-ounce charges that are flushed through a hose into a perforated cast-iron sphere, where they are detonated. Figure 3 shows a Flexotir sphere.

• Aquaseis(ChemicalIndustries,Ltd.)usesPrimacord® (Dyno Nobel, Inc.) to make a linear array. This feature results in re-duced bubble oscillations.

• Seisprobe(SeismographService,Ltd.)isan array of four exploders using a mix-ture of propane and oxygen.

• AswithSeisprobe,Dinoseis(SinclairResearch, Inc.) uses a propane-and-oxy-gen mixture detonated by high voltage in a chamber between a heavy top element and a lighter bottom. Figure 4 shows a 7000-lb Dinoseis unit on a rear deck.

Nonexplosive marine sources

The dramatic impact of explo-sive sources on the environment, along with major safety issues, led to the progressive ban of explo-sives. Restrictions started as early as 1949 offshore California, but explosives were still in use offshore in some countries in the early 1980s. Before I describe the air gun (the dominant marine source used today), it is worthwhile to devote a few lines to other nonexplosive sources. As mentioned above, developers were concerned with

Figure 4. Marine Dinoseis: A mixture of propane and oxygen is detonated in a chamber formed by two plates. From Jones, 1968. Used by permission of the Geophysical Society of Houston.

Figure 3. Flexotir sphere. A two-ounce charge is detonated in the sphere submerged at a depth of 12 m. Used by permission of CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

safety and the environment. Another concern was bubble control, arising from the increased depth at which the sources were operated.

Sources presented at the 1968 symposium organized by the Geophysical Society of Houston

In 1968, the Geophysical Society of Houston organized a symposium on marine nondyna-mite energy sources. Dinoseis and Flexotir (classified above as explo-sives) were presented at the sympo-sium, along with the following nonexplosive sources:

sparker: A high-discharge cur-•rent between two electrodes in the water creates a bubble made of steam and ionized particles, resulting in a sharp pressure pulse (Luehrmann, 1968).Hydrosein (Marine Geophysical •Services): A void is created by an air-powered piston between two plates. Figure 5 shows a Hydrosein source (Schempf, 1968).air gun: This source will be •described in more detail below.marine vibrator: A modern •vibrator consists of a pulsating sphere made of two half shells connected by a soft envelope. A hydraulic actuator moves the half shells relative to each other. Figure 6 represents the marine vibrator ready for use (Lee, 1968).

Later sources

Other sources were developed later, including

Flexichoc(InstitutFrançaisdu•Pétrole):Thevolumeformedbytwo rigid plates inside a flexible

Figure 5. Hydrosein. A void is created by an air- powered piston between two plates. From Schempf, 1968. Used by permission of the Geophysical Society of Houston.

Figure 6. Marine vibrator. Two half shelves connected by a soft ring are moved in a sinusoidal motion by a hydraulic actuator. From Lee, 1968. Used by permission of the Geophysical Society of Houston.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

envelope is squeezed by using differential pressure between the outside and inside of the envelope.Vaporchoc(CompagnieGénéraledeGéophysique):Abubbleisgrownduringapprox-•imately 40 ms by progressive steam injection into the water. When injection stops, the bubble decreases while steam condenses, and the water rushing into it creates very high pressure and eventually implosion. The bubble fully collapses and does not oscillate. Figure 7 represents a Vaporchoc signature with two peaks, correspond-ing to the start of steam injection and the bubble implosion, respectively. This source,withvariousimprovements,wasusedbyCompagnieGénéraledeGéo-physique until the mid-1980s.water gun: A few liters of water are injected into the sea in a very short time by a •system of pistons and valves. Figure 8 represents the water-gun principle. Figure 8a shows the gun at rest. High air pressure builds up in the firing chamber sealed by the shuttle top. Figure 8b depicts water ejection. The solenoid causes the shuttle to move down, exposing a larger area to the high pressure in the chamber, which then gives it the power to expel water quickly from the chamber through exhaust ports into the sea. Figure 8c shows a void created behind the expelled water. The void then is filled with high-velocity seawater, resulting in an implosion with emission of a sharp pressure pulse. As with Vaporchoc, the water gun does not create an oscillat-ing bubble. A water-gun signature is shown in Figure 9. The initial injection peak barely can be seen 12 ms before the implosion peak. Water guns are still in use today.

Air gun

The air gun emerged as an alternative marine source in the 1960s, and it is now virtually the only source used in offshore seismic operations, although growing envi-ronmental concerns exist. The air gun began with an advantage (it is relatively simple and safe) and with three major problems, illustrated in Figure 10.

The top curve in Figure 10 represents the far-field signal gener-ated by exploding 33 lb of dynamite in the vicinity of the sea surface. Gas generated by the explosion is dispersed immediately into the atmosphere, and consequently, no bubble is formed in the sea.

The bottom curve of Figure 10 is the far-field signal generated by the air gun. The three problems with the air gun can be seen on this curve. The first issue is that energy released by the air gun is much lower than energy released

Figure 7. Vaporchoc signature. The first energy peak corresponds to the beginning of steam injection in the water. The second peak occurs at implosion of the steam bubble. Used by permission of CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

by dynamite. The ratio between the peak amplitudes of dynamite and the air gun on Figure 10 is 200.

The necessity of operating at a certain depth is the origin of the other two problems with the use of air guns. The greater depth prevents air from reaching the surface during its initial expansion, and this leads to a series of bubble oscillations that can be seen on the bottom curve of Figure 10. In addition, the depth is responsible for a separation in time between the initial, positive pressure peak and its negative ghost reflected by the sea surface.

The first two problems are solved by using arrays of several guns. If they are well designed, the arrays result in the addition of initial peak energies and the reduction of bubble energy. The last problem most often is solved by using the gun at a depth shal-low enough to mitigate the negative effect of the ghost.

Figure 9. Water-gun signature. There is no bubble oscillation after the sharp implosion pulse. After Sercel, 2010a, p. 25. Used by permission of Sercel Corporation.

Figure 8. Water-gun principle. (a) High air pressure builds up in the firing chamber. (b) Water ejection. High pressure in the chamber provides the energy necessary to expel the water from the water chamber into the sea. (c) A void is created behind the expelled water. It is filled subsequently with seawater, resulting in an implosion. After Sercel, 2010a, p. 26. Used by permission of Sercel Corporation.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Air-gun operation

Several air-gun designs are commercially available. They all share the basic principle of using air pressure to accelerate port opening, allowing the high-pressure air to be vented explosively in the water. Figure 11 describes the particular operation of a G. Gun 150, manufac-tured by Sercel. Figure 12a and 12b represents the corresponding ampli-tude spectrum and far-field signa-ture, respectively.

Peak pressure

For a single gun, peak pressure is proportional to the cubic root of its volume (Giles and Johnston, 1973):

P = CV1/3. (2)

Bubble period

The bubble period is given by

T

PVP

= ( ).

./

/c c

1 3

05 646 1

(3)

Here, Pc is the pressure in the gun chamber in psi, Vc is the vol-ume of the gun chamber in cubic inches, and P0 is the hydrostatic pressure at the gun depth (Giles and Johnston, 1973).

Gun clusters

When air guns are positioned in close vicinity, they interact. The interaction depends on the distance between them, and it obviously decreases when the distance in-

creases. Clustering guns offers the advantage of allowing more efficient usage of com-pressed air. That advantage can be seen in Figure 13, which compares the signatures

Figure 11. Air-gun operation of the G. Gun 150. (a) Compressed air fills up the return chamber in the hollow shuttle to close and seal the main chamber, which is pressurized. (b) When the solenoid valve is energized, the triggering chamber is pressurized, allowing the shuttle to unseal. Because its larger area is pressurized, the shuttle quickly accelerates and uncovers the ports. The main acoustic pulse is created by the release of high-pressure air into the water. After Sercel, 2010a. Used by permission of Sercel Corporation.

Figure 10. Comparison of “ideal” dynamite pulse and 1000-in3 (16.4-liter) air-gun pulse. After Giles and Johnston, 1973. Used by permission of EAGE.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

produced by a single 160-in3 gun with a four-gun cluster of identical total volume.

Clustering provides the designer with additional freedom to propose highly efficient arrays. It is used in almost all air-gun arrays in operation today. Modeling bubble interaction was a subject of debate in the early 1980s. Ziolkowski et al. (1982) and Parkes et al. (1984) introduce a mod-eling method that has proved to be adequate and is accepted widely today. Commercial software pack-ages also have been developed.

Air-gun arrays

The primary reason to fire sev-eral guns at the same time is to increase the source strength, as indi-cated above. An ensemble of guns fired simultaneously is called an array. The geometric arrangement of guns and clusters of guns of different sizes within an air-gun array also allows researchers to shape the result-ing signal and adjust its directivity. This last property will be discussed briefly later in this chapter. For prac-tical reasons, arrays consist of several subarrays, which are groups of guns and gun clusters aligned along the vessel trajectory. Figure 14 shows an air-gun array composed of three identical subarrays.

Signatures

It has become common prac-tice to record near-field signatures continuously. A near-field hydro-phone is deployed in the vicinity of each gun, and it records the signal emitted by all guns. Recording far-field signatures is not as common. The water should be deep enough to allow the signature hydrophone to be placed at a large distance (a few hundred meters) below the source and another

Figure 12. Air-gun signature of the G. Gun 150. (a) Amplitude spectrum and (b) far-field signature. After Sercel, 2010a. Used by permission of Sercel Corpo-ration.

Figure 13. Comparison of the pulse radiated by one large gun and by a coalesced gun. After Giles and Johnston, 1973. Used by permission of EAGE.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

few hundred meters above the seafloor to avoid interference with the sea-bottom reflection. The loca-tion of this hydrophone should be known with good precision. Figure 15 represents the setup for such a recording. Figure 16 compares a modeled signature with a recorded far-field signature (Figure 16a, time-domain response; Figure 16b, ampli-tude and phase spectra).

Land surface sources

Contrary to explosives and air guns, which generate a pressure pulse in a medium, surface land sources generate a force applied at the surface of the earth. The force can be impulsive (such as weight drop, land air gun, or Dinoseis) or vibratory. The respective far-field sig-natures of the two types of forces are essentially similar. The far-field signature will be discussed in Chap-ter 5, which is dedicated to the vibroseis source.

The most popular surfaces sources are:

• weightdrop:Aweightisdroppedfrom a certain height (generally 1 to 4 m), either directly to the ground (Figure 17) or onto a baseplate (Figure 18). In difficult terrain, helicopters have been used to drop weights from larger heights (Figure 19). The advan-tage of weight drop is its simplic-ity. The major problem is the difficulty of predicting the instant of impact, and therefore, several units cannot be used simultane-ously to increase source strength. Weight drop still is used today,

Figure 14. Air-gun array composed of three identical subarrays, each of which has four clusters of two guns and two single guns. Its total volume is 3210 in3. Used by permission of CGGVeritas.

Figure 15. Far-field signature recording setup. A signa-ture receiver is towed at a depth of 100 to 200 m below the air-gun array. Water depth should be sufficient to observe an adequate length of signature before reflection on the seabed. Used by permission of CGG-Veritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

particularly when a small source is desired. Terra Pack (General Dynamics): A •600-kg hammer is driven agai nst the ground by compressed air at avelocityofabout20m/s. land air gun: The land air gun is •essentially a marine air gun in a water-filled tank in contact with the earth.

• Minisosie:Minisosieisbasicallya thumper with an accelerometer on its pad (Figure 20). It is left to the operator to generate a se-quence of pops as randomly as possible. The accelerometer re-cords the sequence of pops and sends them to the recorder, which performs an operation equivalent to correlation. It is possible to use several units simultaneously. The advantage of this source is its very low weight and flexi-bility. It suffers from two problems: relatively low radiated energy and difficulty for the operator in producing a random sequence of pops. Minisosie is used marginally today.

• Dinoseis(Figure21):LandDinoseisusesthesameprincipleasmarineDinoseis:Amixture of propane and oxygen is detonated in a chamber essentially made of a

Figure 16. Comparison between modeled and measured far-field signatures. (a) Time-domain response. (b) Amplitude and phase spectra. Advances in signature modeling reduce the need for far-field signature recording. Used by permission of CGGVeritas.

Figure 17. Weight-drop truck in the early 1970s. The weight hangs at the rear of the truck. Used by permis-sion of CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

steel plate in contact with the ground and a heavy reaction mass. Because the firing instant is controlled perfectly, several units can be used simultaneously. A Dinoseis source could compete with the vibrator during the 1970s, but it could not keep up with the progress of vibroseis for two main reasons: (1) From a technological point of view, no significant progress could be foreseen in Dinoseis, whereas vibroseis was improving quickly. (2) From a logistical and safety point of view, handling of oxy-gen and, to a lesser degree, of propane is considerably more problematic than handling of diesel fuel.

Many other surface sources have been designed for a very particular use or generally for shallow-seismic work. They include:

• marineairgun:Anairgunisfiredin a water-filled hole. The advan-tage of this source is that it does not use dynamite.

• shotgun:Adownward-pointinggunis fired directly into the ground or into a shallow hole drilled to match the diameter of the gun. The charge can be gunpowder only or a single ball. This source develops very low energy, and it is still in use today despite obvious safety issues.

• electromagneticlift:Large-capaci-tance condensers are discharged into a solenoid coupled to a base-plate. The resulting magnetic field is used to lift a heavy reaction mass. In reaction, the baseplate applies a downward force to the surface of the earth.

Figure 18. Accelerated weight drop. A mechanical system prevents the bounce of the mass, lifts the mass, and simultaneously compresses a spring used to accelerate the mass for the next pop. The mass hits a baseplate. Used by permission of CGGVeritas.

Figure 19. A helicopter lifts a few hundred kilograms of chain and drops it from a height of 50 to 100 m. Used by permission of CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

A source point, often called a shotpoint (SP), might comprise one or more individual sources (such as an explosive charge or vibrator), forming a source array with geomet-ric properties similar to the proper-ties of air-gun arrays. However, although marine designers use guns of various sizes to optimize the air-gun array, land-source arrays most often use identical elements.

Source strength

Source strength can be defined as the amplitude of the far-field signal in a given bandwidth. In marine seismic, it usually is called the peak-to-peak amplitude (of the far-field signature).

It is relatively easy to predict signal amplitude in offshore opera-tions by using Cole’s formula for explosives (see equation 1 above) and by using commercial software for air-gun arrays. The comparison of Giles and Johnston (1973) can be extrapolated to predict equivalent far-field amplitudes (284 bar-m) from 1 kg of explosives and from a very large array of 175 decoupled 100-in3 air guns. Note that this is an order of magnitude only because the output of an air gun depends on the particular model and the pressure at which it is operated.

Prediction is much more diffi-cult onshore. However, a few statis-tics can be given. Meunier and Daures (2008) present an experi-ment that shows the equivalence, in the bandwidth of 5 to 25 Hz, among 1 kg of explosives, one 400-kN vibratorsweeping1.25Hz/s,anda20-ton weight dropped from 20 m.

Figure 20. Minisosie. A thumper is operated “ran-domly.” An accelerometer attached to the pad of the thumper transmits a suite of hit times to the recorder. Used by permission of CGGVeritas.

Figure 21. Dinoseis truck in the 1970s. The firing chamber, which can be seen below the frame, con-sists of a baseplate and a reaction mass, between which a mixture of propane and oxygen is detonated. The oxygen reserve in the rear of the truck and two propane tanks behind the spare wheel can be seen. The rope attached to the truck was used to help the driver keep the right distance. Used by permission of CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Again, these numbers refer to a particular experiment in a particular location and are meant only to provide an order of magnitude.

Directivity effects

It usually is considered that the directivity of a single charge of explosives or of a single air gun is spherical in a homogeneous infinite medium. However, two factors break the spherical symmetry and make seismic source directive:

1) the vicinity of reflecting interfaces. For example, Figure 22 represents the directivity diagram of the point surface source (land). For P-waves, amplitude is highest in the vertical direction, and it progressively decreases toward zero in the horizontal direction. It does not depend on frequency.

2) the geometric distribution of individual sources when using source arrays. In land seismic, the resulting directivity can be difficult to evaluate, and most often, the array alone is taken into account. In marine seismic, the situation is simpler, and directivity evaluation is more accurate. Figure 23 represents the source-directivity diagram for the source array shown in Figure 14.

Seismic receivers

It is convenient to distinguish between motion sensors, which measure the projec-tion of a motion (displacement, velocity, or acceleration) along one or several axes, and pressure sensors, which measure pressure.

Motion sensors

Motion sensors are used onshore, on the seafloor and, since 2007, in some marine streamers (see Appendix B). The domination of the electromagnetic geophone was unchallenged until 1999, when a first contender, the microelectrome-chanical-system (MEMS) accelerom-eter was introduced in the seismic industry. A second contender, the optical-fiber accelerometer, also is gaining in use, especially on the seafloor.

Electromagnetic geophone

From the early 1930s, one type of land receiver has dominated the seismic market and still does — the

Figure 22. Small source on “chalk half-space” (P and S-V radiation patterns). The light gray line represents radial displacement of P-wave particle radial displace-ment (m). The black line is the S-V wave transverse displacement (m). Radial distance is 1000 m, VP = 2140 m/s, VS = 1235 m/s, Poisson = 0.25, density 1800 kg/ m3, and source strength is 200 kN at 10 Hz. After Sallas, 2010. Used by permission of EAGE.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

electromagnetic geophone. Its size and mass have decreased from 15 cm and 6 kg, respec-tively (Figure 24), to 3.2 cm and 74 g, respectively (Figure 25). Since the late 1960s, the most notable advancements of the geophone have been increases in sensitivity, reliabil-ity, and similarity; a decrease in distortion; and a notable increase in numbers of units produced (more than 6 million per year). However, the size of the geophone has remained unchanged during that time.

The basic principle of the velocity geophone is shown in Figure 26. A coil is attached to a moving mass suspended by a spring to a structure coupled to the ground. The structure includes a permanent magnet that generates a magnetic field in the region of the coil. Relative motion of the magnetic field and coil generates current in the coil and voltage across the output terminals, proportional to the velocity of the ground motion.

Figure 23. Directivity diagrams of the source array in Figure 14 (a) in the sail-line direction and (b) across the sail-line direction. Angles from the vertical (from −90° to +90°) are marked in degrees. Frequencies (from 0 to 250 Hz) are plotted along the radii. Used by permission of CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Figure 27 represents the response of a modern geophone. It is given by its ampli-tude (Figure 27a) and phase (Figure 27b) spectra for various damping values. Damping is adjusted by inserting a resis-tor in parallel with the geophone coil. When damping is lower than 70%, the geophone response goes through a peak at a frequency that increases progressively when damping increases. For zero damp-ing, the frequency of the peak is the natural frequency (10 Hz in Figure 27). Damping of about 70% is used most often because it is the lowest damping for which there is no peak in the amplitude spectrum.

Note that when damping increases, sensitivity decreases. Above the natural frequency, the response is constant. The constant value defines the geophone sensi-tivity(22V/m/sfor70%damping,asshown in Figure 27). Below that frequency, amplitudedecreaseswithaslopeof12dB/octave. The phase rotates from 0° at low frequency through 90° at the natural frequency to 180° at high frequency.

Natural frequencies range from a few millihertz for earthquake seismology (the corresponding sensors are called seismometers, not geophones) to a few hundred Hertz for special applications. Most seismic operators in the oil industry use a natural frequency of 10 Hz for surface seismic work. At that frequency, depending on the spring design, geo-phones can operate only in the vertical or horizontal direction with some “tilt” tolerance. The tolerance increases with the natural frequency to reach 180° even-tually. Then geophones can operate in any direction and thus are called “omni-tilt” geophones. A 14-Hz omnitilt geo-phone is now available.

Geophone elements are inserted into plastic cases, as shown in Figure 25. Cases

Figure 24. Geophone used in 1930s and 1940s. It weighs 6 kg. SEG Virtual Geoscience Center, 2000.

Figure 25. Modern geophone element with its case. Diameter is 25.4 mm, height is 32 mm, weight is 74 g, and operating temperature ranges from −40°C to +100°C. From ION, 2007. Used by permission of ION.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

for use in swamps and shallow water are totally waterproof.

Geophones can be grouped in strings of several geophones, most often con-nected in series. The strings are deployed on the ground in various geometric arrangements and then are connected to the recorder.

MEMS sensors

After 50 years of incremental enhancements of the conventional (elec-tromagnetic) geophone, Maxwell (1999, p. 1182–1183) introduces a new seismic sensor in these terms: “The MEMS (micro-electro- mechanical-system) tech-nology is similar to that used in car airbag control circuits and allows mass production of very closely matched com-ponents. The new geophone design has low intrinsic noise and ultralow distor-tion. A digital feedback system keeps a reference mass steady during recording, and the output is proportional to the correction force needed to keep the mass station-ary when the ground is accelerated. This output is digital.”

Figure 28 represents the two components of the first seismic microelectromechani-cal-system (MEMS) accelerometer (ION Vectorseis®). Figure 28a represents two micro-machined accelerometers, each of which includes a frame, a mass, and springs. Figure 28b represents the application-specific integrated circuit (ASIC), which performs the feedback loop and generates the digital output.

MEMS sensors present many advantages. Their response is flat in amplitude and constant in phase from zero frequency (DC). They can measure the gravity field, and from that, they can evaluate tilt angles relative to the vertical. In that technology, the output signal is a by-product of the feedback loop. The elimination of a coil makes the MEMS sensor insensitive to electromagnetic noise.

The weakness of the MEMS sensor is its higher noise. The noise floor is the conver-sion of thermal noise into acceleration or into velocity. At any given frequency, signal higher than the noise floor will be seen on an individual record, and signal lower than the noise floor will be buried in the noise.

Figure 29 is a comparison between the noise floors of a geophone-amplifier system (full-scale 1.6-V rms) and of a MEMS accelerometer. At high frequencies, the MEMS noise floor is lower, which makes it ideal for high-resolution single-sensor work. Note that connecting two geophones in series results in a 6-dB decrease in the noise floor. That is not possible with MEMS. Signal addition must be digital, and the overall gain

Figure 26. An illustration of the moving-coil geophone (a velocity geophone). A mass (a piece of material including a coil) is attached to a spring supported by a magnet coupled to the earth. When the earth moves, there is a relative motion between the coil, which tends to remain stationary, and the magnetic field, which is cou-pled to the earth. The resultant voltage output is proportional to the velocity of the motion. After Sercel, [1980s]. Used by permission of Sercel Corporation.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

insignal-to-noiseratio(S/N)ofusing two MEMS is only 3 dB (see equation 19 of Chapter 2).

Optical sensors

The general principle of the fiber-optic sensor is to link motion or pressure variation to a change in fiber length and to measure that change. Figure 30 represents an optical accelerometer. A large quantity of optical fiber is wound around a structure whose size changes in one specific direction when submitted to an acceleration in that direction.

In Figure 30, the structure consists of two half cylinders con-nected by a rod that acts like a spring. When structure size varies, the length of fiber varies accord-ingly. Two Bragg gratings are applied to the fiber on each side of the coil. Bragg gratings are partial reflectors sensitive to one particular wavelength. When light (contain-ing many wavelengths) enters the system, the wavelength associated with that particular sensor is reflected before and after it has traveled through a variable length of fiber. Analysis of the phase dif-ference between the two reflections provides the measure of accelera-tion. The noise floor of the optical accelerometer is very similar to that of a MEMS accelerometer.

Figure 31 represents an opti-cal pressure sensor (hydrophone). The fiber now is wound around a cylinder made of compressible material. The diameter of the cylinder and the length of fiber wound around it change when

Figure 28. Microelectromechanical-system (MEMS) accelerometer components. (a) Bulk-capacitive micromachined accelerometers. (b) Application- specific integrated circuit. From Tessman et al., 2001, Figures 2 and 3.

Figure 27. Geophone response. (a) Amplitude response for various damping factors. For damping values lower than 70%, the response goes through a peak near the natural (or resonant) frequency. A damp-ing of 70% is used most commonly; it is the lowest damping for which the resonance is eliminated fully. (b) Phase response for the same damping factors. It varies between 0° and 180° from 0 Hz to 1000 Hz. The phase shift occurs around the natural frequency; it becomes more progressive when damping increases. After ION, 2007. Used by permission of ION.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

submitted to a pressure change. As with optical accelerometers, Bragg gratings are applied on each side of the coil, and the phase dif-ference between the corresponding reflections provides the measure of pressure. The most remarkable ad-vantages of optical sensors are their insensitivity to humidity and to electromagnetic noise, their high reliability, and the fact that they do not require power.

Pressure sensors

The dominant pressure-sensor (or hydrophone) technology uses a piezoelectric transducer. Piezo-electric materials (crystals such as quartz or ceramics such as barium titanate and lead zirconate titan-ate [PZT ]) have the property of changing their dimension when inserted into an electric field. Recipro-cally, when those materials are submitted to strain, electric charges build up on the surface of the material. If electrodes are in contact with those surfaces, a voltage can be measured. The general principle of piezoelectric hydrophones is to use pres-sure variations to create a geometric dis-tortion of a piezoelectric material such that the measured voltage is proportional to pressure. There are several designs of piezoelectric hydrophones. Figure 32 represents two of them.

The bender (Figure 32a) is more sensi-tive than the compressor (Figure 32b), but the bender is limited in depth. A disc or strip of piezoelectric material is fixed to a support and can bend under pressure. Bend-ers are used in marine streamers and in transition-zone hydrophones. The compres-sor, shown in Figure 32b, is less sensitive, but it can sustain large depths. A cylinder or sphere is squeezed under pressure.

Figure 29. Sensor noise floor represented in velocity units. It decreases continuously for a MEMS acceler-ometer (light gray curve). For a geophone-preamplifier system, it decreases sharply below the natural fre-quency and then remains constant. Used by permis-sion of CGGVeritas.

Figure 30. Optical accelerometer. Optical fiber is wound around a structure whose size changes in a given direction when submitted to acceleration in that direction (two solid half cylinders connected by a spring); as a result, the total length of the fiber changes. Light enters the system from the left. A small por-tion of light is reflected by Bragg grating 1. Another small portion is reflected by Bragg grating 2 after having traveled along the fiber that is submitted to length variation. The phase difference between the two reflections is converted into a measure of acceleration. Used by permission of CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Compressors are used in ocean-bottom cables and ocean-bottom nodes.

Piezoelectric sensors have very high-capacitive impedance and need some impedance adaptation to be used in favorable conditions. The adaptation can be performed by an electronic pre-amplifier (in most marine streamers) or by a transformer (in most transition-zone hydrophones). As a consequence, their wide natural bandwidth is reduced to the desired value for the particular application. Another consequence is that their sensitivity can be adjusted easily. Figure 33 compares the noise floors of a transition-zone hydrophone and of a conventional 10-Hz geophone in a medium of acoustic impedance 1.5 106 kg m−2 s−1. The full scale is 0.4 V.

Receiver arrays

As with sources that can be arranged in source arrays, sensors can be arranged in receiver arrays, also called receiver groups or simply receiv-ers. The aim of the arrangement is noise reduction. In marine seismic, the noise to attenuate is the streamer noise travel-ing along the streamer, and in land seismic, it is ground roll. Properties of these geometric arrangements will be discussed in more detail in Chapter 6, although they are not essentially differ-ent from the properties of source geo-metric arrangements briefly men tioned earlier.

Seismic cables

Receivers must be connected to the recorder. Traditionally, they are con-nected by cables. The majority of land seismic operations and all marine-streamer operations still use cables.

Figure 31. Optical hydrophone. Optical fiber is wound around a compressible cylinder. When the pressure changes, the diameter of the cylinder and hence the total length of fiber change. Light enters the system from the left. A small portion of light is reflected by Bragg grating 1. Another small portion is reflected by Bragg grating 2 after having traveled along the fiber that is submitted to length variation. The phase difference between the two reflections is converted into a measure of pressure. Used by permission of CGGVeritas.

Figure 32. Piezoelectric hydrophone designs. (a) Bender. Piezoelectric material is glued onto a metal disc or strip that bends under pressure. (b) Compressor. A cylinder or sphere is com-pressed under pressure. Used by permission of Peter Maxwell and CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

However, two other options remain: (1) radio transmission, which has not solved the data-flow problem yet, and (2) local recording and storage, which seem to be gaining ground.

Streamers

In marine seismic, sensors and cables form a streamer towed by the seismic vessel. A streamer is a polyurethane tube with a diameter of 5 to 6 cm and a length of 4 to 12 km. It is commonly made of 12-station, 150-m sections. Each station includes eight to 16 hydrophones and one analog-to-digital converter on modern streamers. Buoyancy is provided by kerosene (liquid streamer) or by solid filling (solid streamer). The advan-tage of solid streamers is their lower flow noise. The maximum number of streamers towed by a single vessel has increased steadily, from three in 1991 to 17 in 2009. A typical arrangement is shown in Figure 34.

Figure 34. Marine vessel towing eight streamers. Drawn by Denis Mougenout. Used by permission of CGGVeritas.

Figure 33. Noise-floor comparison in a medium of acoustic impedance 1.5 106 kg m−2 s−1 (water). The geophone has a sensitivity of 28 V/(m/s). The hydrophone has a sensitivity of 7.4 V/bar. The recorder noise is 170 nV between 3 and 200 Hz. Used by permission of CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Streamers are fitted with several kinds of positioning devices, shown in Figure 35. Dif-ferential global-positioning-sys-tem (GPS) antennae are attached to the front and tail of each streamer. Along with the vessel and source-array antennae, the GPS antennae provide absolute positioning of the front and tail of each streamer. Acoustic transpon-ders attached to each streamer provide distances between the streamers. Along with compass readings and with the known positions of the heads and tails, the distances allow global recon-struction of streamer geometry. In addition, streamer depths are controlled by birds (with two wings) attached at regular inter-

vals along streamers. Figure 36 shows one such bird and the process of attaching (or detaching) the birds from the streamer.

The latest streamer attachment was introduced in the late 1990s. It is a streamer steering device that can correct about 3° of streamer feathering. Streamer feathering is the deviation of the streamer trajectory from the planned (usually straight) trajectory, caused by crossline currents. Figure 37 represents such a steering device.

Two proprietary streamers must be mentioned. The Q-marine streamer from Western Geco, Inc., uses single hydrophones with an interval of 3 m. Swell noise can be filtered by using numerical filters that are more efficient than analog arrays. A Q-marine example is reproduced in Appendix B. The advantage of single sensors will be discussed in Chapter 6.

Figure 35. Streamer positioning typically combines three types of measures. The differential global-positioning system on the front and tail of each streamer provides absolute positioning. Compasses along each streamer provide relative positioning. An acoustic network among stream-ers provides relative positioning. Light gray lines represent acoustic ranges. Used by permission of CGGVeritas.

Figure 36. Birds for streamer depth control. Photograph used by permission of ION.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

The GeoStreamer® from Petroleum GeoServices uses the dual-sensor tech-nique (hydrophones and accelerom-eters) to attenuate the receiver ghost (discussed in Chapter 4) and therefore can be towed deeper to record lower frequencies in a quieter environment. Implementation of that technology and a first result were presented by Teng-hamn et al. at the SEG 77th Annual International Meeting (see Appendix B).

Ocean-bottom cables

Ocean-bottom cables (OBCs), which are used to record data on the seafloor, are characterized by two fea-tures: (1) The high cost of operation reserves the use of OBCs to cases in which streamers cannot be used, particularly in obstructed areas. (2) The possibility of recording shear waves gives the use of OBC tech-nology an advantage when con-verted shear waves are contemplated.

As a consequence, most OBCs use four-component single sensors (velocity and pressure), and the number of receivers on a 3D OBC crew is usually lower than on a conventional 3D land or marine crew. Ocean-bottom-cable acquisi-tion geometries are similar to land acquisition geometries. The most common technique, but not the only one, is orthogonal acquisition (Figure 38).

The inset in Figure 38 shows a single-sensor 4C receiver from Ser-cel. Because of the absence of ground roll, the group interval is chosen to sample signal properly, and it is larger than on land and marine single-sensor acquisition. For economic reasons, the receiver-line interval is often larger than the source-line interval.

Figure 37. Streamer steering device. The Nautilus® is one of the newest steering device available. From Sercel, 2010b. Used by permission of Sercel Corporation.

Figure 38. Ocean-bottom-cable acquisition geometry. Inset photograph from Sercel, 2010c. Used by permission of Sercel Corporation.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Node technology competes with OBC. Nodes are autonomous ocean-bottom seis-mometers with local recording and storage capability. Clocks are incorporated into nodes because periods of immersion range from weeks to months, and precise measure-ment of time is necessary.

Land seismic cables

Today, virtually all seismic cables used for 3D acquisition are digital. That means preamplification and analog-to-digital conversion operations have migrated from the instrument truck to the line. To reduce the number of connections (which are the cause of a large proportion of problems), some systems have inserted the electronic compo-nents that perform those operations into the cable itself. The technological challenge was to enable the electronic components to work in the field (in temperatures ranging from −35°C to 50°C and in dry air and under a few decimeters of water) with the same performance as in an air-conditioned cabin. That was the necessary condition for the deployment and subsequent recording of very large numbers of receivers.

A rich 3D acquisition geometry is represented in Figure 39. Although the intervals used were at about 50 m until about 2005, receiver and SP intervals show a definite decreasing trend. Intervals closer to 25 m are more common, even for deep targets. Deep targets have contributed to the lengthening of source-receiver offsets, which often reach 8 km or more. Finally, reduction in line intervals is made possible by progress in seismic recording technology (because of the use of more receivers) and the new trend of simultaneous acquisition that will be discussed in Chapter 5.

Various cableless systems are the obvious competition for land seismic cables. It is clear that despite remarkable technological achievements, deploying and moving large

Figure 39. Land acquisition geometry. Receiver lines are represented in black. Source points (SP) are represented in gray (center). More than 50,000 channels are necessary. For each SP, the receiver patch covers an area of 144 km2. Used by permission of CGGVeritas.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

quantities of cables are among the major challenges for modern seismic crews. Replac-ing cables by autonomous nodes (similar in principle to the nodes used in sea-bottom operations) is an idea that manufacturers have investigated. A major advantage of land nodes over seabed nodes is their access to GPS time. The modest cost of access (equip-ment cost and power consumption) is made possible by the remarkable progress of GPS technology.

An interesting contribution to the cable-versus-cableless debate is given by Lansley et al. (2008). A major component in the debate is the weight of field equipment. Figure 40 compares total weights of ground equipment for three scenarios: (1) a cable system with arrays of six geophones, (2) a cable system with 3C digital point receivers, and (3) a cableless system with 3C digital point receivers. Weights are given as a function of receiver interval.

Whereas comparison between 3C point receivers with and without cables is straightforward, comparison between point receivers and geophone arrays should be made at different receiver intervals. The simplest assumption (made by Lansley, 2008) is that point-receiver intervals are 50% of receiver-arrays intervals. Under that assumption, equal weight for cable-array geometry and cableless point-receiver geometry is obtained for a receiver-array interval of 50 m corresponding to a point-receiver interval of 25 m.

Other factors must be taken into account in the debate. Cableless systems need one battery per receiver. For large number of receivers, battery management can become a problem. In addition, high-capacity battery technology might not be compatible with the requirement of operating in all temperature conditions (e.g., lithium ion). Another factor is the type of survey. For vibroseis surveys, recording should be continuous or

Figure 40. Total ground-equipment weight as a function of receiver interval for a recording spread that is 10 × 10 km with a 400-m receiver-line interval. All necessary equipment, including trans-verse data-transmission cables, boxes, and batteries, is incorporated. After Lansley et al., 2008, Figure 5.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

almost continuous; therefore, battery life might become a problem. A final factor is quality control. Although cable systems allow data observation and analysis in real time, that is not possible (at least not yet) for a cableless system with a large number of receivers.

The conclusion of Lansley et al. (2008, p. 894) is worth citing: “In areas with dif-ficultaccessand/orhighpopulationdensities,suchascities,therewillbeabenefitinusing cable-less systems. For low trace density, coarsely sampled 3D surveys with large receiver-station intervals, there will also be a slight weight advantage in recording with-out cables. However, as the spatial sampling becomes smaller and the trace density greater, significant operational and recording efficiency benefits are gained using cables. Uncommitted systems that offer the flexibility to use either cables, or cable-less technology, or a combination of both within the recording spread will permit recording optimiza-tion under all conditions.”

An example of acquisition in a very difficult area was presented by H. Yongqing et al. at the 73rd Annual SEG International Meeting (see Appendix C).

Seismic recorders

Modern seismic recorders face several challenges. Methods to address those chal-lenges are either specific to one manufacturer or are used by all manufacturers.

The first challenge is to increase the system capacity to record larger numbers of seismic channels (a seismic channel is the electronic equipment necessary to record signal sent by one receiver). Telemetry is a necessary condition. Cableless systems have no technical limitation. For cable-based systems, data rates (the amount of data that can be transmitted in a cable in one second) have increased steadily, and some systems use optical fiber to increase the rate further.

The second challenge, sensitivity, is not new. However, the use of single-sensor acquisition has resulted in a lower receiver sensitivity that might have to be compen-sated for by a higher preamplifier gain.

The third challenge, dynamic range, is not new either, but single-sensor or shorter arrays do not provide the same attenuation of surface waves as receiver arrays provided. That effect results in a net increase of overall data dynamic range, which is addressed throughout the industry by the use of 24-bit, fixed-gain, delta-sigma ana-log-to-digital converters. These converters provide as much as 130 dB of instantaneous dynamic range.

Nonseismic noise is the fourth challenge. It essentially consists of electromagnetic leakage onto seismic circuitry. In principle, leakage is restricted to analog circuitry. Telemetry results in a significant size reduction of the circuitry and hence in electro-magnetic noise contamination.

The above four challenges have a direct effect on seismic image quality. The next three have purely operational effects.

First, we consider efficiency. Cost control is a permanent concern of seismic opera-tors. Smooth recording operations can fix a large share of that concern. Cable and bat-tery management are essential. Reduction in power requirement will simplify the latter and will aid in the use of cableless systems. Improved hardware and software to reduce



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

or eliminate dead times have contributed significantly to an improvement in productiv-ity. Besides enabling better signal processing, continuous recording can help marine and vibroseis operations.

Reliability is another challenge. With an increasing number of channels, reliability requirements have become more stringent. The requirements have been satisfied by a reduction in the number of connectors and by the generalization of redundant data transmission through enhanced system architecture and adapted acquisition software. Selection of electronic components has become more careful, and specific integrated circuitry has been developed.

Flexibility also is needed. In most field operations, especially on land, often it is not possible to predict the next problem that will have to be solved. Therefore, flexibil-ity in the use of seismic equipment is a quality that operators are seeking with growing interest. Flexibility, as suggested above by Lansley et al. (2008), will become a determi-nant parameter of seismic recording systems. That condition will require developments in system architecture and in acquisition software.

Conclusion

A seismic experiment consists of detonating a source and deploying seismic receiv-ers to observe the ground motion. Behind this apparent simplicity, huge amounts of time and imagination have been required to design adequate and reliable sources that also are respectful of the environment. Much patience and careful experimentation have been necessary to bring the electromagnetic geophone so close to perfection that it has become difficult to surpass. Highly skilled engineers who have taken advantage of progress in the electronics industry have built recorders that can record simultaneously, in any environment and at any latitude, the thousands of seismic channels that are needed to describe ground motion adequately.



Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

Dow

nloa

ded

01/1

7/13

to 1

92.1

59.1

06.2

00. R

edis

trib

utio

n su

bjec

t to

SEG

lice

nse

or c

opyr

ight

; see

Ter

ms

of U

se a

t http

://lib

rary

.seg

.org

/

3. chapter 3 components of seismic acquisition

Documents

seismic bandwidth lavergne

causes of mallets failure

seismic source point

weak source

artificial source

mallets experiment

large ground motion

repartition of energy