VERTICAL SEISMIC PROFILING

Vertical Seismic Profiling (VSP) is a high resolution seismic method. It brings

detailed information in the vicinity of a bore well on the propagation of seismic waves

through the earth, on the analysis of multiples and the horizons that are not accessible by

surface seismic studies.

In VSP a velocity geophone anchored to the borehole wall receives information

from two opposite directions: the down going and upcoming waves/reflected waves

(Fig.1). This technique of recording simultaneously two wave trains is a major advantage

when compared to the conventional seismic methods, which access only

reflected/refracted waves. These two waves can be separated by processing and can be

used to extract detailed information from both of them. For allowing a detailed analysis

of the downgoing wave propagation and for precise separation of the up and down going

signals, recording was carried out at a large number of levels in the wall (i.e., 50 to 400).

Precise knowledge of down going wavetrains at all depths allows the computation of

powerful deconvolution operators that will be applied to the upcoming wavetrains

allowing high resolution processing of VSP data with minimum assumptions concerning

the earth response.

Data are sampled every 20 milliseconds at large number of levels as mentioned

above in wells drilled at locations determined from surface seismic sections and stored

on 9 track compatible tapes. Simultaneous measurement of travel time and depth

provides the required correlation, and also gives information on average and interval

velocities.

In the absence of sever structural effects, the surface seismic section at the well,

a synthetic seismic section (or theoretical earth response at the well), and the VSP (the

measured earth response at the well) provides sufficient information for a complete

interpretation. However, all the three traces are obtained with an assumption that the

earth is horizontally stratified near the well. When this is not the case, reflections no

longer takes place at normal incidence and reflection points may be distant from the well.

This is also the case of the offset VSP, where the source and geophone are laterally

separated, in order to achieve more extensive sub-surface coverage of the target horizons.

Thus to cope with non-vertical rays and the mode conversions that occur when

rays scatter at non-normal angles, a specialized acquisition tool like Schlumberger

Acquisition Tool (SAT) is employed to record all the three components of the wave field.

Principles of Measurement

An elastic medium, like earth’s crust allows both P and S waves to pass through

it. In a homogeneous medium plane waves of all types (P, SV and SH) are propagated

independently without interaction. This is also the case when the direction of travel is

perpendicular to the layering in a layered medium. However, because the direction of

travel is rarely normal to the boundaries between formations, P and SV wave couple. For

example, an incident P wave incident on a horizontal boundary will be scattered into P

and SV reflected and transmitted waves, while an incident SH wave is reflected and

transmitted without any mode conversion. The amount of P-to-S wave conversion

increases with increasing impedance contrast between layers and with the increasing

angle of incidence. In marine surveys, sea bed with large impendence contrast generated

downwardly scattered P and SV wave fields and on land P-waves usually emit significant

SV component.

Thus with single vertical component recording, the amplitudes are lost and the

angles of arrival of the waves are difficult to determine. Only three component recording

and vector processing permit the recovery of both angles of incidence and amplitudes, as

well as the separation of P and S waves, which is possible with the SAT tool.

SAT tool description:

The downhole part of the tool consists of power supply, mechanical and

acquisition sections.

Mechanical section: The function of this section is to provide the best possible

tool-to-formation coupling. This is achieved by mechanical arm, which can be opened

against the formation to push the SAT tool against the borehole wall with a variable force

(usually between 130 to 400 lb) which can be monitored and controlled from the surface

to achieve optimum coupling between SAT tool and the formation. At the end of the arm

is a micro-resistivity pad, used when moving between levels, to record both caliper and

micro-resistivity curves for the depth correlation and the accurate positioning of the tool.

Acquisition Section: The acquisition section contains the tri-axial geophone

package and the electronics to acquire and process the geophone signals.

Geophone package: A SAT tool uses three SM-4 geophones, which consists of an

electrical coil suspended in a permanent magnetic field by a spring. Any relative

movement between them induces a current in the coil. With a change in the

spring/coil mass equilibrium the proportionality of induced current to coil

movement changes hence reduces the data quality. Thus to reduce the

dependence of data quality on the angle of the tilt of the geophone assembly,

geophones are placed in a gimbaled geophone assembly. All the three geophones

are mounted in an unbalanced cylinder. Of the three geophones, the Z-axis

geophone is kept permanently vertical by the effect of gravity on its gimbaled

mounting and the X-axis geophone is maintained in a horizontal plane by its

gimballed mounting and is oriented in the direction of the azimuth of the

tool/borehole. The third geophone, called Y-axis geophone, is fixed in the

unbalanced cylinder but, due to the movement of the cylinder, will always lie with

its sensitive axis in a horizontal plane and perpendicular to the azimuth of the

borehole. Thus three geophones are maintained mutually orthogonal inside the

SAT tool and are free to find their rest positions when the tool is anchored prior to

the data acquisition.

Two potentiometers in the assembly allow the measurement of the angle

between the low side of the tool and the azimuth of the anchoring arm, and its

deviation from the vertical. If the azimuth of the borehole is known, the

orientation of tri-axial geophone system can be determined.

Electronics and Telemetry: This part is to implement all the functions involved in

the acquisition of the data and their transmission uphole. Its operation is

governed by the microprocessor which controls the sampling of the geophone

outputs, the amplification, digitization and transmission of the data to the surface.

Each geophone signal passes through a pre-amplification stage with a fixed gain

of 30 dB. After pre-amplification the signal passes through a programmable

amplifier, the gain to be applied may be selected in order to maximize the signal

quality.

The amplified signal is now passed through an antialiasing filter prior to

sampling to avoid the ambiguity of the frequencies represented by sampled data.

The signal is now digitized every 1 millisecond and passed through an autoranger,

wherein amplitude of the input signal is used to control the gain. Thus a small

input signal will be amplified and the output signal will lie within an optimized

range (for example the dynamic range possible with SAT tool is 90 dB). Finally

the analogue samples of the waveform are digitized and transformed into a 12 bit

number (11 bits for the amplitude of the sample, 1 for its sign). After adding

another four bits for the autoranger gain, sixteen bit words are sent uphole by the

telemetry system.

A unit known as the Cyber Service Unit (CSU) acquires the signal sent

uphole by the telemetry system and also by geophones situated at the surface. It

also controls the seismic energy sources. There are facilities in CSU to speed up

the acquisition process during Vibrosies surveys. A downhole shaker assembly in

SAT enables a complete check of the tool system while it is in the well. Analysis

of the resulting signal, acquired by the SAT tool, will indicate if it is functioning

correctly, and also about the quality of the coupling between the tool and the

formation.

VSP Data Processing

Vector Wave Processing: The data recorded with the SAT tool is split into three sets of

files: X, Y and Z, one from each orthogonal component. Any single component of the

data can be processed by the standard methods developed for the purpose. At any given

interface, P-waves and SV-waves are coupled together, while SH-waves are decoupled

from the P-SV system. This decoupling of the different wave types at a single interface

will hold for the medium as a whole if a single plane can be defined that is perpendicular

to all interfaces. Although P-waves and S-waves are coupled at interfaces, they travel at

different speeds, and along different paths through the earth, which are sensitive to

different mechanical properties of the rocks. Thus separation of total elastic wave field

that is recorded into its P and S components is useful for their further processing and

interpretation.

Three methods viz., signal-based method, model-based method and wave-

equation method are currently available for analyzing the different wave types present in

the elastic wave field recorded by the SAT tool. The first two methods are not intended

to achieve a rigorous separation of the different wave types, but rather to enhance chosen

events. These methods some times distort the amplitudes of events in the data, but the

phase relationships (arrival times) will be preserved. The third method i.e., wave-

equation method is designed to achieve a full separation of the wave types. This method

preserves both true amplitude and the phase of the events, provided they are recorded

accurately.

Signal-based method determines at each level an orthogonal three-component

reference frame to which the initial data is referenced to enhance the different types of

waves. Two steps are involved rotation in the horizontal plane and rotation in the vertical

plane.

Model-based method uses a predefined model of the subsurface in order to locate

in time and separate different kinds of waves. It uses ray-tracing to compute various

arrival angles versus versus time at a given level for both P- and S-wave events. The

model-based method works very well when accurate amplitudes are not required for the

separation of P- and S-wave fields. However, as the method is model dependent, it has

its own limitation in the areas of complex tectonics.

Wave-equation method aims to perform a rigorous separation of P- and S-waves

using the elastic wave equation. Few assumptions like the particle motion on the Z axis

is due solely to P-waves and SV-waves travelling in a single plane; all the events are

travelling across the well in a consistent direction; the formation is locally homogeneous

and isotropic etc. were made. Mathematically the recorded data is represented as a

spectrum of plane P-wave and SV-wave propagating across the well. Amplitudes of

individual plane P- and S-waves are determined using the assumptions made and then

separation is achieved by resynthesizing the wave field from either its plane P- or its S-

wave components. The vector nature of the wave fields are preserved during the

separation, so that both horizontal and vertical components can be studied individually.

The separation of upcoming and downgoing waves can be made simultaneously with the

P/S separation.

In general the processing of VSP data consists of the following steps (Fig.2);

1. Stacking and bandpass filtering to improve S/N ratio.

2. Velocity filtering to separate up and down going wavetrains

3. Removal of multiples and to adapt the wavelength to the desired output in

order to match the seismogram to the seismic section.

The VSP processing sequence usually includes most of the following steps:

Shot selection to reject the noisy poor-quality shots.

Editing of individual shots.

Consistency check of the surface hydrophone/geophone signal

Stacking of individual shots

Monitoring of phase shifts and acoustic impedance at all levels

Bandpass filtering to eliminate noise and remove aliased frequencies

TAR

Velocity filtering to separate the upgoing and downgoing components of the total

wavefield

Autocorrelation of the downgoing wave after velocity filtering in order to select

the proper deconvolution parameter

Predictive deconvolution to remove multiples. Detailed knowledge of the

complete wavefield, which contains all multiples, allow the design of long and

powerful deconvolution operators.

AGC

Time-variant filtering to match the surface seismic data.

The processing of fine sampled VSP data permits separation of the strong, down-

going incident pulse, and its multiples, from the weaker, upgoing, reflected energy. After

suitable deconvolution, the upgoing wave train may be correlated with the surface

seismic section, to aid the identification of primary reflectors in terms of the acoustic

impedance contrasts detected from well logs.

Applications of VSP