vertical seismic profiling
TRANSCRIPT
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