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GCVSP – The Link Between Geology and Geophysics*
By
Ronald C. Hinds1 and Richard Kuzmiski2
Search and Discovery Article #40552 (2002)
*Adapted for online presentation from the article by the authors in AAPG Explorer (January, 2000), entitled “VSP Links Geology, Geophysics.” Appreciation is expressed to the author and to M. Ray Thomasson, former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation, AAPG Communications Director, for their support of this online version.
1Senior geophysicist, EnCana Corporation, Calgary, Canada
2Computalog, Calgary
Geophysicists interpret surface seismic reflection data presented in time, and geologists construct models, drill wells, and acquire well logs in depth. An accurate velocity model of the subsurface is required to link the two types of data (time and depth).
Vertical
seismic profile (VSP) data is generated by a surface source and recorded by
geophones located at many depth levels spanning the entire borehole
. The VSP and
surface seismic data invariably match in character due to the common source and
receiver type used in both surveys. This may not always be the case for the
sonic-log-derived synthetic seismogram, which is often used to tie well logs to
surface seismic data.
In this article we review the acquisition, basic data objectives, and interpretation of VSP data.
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VSP Data AcquisitionThe operation of the VSP survey is as follows: · The sonde containing the geophone package of three orthogonal X, Y, and Z geophones (Figure 1) is lowered to a prescribed depth location.
·
A locking arm on the sonde pushes
the geophone assembly against the · The surface source energy source is fired. Acoustic energy from the source is recorded at the geophone sonde. The locking arm is then retracted and the sonde is moved to the next depth location.
Figure 2 illustrates VSP source and
receiver geometries. The near- or zero-offset VSP geometry occurs when the
source lies vertically above the geophones (source S1 and
receiver A in
Figure 2A). A far-offset VSP occurs when
there is substantial offset distance between the vertical projection of
the sonde to the surface and the source (source S2 and geophone
A in
Figure 2A). In deviated boreholes, source
S3 in
Figure 2B can be zero-offset for location
A, but far-offset for location B. In general,
the zero-offset VSPs will seismically image the geology at the Zero- or Near-Offset VSPIn Figure 2, the raypaths of the acoustic energy shown are reflections up from interfaces located below the sonde. Surface seismic surveys also record energy arriving from below the geophones. Unlike surface seismic surveys, VSP data also contain acoustic energy traveling downward toward the geophones in the sonde. "Upgoing" VSP events are
defined as VSP events that decrease in traveltime as the sonde is lowered
down the An example of zero-offset VSP data is shown in Figure 3A. Note that the downgoing events are much higher amplitude than the upgoing events, which dip in the opposite direction. The first arriving event (first break curve) is the primary P-wave downgoing event. A downgoing event arriving later in time than the primary must be a multiple. The VSP downgoing wavefield contains all of the multiple events that contaminate our surface seismic data. Since the downgoing and upgoing events are linked at the interfaces, we can use the downgoing events to eliminate multiples from our upgoing VSP data. The difference in traveltime between zero-offset VSP upgoing events (shown in Figure 3B) and the two-way traveltime of a surface seismic event is the traveltime along a raypath connecting the sonde location to a surface geophone. This is equivalent to the traveltime of the primary downgoing event (Figure 4). Bulk shifting each zero-offset VSP trace by its first break time aligns the upgoing events into pseudo two-way traveltime (Figure 3C). One can determine the depth of the geological interface that created the upgoing event by: · Interpreting the upgoing event on the shallow depth traces out to the trace where the event intercepts the first break (time of first recorded data). · Following the trace up to the top of the plot to read off its depth value. (Look at Figure 3 and do this for the orange colored upgoing event in panel C. This is the interpretive link between the geophysical seismic event and its associated geological interface.) Multiple identification can be easily done using VSP data. An upgoing multiple is an upgoing VSP event whose raypath undergoes more than one reflection bounce during its travel to the sonde. Find the primary upgoing event in Figure 3C (colored blue) that terminates at the first break time of the 750-meter depth trace. A multiple upgoing event whose last upgoing reflection occurred at the 750-meter interface arrives later in time but also terminates at the 750-meter trace. Why? -- When the sonde is lowered below 750 meters, rays traveling upwards from the 750-meter interface never reach the sonde. The multiples of our upgoing primary event (blue) in Figure 3C are highlighted in yellow. This allows one to interpret multiples, which may be contaminating later arriving primary upgoing events. Can you see one? The green-colored upgoing primary generated at 1,180 meters can be seen to extend from the first break curve to the multiple contaminated data highlighted in yellow and change in character.Multiple elimination can be achieved by using the downgoing events. In Figure 3A, the multiple downgoing events parallel the first break curve. We design an operator that will collapse all of the downgoing events arriving after the primary downgoing event (first break curve). This operator can be applied to the data in Figure 3C. The deconvolved upgoing events can be seen in Figure 5B. The deconvolved data can be compared to the surface seismic data to evaluate the residual multiple contamination left in the processed surface seismic data.
Far-Offset VSP DataWhen the surface source is not located vertically above the downhole receivers, the up- and downgoing traveling seismic energy arrive at the sonde at angles other than vertical. At any given sonde location, the up- and downgoing events are distributed onto all three geophones (two horizontal, X and Y and vertical Z). In the processing of the
far-offset data, our aim is to separate the downgoing events from the data
and then isolate the upgoing events on a single data panel for
interpretation. In
Figure 2, the far-offset raypaths show
that interfaces will be imaged from the We want to isolate the upgoing events. To do this, we process the X, Y, and Z data using polarization filters (mathematically redistributing the up- and downgoing events into the plane defined by the wellbore and source). Wavefield separation is performed to isolate the upgoing events. A final round of polarization processing is performed to isolate the upgoing events onto a single data panel. Using the X, Y, and Z data contained in Figure 6 as input, the final isolated upgoing events are presented in Figure 7A. In
Figure 2A, we saw that the far-offset VSP
geometry resulted in reflections along the interface laterally away from
the To
transform the data in
Figure 7A into a pseudo-seismic section,
we use a model of the velocity around the The output of
this process is shown in
Figure 7B. The horizontal axis is now
distance from the well in meters. In
Figure 7B, two faults can be interpreted.
The distance from the well location where the faulting occurs can be
determined using the horizontal axis. The fault nearest the well can be
interpreted to be 75-80 meters away, and the farthest fault is 205 meters
from the well. A seismic event -- highlighted in
green -- can be seen to truncate against the fault nearest the
ConclusionsThe
zero-offset VSP gives us a link between surface seismic and reflector
depths at the If one has access to VSP data in an area where he/she wants to drill an exploration well, a quick check for the existence of multiples on the VSP data should be done. This could prevent drilling a dry hole if the interpretation was based on surface seismic multiples. The far-offset VSP gives information of the subsurface away from the well. The lateral imaging can be used to locate missed targets such carbonate reef edges or missed sand channels. |