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).
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 column
we review the acquisition, basic data objectives and interpretation
of VSP data. A later column will show examples of how VSP data can be
integrated into our everyday exploration effort.
VSP Data Acquisition
The operation of
the VSP survey is this:
- The sonde
containing the geophone package of three orthogonal X, Y and Z geophones
(see Figure 1) is lowered to a prescribed
- A locking arm on the sonde
pushes the geophone assembly against the borehole wall.
- The surface source energy
source is fired.
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
In general, the
zero-offset VSPs will seismically image the geology at the borehole,
and the far-offset VSPs will image laterally from the borehole in the
direction towards the surface source.
Zero- or Near-Offset VSP
In 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.
seismic surveys, VSP data also contain acoustic energy traveling downwards
toward the geophones in the sonde.
VSP events are defined as VSP events that decrease in traveltime as
the sonde is lowered down the borehole -- and cease to exist once the
sonde is below the interface from where the reflection took place.
events are defined as events whose traveltime increases as the recording
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 (see Figure
Bulk shifting each zero-offset VSP trace by its first break time aligns
the upgoing events into pseudo two-way traveltime (Figure
One can determine
the depth of the geological interface that created the upgoing event
- 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.)
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.
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
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 Data
When 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
In Figure 2, the far-offset raypaths show
that interfaces will be imaged from the borehole out to half the source
receiver offset. The final upgoing event panel will be processed to
take on the appearance of a seismic section.
Look at the X, Y and Z data in Figure 6.
The primary downgoing event is distributed onto all three panels. As
the sonde is lowered to different depth levels during the VSP survey,
the sonde rotates. This rotation effect can be seen in the inconsistent
first break wavelets on the X and Y panels.
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
In Figure 2A, we saw that the far-offset
VSP geometry resulted in reflections along the interface laterally away
from the borehole. In fact, the coverage extends from right at the intersection
of the interface with the borehole (sonde at depth of interface) out
to half the source/well offset (sonde at borehole surface).
As the data is
recorded at various downhole locations starting from the surface down
to the depth of the interface, the geology along the interface is continuously
To transform the data in Figure 7A into
a pseudo-seismic section, we use a model of the velocity around the
borehole. With this model, we stretch or transform every depth trace
into the offset from the well/traveltime domain. This procedure, called
the VSPCDP mapping, is shown in Figure 8
for the deepest and shallowest depth traces. We apply this process to
all the traces and then re-bin the data to look like seismic traces.
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 furthest fault is 205
meters from the well.
A seismic event -- highlighted in green -- can be seen to truncate against
the fault nearest the borehole. The borehole itself is located along
the right edge of the plot in Figure 7B.
VSP gives us a link between surface seismic and reflector depths at
the borehole location. Interpretation is easy and the geological logs
can be tied confidently to the VSP and then directly to the surface
seismic. The VSP data illuminate multiples clearly.
If one has access
to VSP data in an area where they want 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.
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.