Geology Reference
In-Depth Information
carbon fields where 4D seismic surveys are routinely
used for the management of the production from the
field. It can be argued that there is more hydrocarbon re-
source to be recovered by careful monitoring of known
fields, than by exploration for new fields.
Source
Detector
4.13 Vertical seismic profiling
Vertical seismic profiling (VSP) is a form of seismic re-
flection surveying that utilizes boreholes. Shots are nor-
mally fired at surface, at the wellhead or offset laterally
from it, and recorded at different depths within the bore-
hole using special detectors clamped to the borehole
wall. Alternatively, small shots may be fired at different
depths within the borehole and recorded at surface using
conventional geophones, but in the following account
the former configuration is assumed throughout. Typi-
cally, for a borehole 1 km or more deep, seismic data are
recorded at more than 100 different levels down the
borehole. If the surface shot location lies at the wellhead
vertically above the borehole detector locations, so that
the recorded rays have travelled along vertical ray paths,
the method is known as
zero-offsetVSP
. If the surface shot
locations are offset laterally, so that the recorded rays have
travelled along inclined ray paths, the method is known
as
offset VSP
(Fig. 4.43).
VSP has several major applications in seismic explo-
ration (Cassell 1984). Perhaps most importantly, reflec-
tion events recorded on seismic sections obtained at
surface from conventional reflection surveys can be
traced byVSP to their point of origin in the subsurface,
thus calibrating the seismic sections geologically. Ambi-
guity as to whether particular events observed on con-
ventional seismic sections represent primary or multiple
reflections can be removed by direct comparison of the
sections withVSP data.The reflection properties of par-
ticular horizons identified in the borehole section can be
investigated directly using VSP and it can therefore be
determined, for example, whether or not an horizon re-
turns a detectable reflection to the surface.
Uncertainty in interpreting subsurface geology using
conventional seismic data is in part due to the surface
location of shot points and detectors. VSP recording
in a borehole enables the detector to be located in the
immediate vicinity of the target zone, thus shortening
the overall path length of reflected rays, reducing the ef-
fects of attenuation, and reducing the dimensions of the
Fresnel zone (Section 4.4.1). By these various means,
the overall accuracy of a seismic interpretation may be
Reflector 1
Reflector 2
Fig. 4.43
An offset VSP survey configuration.
markedly increased. A particular uncertainty in conven-
tional seismics is the nature of the downgoing pulse that
is reflected back to surface from layer boundaries. This
uncertainty often reduces the effectiveness of deconvo-
lution of conventional seismic data. By contrast, an in-
trinsic feature of VSP surveys is that both downgoing
and upgoing rays are recorded, and the waveform of the
downgoing pulse may be used to optimize the design of
a deconvolution operator for inverse filtering of VSP
data to enhance resolution. Direct comparison with
suchVSP data leads to much improved reliability in the
geological interpretation of seismic sections recorded at
the surface in the vicinity of the borehole.
The nature of VSP data may be considered by refer-
ence to Fig. 4.44, which illustrates a synthetic zero-offset
VSP dataset for the velocity-depth model shown, each
trace being recorded at a different depth. Two sets of
events are recorded which have opposite directions of
dip in the VSP section. Events whose travel time in-
creases as a function of detector depth represent down-
going rays; the weaker events, whose travel time reduces
as a function of detector depth, represent upgoing, re-
flected rays. Note that the direct downgoing pulse (the
first arrival, D0) is followed by other events (DS1, DS2,
DS3) with the same dip, representing downgoing near-
surface and peg-leg multiples. Each reflected event (U1,
U2, U3) terminates at the relevant reflector depth,
where it intersects the direct downgoing event.
For most purposes, it is desirable to separate downgo-
