Geology Reference
In-Depth Information
x
across a series of closely-spaced survey lines or around a
grid of lines. However, as discussed later in Section 4.10,
three-dimensional surveys
provide a much better means of
mapping three-dimensional structures and, in areas of
structural complexity, they may provide the only means
of obtaining reliable structural interpretations.
Reflection profiling is normally carried out along
profile lines with the shot point and its associated spread
of detectors being moved progressively along the line to
build up lateral coverage of the underlying geological
section. This progression is carried out in a stepwise
fashion on land but continuously, by a ship under way, at
sea.
The two most common shot-detector configurations
in multichannel reflection profiling surveys are the
split
spread
(or
straddle spread
) and the
single-ended spread
(Fig.
4.7), where the number of detectors in a spread may be
several hundred. In split spreads, the detectors are dis-
tributed on either side of a central shot point; in single-
ended spreads, the shot point is located at one end of the
detector spread. Surveys on land are commonly carried
out with a split-spread geometry, but in marine reflec-
tion surveys single-ended spreads are the normal con-
figuration due to the constraint of having to tow
equipment behind a ship. The marine source is towed
close behind the ship, with the hydrophone streamer
(which may be several kilometers long) trailing behind.
0.5
x
Fig. 4.10
The horizontal sampling of a seismic reflection survey
is half the detector spacing.
lower dominant frequency due to the progressive loss of
higher frequencies by absorption (Section 3.5) and
higher velocity due to the effects of sediment com-
paction, vertical resolution decreases as a function of
depth. It should be noted that the vertical resolution of a
seismic survey may be improved at the data processing
stage by a shortening of the recorded pulse length using
inverse filtering (deconvolution) (Section 4.8).
There are two main controls on the horizontal resolu-
tion of a reflection survey, one being intrinsic to the
physical process of reflection and the other being deter-
mined by the detector spacing. To deal with the latter
point first, the horizontal resolution is clearly deter-
mined by the spacing of the individual depth estimates
from which the reflector geometry is reconstructed.
From Fig. 4.10 it can be seen that, for a flat-lying reflec-
tor, the horizontal sampling is equal to half the detector
spacing. Note, also, that the length of reflector sampled
by any detector spread is half the spread length.The spac-
ing of detectors must be kept small to ensure that reflec-
tions from the same interface can be correlated reliably
from trace to trace in areas of complex geology.
Notwithstanding the above, there is an absolute limit
to the achievable horizontal resolution in consequence
of the actual process of reflection.The path by which en-
ergy from a source is reflected back to a detector may be
expressed geometrically by a simple ray path. However,
such a ray path is only a geometrical abstraction.The ac-
tual reflection process is best described by considering
any reflecting interface to be composed of an infinite
number of point scatterers, each of which contributes
energy to the reflected signal (Fig. 4.11). The actual re-
flected pulse then results from interference of an infinite
number of backscattered rays.
Energy that is returned to a detector within half a
wavelength of the initial reflected arrival interferes con-
4.4.1 Vertical and horizontal resolution
Reflection surveys are normally designed to provide a
specified depth of penetration and a particular degree of
resolution of the subsurface geology in both the vertical
and horizontal dimensions. The vertical resolution is a
measure of the ability to recognize individual, closely-
spaced reflectors and is determined by the pulse length
on the recorded seismic section. For a reflected pulse
represented by a simple wavelet, the maximum resolu-
tion possible is between one-quarter and one-eighth of
the dominant wavelength of the pulse (Sheriff & Gel-
dart 1983). Thus, for a reflection survey involving a sig-
nal with a dominant frequency of 50 Hz propagating in
sedimentary strata with a velocity of 2.0 km s
-1
, the
dominant wavelength would be 40 m and the vertical
resolution may therefore be no better than about 10 m.
This figure is worth noting since it serves as a reminder
that the smallest geological structures imaged on seismic
sections tend to be an order of magnitude larger than the
structures usually seen by geologists at rock exposures.
Since deeper-travelling seismic waves tend to have a
