Geology Reference
In-Depth Information
1.0
n
= 2
n
= 4
n
= 8
0
0
0.5
1.0
Detector spacing
Δ
x
Fig. 4.13
Response functions for different
detector arrays. (After Al-Sadi 1980.)
Wavelength
λ
Arrays comprising areal rather than linear patterns of
geophones may be used to suppress horizontal noise
travelling along different azimuths.
The initial stage of a reflection survey involves field
trials in the survey area to determine the most suitable
combination of source, offset recording range, array
geometry and detector spacing (the horizontal distance
between the centres of adjacent geophone arrays, often
referred to as the group interval) to produce good seis-
mic data in the prevailing conditions.
Source trials involve tests of the effect of varying, for
example, the shot depth and charge size of an explosive
source, or the number, chamber sizes and trigger
delay times of individual guns in an air gun array.The de-
tector array geometry needs to be designed to suppress
the prevalent coherent noise events (mostly source-
generated). On land, the local noise is investigated by
means of a
noise test
in which shots are fired into a spread
of closely-spaced detectors (
noise spread
) consisting of in-
dividual geophones, or arrays of geophones clustered to-
gether to eliminate their directional response. A series of
shots is fired with the noise spread being moved progres-
sively out to large offset distances. For this reason such a
test is sometimes called a
walk-away spread
. The purpose
of the noise test is to determine the characteristics of
the coherent noise, in particular, the velocity across the
spread and dominant frequency of the air waves (shot
noise travelling through the air), surface waves (ground
roll), direct and shallow refracted arrivals, that together
tend to conceal the low-amplitude reflections. A typical
noise section
derived from such a test is shown in Fig.
4.12(a). This clearly reveals a number of coherent noise
events that need to be suppressed to enhance the SNR of
reflected arrivals. Such noise sections provide the neces-
sary information for the optimal design of detector
phone arrays. Figure 4.12(b) shows a time section ob-
tained with suitable array geometry designed to suppress
the local noise events and reveals the presence of reflec-
tion events that were totally concealed in the noise
section.
It is apparent from the above account that the use of
suitably designed arrays can markedly improve the SNR
of reflection events on field seismic recordings. Further
improvements in SNR and survey resolution are achiev-
able by various types of data processing discussed later in
the chapter. Unfortunately, the noise characteristics
tend to vary along any seismic line, due to near-surface
geological variations and cultural effects.With the tech-
nical ability of modern instrumentation to record many
hundreds of separate channels of data, there is an increas-
ing tendency to use smaller arrays in the field, record
more separate channels of data, then have the ability to
experiment with different array types by combining
recorded traces during processing. This allows more so-
phisticated noise cancellation, at the cost of some
increase in processing time.
4.4.3 Common mid-point (CMP) surveying
If the shot-detector spread in a multichannel reflection
survey is moved forward in such a way that no two re-
flected ray paths sample the same point on a subsurface
