Geology Reference
In-Depth Information
direct wave containing frequencies between 200 and 35 Hz would be strongly
attenuated.
Non-linear arrays produce more complicated curves, and may perform
better, but simple arrays are preferred since mistakes are less easily made
in setting them out. The range of frequencies over which the direct wave is
attenuated is proportional to array length, and it may be necessary to overlap
the geophones in adjacent arrays. It would be unusual in a shallow survey
to have more than five geophones in one array.
12.2.3 Shot arrays
Seismic cables for use with 12- or 24-channel seismographs are not designed
with arrays in mind, and non-standard connectors may have to be fabricated
to link the geophones to each other and to the cable. It may be easier to use
arrays of shots instead.
A shot array using explosives usually involves simultaneous detonation
of charges laid out in a pattern resembling that of a conventional geophone
array. If an impact source is used with an enhancement instrument, the same
effect can be obtained by adding together results obtained with the impact at
different points. This is the simplest way of reducing the effects of surface
waves when using a hammer.
12.2.4 Common mid-point shooting
Improving signal-to-noise ratios by adding together several traces (
stacking
)
is fundamental to deep reflection surveys. In shallow surveys this technique
was originally used only to stack (enhance) results obtained with identical
source and detector positions. However, now that data are routinely recorded
digitally, NMO corrections can be applied to traces produced with different
source-receiver combinations. The technique normally used is to collect
together traces that have the same source-receiver mid-point (
common mid-
point
or CMP), apply the corrections and then stack.
The number of traces gathered together in a CMP stack defines the
fold
of
coverage. Three traces forming a single synthetic zero-offset trace constitute
a three-fold stack and are said to provide
300% cover
. The maximum fold
obtainable, unless the shot-point and geophone line are moved together by
fractions of a geophone interval (as can easily be done in marine surveys
but not on land), is equal to half the number of data channels.
Figure 12.6 shows the successive geophone and source positions when
a six-channel instrument is used to obtain 300% cover. Special cables and
switching circuits are available for use in deep reflection surveys, but CMP
fieldwork with the instruments used for shallow surveys can be very slow and
laborious. Because traces from several different shots have to be combined,
CMP processing can generally not be done in the field.
