33 shot arrays seismic cables for use with only 12 or

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3.3 Shot arrays Seismic cables for use with only 12 or 24 channels 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. 3.4 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 is normally used only to stack (enhance) results obtained with identical source and detector positions. If, however, the data are recorded digitally, NMO corrections can be made (although not in the field) to traces produced with different source– receiver combinations. The technique normally used is to collect together a number of traces that have the same mid-point between source and receiver ( common midpoint or CMP traces), 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 3-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, is equal to half the number of data channels. Figure 2.6 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 is very slow and laborious. The need to combine traces from several different shots makes it difficult to do CMP processing in the field.
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40 Figure 2.6 CMP schematic, for 3-fold cover with a 6-channel system. Shot points A, B, C and D are progressively one geophone group interval further to the right. Note that the distance between reflection points (depth points) on the interface is only half that between geophone groups on the surface. Shots A and D have no depth points in common .
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41 Figure 2.7 Effect of dip in CMP shooting. In contrast to single-fold shooting (Figure 2.3), the shot points as well as the geophone locations are different for the different traces. Shot points and detector locations are equivalent and the ‘depth point’ on the reflector moves up dip as the offset increases. The move-out equation is most easily derived by noting that the path from source to detector is equal in length to the path SG ˡ from the source to the detector ‘image point’, and that the geometric relationships between similar triangles imply the equality of all the lengths marked ‘y’. The Pythagoras relationship can be
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