Effect of too many groups connected to the cable

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Problem

Land cables are built in sections that are identical. The connections to plugs at each end of the section effectively rotate the system by the number of groups to be connected to the section. Thus, if pins 1, 2, and 3 at one end of the section are connected to takeouts for 3 groups in a section, at the other end of the section they will connect with pins 4, 5, and 6 of the next section. Usually there are more sets of wires than channels being used at any one time, for example, perhaps 96 independent pairs of wires for use with 48 channels. However, occasionally so many sections and geophones are laid out that a distant group of phones is connected to the same channel as a nearer group. Sketch a possible arrangement for the connections in one section with takeouts for three channels and explain how this would appear on the seismic record.

Solution

For simplicity we assume 9 channels only with 3 groups per section, as diagramed in Figure 8.1a. The circles at the left represent the intake to the recording system, the arrows and represent the connectors at each end of the sections. When two sections are connected, the takeouts for the right-hand section are rotated three channels as shown in the figure. Note that the section is symmetrical; the end pin on the left (#1) relates to the closest takeout, as does the end pin on the right (#9). So flipping the section end-for-end makes no difference. If we connect three sections with nine groups as shown, the profile will be recorded correctly. However, if a 4th section is connected at the right, the rotation will have gone full circle and we will have distant geophones connected to the same channels as near geophones, as shown in Figure 8.1a.

Assuming that the source is to the left of the cable, the signals from groups 10−12 will differ from those from groups 1−3 in NMO and will be weaker due to the longer path. The NMO corrections appropriate to groups 1 to 3 are incorrect for groups 10 to 12 so that, after these corrections have been applied, the signals from groups 10−12 will not reinforce those received by groups 1−3 and so will constitute noise on the record. The effect will be most serious in weak portions of the record.

Figure 8.1a.  Relation between groups and channels.

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