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FIG. S-18. Spread arrangements for 24 inline geophone groups. Split spread or split dip if the source is at A, offset split dip if the source is at B, end-on if the source is at C, inline offset if the source is at D, broadside T if the source is at E, broadside L if the source is at F, and cross if some geophones are at right angles (G to H).

1. Arrangement of geophone groups in relation to the source point. Various arrangements are used; see Figure S-18. See also Figure F-1 for fan shooting and Figure T-4 for 3D spreads. Spreads are interlocking if the geophone group location and the source for one profile are located at the source and geophone group location (respectively) for another profile (for example, source at A into geophone 24 and source at C into geophone at 13 in Figure S-18). Spreads are reversed if the same array of geophones is shot into from sources in opposite directions inline (for example, spread from 1 to 24 from both sources at C and J in Figure S-18). A microspread has very small geophone group intervals (2 to 15 ft).

FIG. F-1. Fan shooting. Detectors are located roughly on the arc of a circle centered at the source point in different directions. Expected arrival times are determined from a normal traveltime curve (versus distance) where no local high-velocity body is present. An early arrival (lead) with respect to the normal indicates that part of the travelpath is at an abnormally high velocity, signifying the presence of a local high-velocity body such as a salt dome. This method was used in early seismic exploration to locate salt domes within the thick low-velocity Gulf Coast sediments. (From Nettleton, 1940.)
FIG. T-4. 3-D surveying (Continued)

2. The layout of electrodes or antennas in resistivity or electro-magnetic surveying. See array (electrical).