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Downhole Survey - A sturdy hardwood board is laid beside the wellhead, and a downhole seismic, three – component, clamping geophone is lowered into the hole. It is then clamped at intervals, commonly five feet, and at each interval the board is hammered. First it is hammered on one of its ends and a record is made. Then the procedure is repeated at the other end. This is necessary to identify the shear wave arrival by virtue of the two arrivals having opposite polarity. The geophone is then lowered, incrementally, down the borehole and more measurements are done. By this process the compression and the shear waves’ velocities are determined at intervals as a function of depth in the vicinity of the borehole.
Crosshole Survey – The difference between this survey and the Downhole one is that in the Crosshole approach there are two boreholes. A downhole seismometer is in one of the holes and the shear wave source in the other. This approach is a little more definitive for acquiring information on discrete layers. The information applies to a little larger volume of sediment/rock between the holes as opposed to the near vicinity of one borehole. |
