MULTIPLE COHERENCE OF NOISE AT 3 VERTICAL ARRAYS UBSO, GV-TX. AP-OK
Seismic data lab. rept.
TELEDYNE INDUSTRIES INC ALEXANDRIA VA EARTH SCIENCES DIV
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Multiple coherence gives a quantitative measure versus frequency of how well a linear combination of n input channels can match the n 1st channel in a seismic array. If the inputs can match the output exactly, then the multiple coherence is unity and only n channels are necessary to describe the noise field. This report shows multiple coherence versus frequency at three vertical arrays of short period, vertical component seismometers. One of the seismic traces, either the surface trace or the deepest trace, is used as the output and 2 to 6 of the others are used as inputs. The multiple coherence properties of the noise are similar at the three vertical arrays with similar geometries. At all three sites the downhole channels correlate with each other better than with the surface channel. At all three sites the coherence between downhole seismometers decreases markedly as their separation increases. When the deep trace is the output and all seismometers are added as inputs, the multiple coherence is highest at GV-TX. The multiple coherence at GV-TX is greater than . 85 for all frequencies from .1 to 2.5 cps. For the AP-OK and UBSO wells the multiple coherence varies between .6 to .9 for most frequencies greater than .5 cps.