DETECTION OF DISTANT EXPLOSIONS BY SEISMIC MEASUREMENTS IN DEEP BOREHOLES

Simultaneous measurements of seismic signals in adjacent cased and open boreholes are compared. In general, the same earth noise is recorded in both holes, and the response to a high amplitude unidirectional signal is the same in both holes. The conclusion is that the casing in a borehole has no detectable effect upon a seismic signal if the casing is cemented to the borehole wall and wall-coupled geophones are used. Design and performance features of a moving coil geophone for borehole use are described. The natural frequency is 1.2 cps and the sensitivity is 50 v/cm/sec. Including the amplifier, the line output sensitivity is 17,000 v/cm/sec. The input noise level of the amplifier is about 0.2 microvolt, which corresponds to ground motion of about 10 to the minus 8 cm. at 1.0 cps. A balldrop method of geophone calibration is described. The improvement of signal-to-noise ratio with depth for the first ''P'' arrival was determined for the Prater No. 1 well in southwest Oklahoma. During pre-dawn periods when the local noise was at a minimum, a two times improvement in signalto-noise ratio was obtained between depths of 1,500 and 4,500 feet. Below 4,500 feet the improvement was less, and no improvement was noted at 10,000 feet. A greater improvement was obtained during daylight hours. (Author)