Accession Number : AD0287161

Title :   DETECTION OF DISTANT EXPLOSIONS BY SEISMIC MEASUREMENTS IN DEEP BOREHOLES

Corporate Author : AIR FORCE CAMBRIDGE RESEARCH LABS HANSCOM AFB MA

Personal Author(s) : VAN SANDT, D.R.

Report Date : 31 JUL 1962

Pagination or Media Count : 1

Abstract : 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)

Descriptors :   *SEISMIC WAVES, *UNDERGROUND EXPLOSIONS, AMPLIFIERS, CALIBRATION, EARTHQUAKES, EXPLOSIONS, EXPLOSIVES, INSTRUMENTATION, LOW FREQUENCY, SEISMOGRAPHS, SENSITIVITY, SIGNAL-TO-NOISE RATIC.

Distribution Statement : APPROVED FOR PUBLIC RELEASE