Instrumentation for Performing Cross-Correlation of Acoustic and Seismic Data on an Analog Computer

Abstract

In an experiment designed to study acoustic background noise at high altitudes, a vertical array of two microphones spaced 300 ft apart was hung from a balloon. To determine whether the noise was propagating upward from the jet stream or whether it was caused by local hydrodynamic flow, the signals from the two probes were cross-correlated on an analog computer. The PACE analog computer was chosen for the application because of its ability to handle frequencies as high as 150 cps. An adjustable time delay between channels was provided by a special magnetic tape transport with movable playback heads. Frequency analysis of seismic signals from two large explosions was also performed on the computer by cross-correlating the data with artificially generated sinusoids of various frequencies. Because of the nature of the seismic signal, it is necessary to use special circuitry to permit the accurate measurement of the amplitude of any specified frequency component for any portion of the signal, thus giving an accurate indication of time of occurrence of this particular frequency. Instrumentation for each of these tests is explained and some results are shown for each case. The results of the frequency analysis of the seismic signal are compared with those obtained by conventional filtering methods.