Deep crustal reflection recordings using "Vibroseis" methods; a feasibility study

This work on deep crustal reflections started in 1972. Possibilities for improvement in signal quality were noted and, in 1973, were incorporated in a new experiment.All work was done with 15.0-2.0 hz or 20.0-5.0 hz swept frequency signals 30 sec long. On most occasions, 240 sweeps were used--all recordings of each trace were added together. In order to further improve the signal/noise, six large vibrators, spaced closely together along a line at right angles to the source-receiver line, were used simultaneously. The basic detector was an 8-arm star, with radius 300 ft, each arm containing thirty 4.0 hz geophones. Two very good refraction events were recorded; they came from velocity discontinuities at depths of only a few thousand feet. Other long delayed energy, which might be deep crustal reflections, was observed. Due to the lack of complete sub-surface coverage and possibly due to complexity of the structure, these events cannot be certainly identified as reflections.