Multiple borehole radar. Volume 1: Subsurface site investigation by radar, phase 3

Phase 2 is part of a projected 5-phase program to develop new techniques for subsurface site investigation by remote sensors in boreholes and tunnels. The objective of the overall program is to evaluate the benefits of advanced computer-based geophysical methods, as supplements to other pre-excavation site investigation methods. This grant covered the development and initial field evaluation of a short pulse electromagnetic borehole radar system. The borehole antenna(s) can acquire data from either a single or a crosshole mode. Single-hole measurements are obtained by a transmit-receiver antenna, whereas cross-hole data is acquired with a transmitter and receiver in separate holes. The principal energy of the radar antenna is in the range of from 50 MHz to 150 MHz. The borehole radar system was field evaluated at two sites in rock. One test site featured an array of four subvertical boreholes on a line about 15' to 50' apart and about 250' deep. The measured geologic profile was of weathered to unweathered quartz mica schist to gneiss. Both single and cross-hole measurements were correlated to the core data. At the other test site three close spaced (8' to 20') subhorizontal holes were drilled to a depth of 100'. Here a detailed geologic map of the rock volume included was developed. The rock mass was an unweathered diabase. Field measurement results are strongly correlated to specific known rock structures.