A unique Crosshole Electromagnetic system has been developed to image geological conditions between boreholes. The equipment is based on the electric field technique which provides greater sensitivity and detectability than systems based on the magnetic field technique. At the low conductivity range, where most geological situations occur, the electric field is more sensitive to variations in conductivity. The system measures amplitude and relative phase of the received signal. Attenuation of the signal relates to conductivity between boreholes, which in earth materials has a much larger range of variation over other physical properties such as density or acoustic velocity. Antenna frequencies vary from the high kilohertz to low megahertz range which allow for high resolution and significant penetration range especially in more resistive environments such as bedrock.
The electric field technique uses a transmitter antenna which radiates an electric field voltage and dipole receiver antenna with associated electronics. The transmitter and receiver antennae may be in boreholes separated by a few metres to as much as 120 metres, and in holes up to 600 metres in depth. These antennae are controlled by a cable and winch system that allows the generation of a very large number of transmitter to receiver measurements that sample the volume between the holes. The resulting multiple ray path data is then processed to create a tomogram. This tomographic image shows higher levels of attenuation of the signal where more conductive materials are present, and lower values in more resistive areas. In operation, the receiver antenna is held at a constant depth and a complete pass of the borehole is carried out with the transmitter antenna. The receiver antenna is then incremented to the next depth point and the process is repeated. This procedure results in the very large number of ray paths that are required for the tomogram data processing.
The tool has widespread application in mine development, geotechnical, environmental, groundwater and enhanced oil recovery applications. Applications for the tool may include: