Field Test Results of a Borehole Directional Radar

Abstract

The Borehole Directional Radar System is a high power, high resolution tool that is being developed to locate lithologic layers of fractures away from a wellbore. The prototype is a 50 KW peak power pulsed directional radar system that determines fracture location by transmitting powerful radar pulses, 8 nanoseconds in length, in a known direction form a borehole. The frequency spectrum of the pulses ranges up to the VHF band, which is between 30 to 300 MHz The transmitter and receiver rotate in place, permitting the tool to scan for fractures in all direction from the borehole. Because discontinuities in the rock interrupt and reflect radar signals, signals that return to the tool's receiving antenna indicate fractures. The distance of the fracture from the borehole is determined by the time delay of the return signal. The radio frequency signal is sampled and transmitted to the surface by wire line at audio frequencies and then reconstructed at the surface using a personal computer. The key to the tool's potential is its ability to accurately measure distance and direction of a lithologic discontinuity underground. This paper presents field test results that show the capabilities of the tool for locating lithological discontinuities up to 10.5 m (34.5 ft) away from a wellbore. Unique features of the system are discussed. Potential applications of the system area described, such as locating gas and oil reservoirs below a salt dome and possibly detecting a blow out well with or without casing. In the area of national security, this system might be used to evaluate the integrity of underground structure, such as the Strategic Petroleum Reserve and Nuclear Waster Repository Site. It might also be used to locate underground tunnels and is being considered for underground nuclear test verification.