Theory of a directional borehole antenna

Abstract

The theory of a directional antenna constructed for the RAMAC borehole radar system is described. The radar was developed for the International Stripa Project and is presently widely used for mapping rock in mines and tunnels. The radar operates in the frequency range 20-100 MHz and the antenna can locate fracture zones and other reflectors from singel borehole measurements. The antenna design is determined by the small diameter of the antenna compared with wavelength. Four different signals are measured and digitally combined to produce signals corresponding to an ideal directional antenna, a dipole antenna and a checksum testing the function of the system. Tests have shown that the azimuth of fracture zones can be determined within 3". PREVIOUS WORKThe RAMAC borehole radar was developed within the International Stripa Project on nuclear waste storage in order to locate fracture zones in crystalline rock, since fracture zones can bring radioactivity from a storage via ground water flow. The radar has also been used to map mines, survey tunnel constructions, etc. The system has routinely produced radar pictures since 1985.The borehole radar consists of transmitter and receiver built on the antennas and enclosed in two borehole probes. Energy is supplied by electric batteries and the probes communicate with the control unit through optical fibers. Data is transferred to a standard PC with various application programs. The optical fibers are insensitive to external conditions which ensures the accuracy of the radar and avoid waves propagating along the borehole. The radar has a cable winch for measurements down to a depth of more than 1 km. The radar is similar to a previously developed system [ 11. The frequency range is 20-100 MHz and the diameter of the probe 48 mm.The directional antenna is needed to locate fracture zones from measurements in singel boreholes. Dipole antennas based on resistively loaded wires have traditionally been used in borehole radars, but can only measure the distance to a reflector from the borehole. Additional data is necessary to locate reflectors in azimuth and a number of methods have been devised for this purpose, e g correlation with borehole data and seismic measurements, crosshole radar reflections and radar tomography on the amplitude or velocity of the pulse [2,3]. These methods produce excellent results, but become prohibitively expensive at large sites. The maximum radar range in granite is 200-300 meter, which makes crosshole investigations expensive in view of the drilling cost. Complete crosshole measurement were performed in Stripa at the Crosshole site [l] and at the SCV site [3,4] and the capability of the radar to detect and characterize fracture zones was thoroughly proven. In particular the results verified that the radar can detect all fractures of importance. ANTENNA DESIGNThe diameter of a standard borehole is much smaller than the radar wavelength. This makes it impossible to produce a narrow lobe, though Lytle [SI tried to create a beam by placing a wire eccentr...

Analysis of borehole-radar reflection logs from selected HC boreholes at the Project Shoal area, Churchill County, Nevada

“…Directional receiving antennas can provide information on the orientation of reflectors. The directional receiver used for this study is a magneticdipole type described by Falk (1992). Although the directional receiver can determine the orientation of planar reflectors and the azimuths to point reflectors, the antenna is less sensitive than the electric-dipole antenna.…”

“…Therefore, specially designed directional antennas are required to determine the strike, dip, and intersection location of planar reflectors, and the azimuth, depth, and radial distance of point reflectors. The directional receiving antenna used for this study is a magnetic dipole tool containing four orthogonal, resistivelyloaded, transmission line antennas, which have the same directional radiation properties described for the loop antennas, but with improved signal characteristics (Falk, 1992). The antenna contains a three-component magnetometer and a plunge sensor to determine the orientation of the receiver for each measurement.…”

“…4). A complete description of the procedures used to interpret the directional data can be found in Falk (1992). Note that the projected intersection depth is relative to the line formed by the axis of the borehole.…”

Geophysical Characterization of a Fractured‐Bedrock Aquifer and Blast‐Fractured Contaminant‐Recovery Trench

“…Single-hole methods were conducted in the six test wells at the site ( fig. 1) and included omni-directional surveys using 60-MHz electric-dipole transmitting and receiving antennas and directional surveys using a 60 MHz magnetic dipole directional receiver described by Falk (1992). Cross-hole tomography surveys were conducted between the four USGS test wells ( fig.…”

Use of borehole-radar methods to monitor a steam-enhanced remediation pilot study at a quarry at the former Loring Air Force Base, Maine

“…The directional receiver antenna can indicate reflector orientation using dual-loop antennas, which act as four different orthogonal receivers, coupled with a threecomponent magnetometer and a plunge sensor that monitor the orientation of the tool in the borehole. The sensitivity and effective radial penetration of directional dual-loop receiver antennas are inferior to those of omni-directional dipole receiver antennas (Falk, 1992;Lane and others, 2001). Because of the severe rotation of the antennas in the borehole during acquisition, the results were difficult to interpret and are not included in this report.…”