Minneapolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 378, 1991 Mar; :1-2
Objective: Develop an improved downhole geophone system with enhanced signal/noise ratio for collecting cross-hole seismic data used in tomographic reconstructions. Background: The Bureau of Mines is conducting research in the use of cross-hole seismic tomography for detecting and monitoring leach solution flow during in situ mining. This technique requires the use of a downhole seismic energy source in one borehole and a downhole wall-locking receiver (geophone) in another borehole separated from the first by the region to be investigated. Bureau research personnel chose a commercially available air gun for use as the downhole seismic energy source. Unfortunately, commercially available downhole wall-locking geophones that the Bureau tested were difficult to operate in the field, often unreliable and generally inadequate in terms of their signal/noise ratio. Therefore, research personnel began to experiment with an alternative downhole geophone design that could be used to collect adequate data in a wide variety of borehole and cross-hole field situations. Approach: An attractive alternative geophone design would be one that would incorporate a pneumatically operated wall-locking system. As an integral part of the Bureau's existing field system, a portable, high-pressure/low-volume air compressor would have sufficient capacity to operate the air gun as well as several small high-pressure pneumatic linear actuators. This approach was the simplest and most compatible with the research activities being conducted at the site. How It Works: The wall-locking geophone system has three major subassemblies. The components of each sub-assembly are two opposing borehole wall contact plates, each with a pneumatic, linear actuator and a horizontally-mounted geophone element. When the actuators are pressurized, their linear piston movements press the contact plates against the borehole wall. Thus, six horizontal geophone elements are locked at 60 degree intervals around the circumference of the borehole wall. To make sure the tool can be recovered from the borehole, the actuators used in this design are double-acting with a spring-assist for the return stroke. Once depressurized, air flow to the actuators can be reversed, providing a backup retraction mechanism should the spring return fail. Test Results: A two-sided prototype of the tool has been used to collect extensive cross-hole seismic data sets at several field sites. Data were collected at these sites both before and during water injection which was done to simulate an in situ mining scenario. It was found that this design inherently improved coupling of the geophone elements to the borehole wall in when compared to other commercially available geophones, resulting in an improved signal to noise ratio. In addition, the smaller size and the use of multiple geophone elements reduced detrimental tool resonance and orientation effects. The tool is simple, reliable and less cumbersome in field operation and has worked in open, cased, dry and fluid-filled boreholes.
Minneapolis, MN: U.S. Department of the Interior, Bureau of Mines, TN 378