Aid mine workers in detecting hazardous roof conditions in underground mines by developing a method of roof sounding which supplements or replaces the traditional roof sounding technique where the miner relies on experience to determine whether or not the rock is sound.
The Bureau of Mines developed an electronic roof sounding device that acoustically determines the integrity of mine roof rock by “listening” to the variation in frequency vibrations within the rock.
How a Rock Stability Tester Works
In previous research, the Bureau of Mines found that loose rock in a mine, when impacted, vibrates at a much lower frequency and for a longer time period than does a solid slab. By quantifying the enhancement of low frequency vibrations in loose rock over solid rock, a repeatable, objective measure of the rock’s soundness can be obtained. This approach was used in developing the rock stability tester.
The compact, battery-powered tester receives a signal from an accelerometer which is held against the rock by a spring-loaded pole. The signal is created by striking the rock near the transducer with a sounding rod. The received signal is amplified with an automatic gain control amplifier. The broadband frequency response is then separated into the two frequency bands to be compared. The energy in each band is determined and the low band energy level is subtracted from the high band energy level. This process minimizes any striking force dif-
ference since a higher force elevates the energy in both bands equally. A high relative number on the instrument’s liquid crystal digital (LCD) readout indicates less competent or “drummy” rock and a low reading indicates rock that is more stable. The instrument can be calibrated with high precision in terms of numeric output versus energy level difference in the two frequency bands. However, interpreting the reading in terms of a given rock mass at a mine site requires calibration at that mine Known loose and known solid material must be tested at the mine to interpret the instrument reading.
The prototype was tested in a variety of underground mine environments, including coal, uranium, molybdenum, silver, and salt mines. In these tests, an accelerometer was placed in contact with the rock to be tested. The rock was then lightly tapped with a scaling bar or sounding stick and the ratio between power spectrum (dominance of frequency components) in the low- and high- frequency bands was numerically displayed on the LCD. Depending upon where the slab was impacted with respect to the
detector accelerometer, a certain amount of scatter was present in all data, and represented a measure of local rock conditions. In mines developed in softer rock, the difference between power ratios for solid and drummy rocks was reduced due to the effect of damping, but once the instrument response became familiar, a determination of slab conditions could be made rapidly. The Department of Labor’s Mine Safety and Health Administration is currently evaluating a new prototype instrument for use underground.
The U.S. Department of the Interior’s Bureau of Mines holds the patent on this device (U.S. Pat. No. 4,598,588, July 8, 1986). Further information concerning the patent or licenses can be obtained by writing the Bureau’s Branch of Technology Transfer, 2401 E Street, N.W., Washington, D.C. 20241.