Locating Abandoned Mines Using the Active Mining Operation as the Seismic Energy Source – Demonstration of a New Method
This study demonstrates a new method for detecting the margin of old mine works using the seismic energy generated by active mining in the coal seam. As the first test of this new concept, the method should not be viewed as a proven, off-the-shelf technology, but rather a demonstration of the fundamental concept which has promise for detecting the margins of old mine works. For the primary method a linear spread of vertical 4.5Hz geophones on the earth’s surface was distributed across the margin of old mine works (both flooded and air-filled locations) and to a position over the active mining nearby. The seismic vibrations associated with the underground mining were passively recorded across this spread, with individual records as long as 32 seconds. Through a process of crosscorrelation, using each seismic trace sequentially as the pilot sweep for correlation with each other trace in the recording, two distinct seismic wave arrival patterns emerged from the data. The direct arrival from the mining operation to the surface through the rock overlying the coal seam was distinct and well represented in
APPLICATION OF GEOPHYSICAL METHODS TO DETERMINE BEDROCK CONDITIONS WITHIN A TUNNEL ALIGNMENT
The Upper Scioto West Interceptor Sewer is a 10-km long bedrock tunnel under construction in Columbus, Ohio. Tunneling progress was halted when the tunnel boring machine (TBM) encountered a clay-filled solution valley measuring approximately 75 m (240 ft) wide. A long and expensive delay followed while a vertical shaft was excavated ahead of the TBM, and a horizontal tunnel was hand dug through the soft material to reach the TBM. A geophysical survey was contracted to identify additional problem areas in advance of further tunneling. The survey included two conventional geophysical techniques, seismic refraction and microgravity, and a new technique named Forward Looking Seismic (FLS). Seismic refraction provided a profile of the bedrock surface along the tunnel alignment. Microgravity also detected bedrock valleys as well as low-density bedrock zones attributed to fracturing, solutioning, etc. The third method, FLS, was conducted as a preliminary test. The FLS test detected two geological features ahead of the TBM, a low-density bedrock zone and a clay-filled void. The low-density zone corresponded to an area where jet grouting was required to facilitate tunneling. An estimate of the location and orientation of the void was successfully made.
Simulation of stress waves in attenuating drill strings, including piezoelectric sources and sensors
A key element in drill steering and prediction of lithology ahead-of-the-bit is the transmission of while-drilling information from the bottom of the well to the rig operator and the geophysicists. Mud-pulse telemetry, based on pressure pulses along the drilling mud and extensional waves through the drill string, is the most used technique. The last method, properly designed, could transmit data rates up to 100 bits per second, against the 1 or 2 bits per second achieved with pressure pulses. In this work, a time-domain algorithm is developed for the propagation of one-dimensional axial, torsional, and flexural stress waves, including transducer sources and sensors. In addition, the equations include relaxation mechanisms simulating the viscoelastic behavior of the steel, dielectric losses, and any other losses, such as those produced by the presence of the drilling mud, the casing, and the formation. Moreover, the algorithm simulates the passbands and stopbands due to the presence of the coupling joints and pulse distortion and delay due to nonuniform cross-section areas. Acoustic and electric pulses, generated at one location in the string, can be propagated and detected at any other location by piezoelectric and acoustic sensors, such as PCB accelerometers, clamp-on ammeters, force, and strain transducers.