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.
Numerical modeling and interpretation of drillstring waves
Drill-bit seismic and pilot seismograms contain drillstring periodicities from signal and environmental noise sources. These coherent components are similar in seismic-while-drilling correlations, and their joint contribution may cause distortions in signal processing. Numeric modeling of the drillstring transmission line is used to correctly interpret the axial and torsional events with similar propagating modes of signal and noise. The reflection coefficients are computed for drillstrings of arbitrary materials, and their average mechanical features are related to the axial and torsional group velocities obtained by the dispersion equations. Fitting of the periodical components in the real seismograms is used to automatically correct the drill-bit pilot delays and to estimate the surface and downhole boundary conditions.