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
The Vibroseis source
Introduction. The Wavefield of a Surface Source in a Horizontally Layered Elastic Medium. Configuration. Basic relations. Decomposition into upgoing and downgoing wavefields. The local reflection of the wavefield at a layer interface. The propagation of the wavefield within a single layer. The global reflection response. The particle velocity and traction components at an arbitrary point in the medium. The transformation to the space domain. The elastic halfspace. The horizontally layered acoustic medium. Far field relations. The Seismic Vibrator. Mechanics of the land vibrator. The vibrator signal. The baseplate behaviour. The Combined Earth-Vibrator Model. The earth model. The combined earth-vibrator model. The numerical procedure. Numerical results. Test of the Theory By Experiment. The data set. The earth model. The vibrator model. The feedback signal on the seismic vibrator. The Marine Vibrator Source. Configuration. Vibroseis: land and marine. The earth model. The mechanical model of the marine vibrator. The combined earth-vibrator model. The numerical procedure. The distribution of pressure and displacement at the surface of the vibrator. The average power factor. Far field relations. Test of the theory by experiment. Processing of Vibroseis data. Description of the data set. Conventional processing. Proposed processing sequence. Concluding remarks. Appendices: Determination of Green's Matrices for Acoustic and Elastic Media. Solution of the Differential Equation Describing the Plate Bending. The Numerical Procedure for the Combined Earth-Vibrator Model. References. Author Index. Subject Index.
VLA stereoscopy of solar active regions. 2: Altitude, relative motion, and center-to-limb darkening of 20 centimeter emission
A newly developed method of stereoscopic correlation is applied to solar radio maps at a wavelength of 20 cm. On the basis of stereoscopic correlations between radio maps obtained on 6 different days we establish the presence of 66 radio source components associated with 22 active regions. We find the following statistical results for active region source structures at 20 cm: 1. The observed lifetime is consistent with an exponential distribution having an e-folding time scale of greater than 18 days. 2. The average altitude of 20 cm sources is 25 +/- 15 Mm; 90% of the sources are found in heights less than 40 Mm. 3. The average diameter of discrete source structures is 48 +/- 15 Mm, implying a vertical/horizontal aspect ratio of q(sub A) approx. = 0.5. 4. No significant source motion has been found with respect to the standard differential rotation rate of Omega = 13.45 deg - ((3 deg (sin(exp 2)B)). 5. We find a statistical limb darkening, which can be described by the relation (T(sub B)(alpha))/T(sub B)(0)) = 0.4 + ((0.6(cos(exp 2) alpha)) for sources with T(sub B) greater than 0.5 MK. 6. The degree of source polarization is 15% +/- 10% and is independent of source location. 7. Bright sources (greater than or approximately equal to 0.5 MK), or equivalently, long-lived sources (greater than or approximately equal to 5 days) show a systematic variation of their altitude as function of the center-limb distance.