Remote Induction Sounding of Hanna Underground Coal Burn II, Phase 1, Employing Pseudonoise/ Cross-Correlation Techniques
An underground coal gasification site near Hanna, WY, was remotely sounded with an induction system at the surface one year after the burn. An estimated 1800 tons (40 000 cu. ft.) of coal 275 ft. deep had been converted to low-Btu gas. The wideband loop-loop system employed pseudonoise signals at the transmitter and cross-correlation over 0-50 kHz at the receiver to obtain time signatures in the field. These time-domain impulse responses were converted to frequency responses on a Fourier analyzer system for ease of interpretation. Single-frequency responses at approximately 1 and 5 kHz were extracted from these wideband responses for further investigation. Skindepth calculations showed these to be effective for probing at these depths. Fou in-line traverses conducted over the Hanna II, Phase 1 Site produced strong anomalies of at least 8-to-1 at approximately 5 kHz. Interpretation of these anomalies agrees quite well in location and shape with chemical modeling estimates from north to south. However, these soundings support the conclusion that the burn may have protruded another 5 m to the west from the chemical estimate. The centroid of the burn region was obtained by averaging anomaly values weighted by their location.
Time-Domain Spectral Induced Polarization Based on Pseudo-random Sequence
To reduce noise during electrical prospecting, we hereby propose a new method using correlation identification technology and conventional electrical exploration devices. A correlation operation can be carried out with the transmitted pseudo-random sequence and received time signal to suppress the random noise, and the time-domain impulse response and frequency response of the frequency domain of the underground media can be obtained. At the same time, using a dual Cole–Cole model to fit a complex resistivity spectrum, which is close to the frequency response, we can get a variety of induced polarization parameters and electromagnetic parameters of subsurface, which can provide more useful information for the exploration of mineral resources. This time domain prospecting method can effectively improve the efficiency of the spectral induced polarization method. In this article, we have carried out theoretical calculations and a simulation to prove the feasibility of such a method.
Propagation of low and medium frequency radio waves in a coal seam
A theoretical model has been developed to explain the unexpectedly long communication ranges of over 300 m obtained, and the marked polarization effects observed, during recent radio transmission measurements in a conductor-free area in a coal mine at frequencies in the range of 57.5 to 920 kHz. The model is based on a dipolar cylindrical TEM mode of propagation, with E vertical and H horizontal, in a low-conductivity horizontal coal seam bounded by high-conductivity rock. The best overall fit to the experimental data over the frequency range 57.5 to 920 kHz is found for coal and rock conductivities of 1.4 × 10−4 mho/m and 1.0 mho/m, respectively, for an assumed coal dielectric constant of 7. A better fit to the data at 57.5 kHz can be obtained if, in the model, the conductivity of the surrounding rock is allowed to be high for only a short distance above and below the coal seam and of a substantially lower value elsewhere. If the coal conductivity is changed from 1.4 × 10−4 mho/m, which is near the lower end of the range of expected values for bituminous coals, to 1.0 × 10−2 mho/m, which is near the upper bound of values for such coals, the attenuation is markedly increased with corresponding reductions in communication range.