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.
Note on the theory of transmission of electromagnetic waves in a coal seam
A coal seam is idealized as a resistive slab bounded by more conductive rock. The source is taken to be a vertical electric dipole. It is shown that the dominant mode has relatively low attenuation.