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.
Electromagnetic modeling of reflections inside an in situ gasified coal seam
As an aid in the design of an electromagnetic system for the detection, monitoring, and mapping of the burn front of an underground (in situ) coal gasification process, an analytical model has been developed that will predict the electromagnetic field reflected from the gasified portion of the coal seam. The coal and gasified section have been modeled electrically as lossy media, surrounded above and below by relatively high conducting overburden and underburden. Vertical and horizontal electric and magnetic dipoles have been investigated, utilizing a geometrical optics formulation which also takes into account the physical roughness of the coal-overburden (underburden) and coal-gasified section boundaries. Indications are that a swept frequency monitoring system is capable of locating the burn front of the in situ gasification process, with the vertical electric dipole being the most attractive element.
Preliminary Field Results on a New Transient Induction System Employing Pseudo-Noise Signals
An experimental system is described which utilizes pseudo-noise (PN) and cross-correlation techniques to characterize subsurface conducting bodies such as those pertaining to natural resources. The system is essentially an extension of the loop-loop, transient-electromagnetic, induction systems used in ore prospecting. In lieu of a low duty-cycle pulse, this system transmits a wide-bandwidth PN digital waveform. The mapping signature is obtained by cross-correlation of the received signal with a replica of the sounding PN waveform. The receiver also averages several signatures to improve the signal-to-noise ratio. This type of system has the potential advantages over previous transient systems of using smaller antennas and requiring less transmitter power for more portable operation. Preliminary field data is shown and discussed for a high conductivity dike of ilmenite magnetite and for the surrounding low conductivity anorthosite. Frequency response data is obtained digitally via the Discrete Fourier Transform operation on the time domain signatures. Interpretations are given for both time and frequency domain representations.