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.
Three-Dimensional Response Maps for a New Wide-Bandwidth Induction System
Three-dimensional response maps are displayed and interpreted in both time and frequency domains. The wide-bandwidth induction system utilizes pseudonoise and cross-correlation techniques to characterize subsurface conducting bodies. In contrast to more typical single-frequency systems, both horizontal-profiling and depth-sounding information are present in these maps. Consequently, interpretation of model surveys is prerequisite to interpretation of field data. Experimental model surveys were performed in the laboratory over a conducting dike and a conducting semi-infinite horizontal sheet. The real part, imaginary part, magnitude, and phase of normalized frequency responses are plotted versus frequency and system location. Raw time signatures are converted to frequency response on 0-50 kHz with the discrete Fourier transform (DFT) and normalized to the response without a conductor present. Normalized transient-time responses (impulse responses) of the conductors are obtained by the inverse DFT and compared with the raw time signatures. Windowing effects in both domains are discussed. The raw-time-signature peak value is quite sensitive to conducting anomalies and can be used to directly delineate conductors in the field. However, more detailed information about size and depth must be obtained by additional signal processing.
Optimum Estimation of Impulse Response in the Presence of Noise
The problem considered is that of estimating the impulse response of a linear system from records of its input and output during a limited interval of time when the system output is obscured by additive random noise. Standard results from statistical estimation theory are applied to derive least squares and Markov estimates which are optimum in the sense of having minimum variance among all linear unbiased estimates. No special assumptions are required concerning the form of the input. Expressions for the variances of the sampling errors are given. The relationships of these estimates to other methods of estimation which have been suggested are discussed.