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43401-G8
Investigations of the Controls of Electromagnetic Wave Propagation Through Fractures
This research investigated the fundamental controls of electromagnetic (EM) wave propagation through discrete fractures. How do formation dielectric properties as well as fracture fluid content and fracture aperture affect EM wave propagation through a discrete fracture? The hypothesis tested was that variable property EM wave propagation (such as variable polarization, angle of incidence, and frequency) through a discrete fracture will introduce characteristic and predictable changes to the recorded radar signal attributes (such as phase and amplitude) that can be related quantitatively to the fracture properties (such as fluid content and aperture).
As proposed, the hypothesis was tested by: a) analytical models (i.e. theoretical predictions), b) 3-D finite difference time domain numerical simulations, and c) controlled experiments. Analytical models were developed to predict the response of variable frequency, polarization and angle of incidence EM wavefields incident to planar fractures of variable aperture and fluid content. The analytical models were shown to be in good agreement with numerical simulations and experimental data. Ground penetrating radar (GPR) signal amplitude and phase were shown to respond in a characteristic and predictable manner to the properties of discrete fractures. Emphasis was placed in the study of changes occurring to the phase of radar signals. GPR investigations have overwhelmingly emphasized the amplitude response of radar waves. We showed that phase is an important signal attribute that should be considered in studies of subsurface properties. This research concludes that GPR signal attributes can be used to characterize quantitatively fractures in geologic formations. The theoretical insights gained through this work and the methods developed can advance the study of the anisotropic fluid-flow properties of fractured formations, which are commonly found in petroleum reservoirs and groundwater aquifers.
Work in this project consisted of three stages. First we developed analytical models and an experimental setup made of thin polycarbonate sheets that provided controlled observations of the effects of discrete fracture properties to the transmitted EM wavefield. The controlled experiments and analytical models showed good agreement with each other and validated the analytical model development. These findings can be summarized as follows: 1) Angle of incidence: Increasing incidence angle to the fracture results in increasing phase and amplitude changes. 2) EM Wave Polarization: Perpendicularly polarized signal amplitudes are consistently higher than parallel polarized signal amplitudes and can be used to detect the presence of a fracture. The phase relationship between orthogonally polarized signals exhibits a signal wavelength (or frequency) to layer thickness dependence and therefore offers the capability to discriminate between varying thickness layers. 3) Layer thickness: For mm-scale layer thickness, lower frequency GPR signals (100 and 200 MHz) result in greater phase lead of the perpendicularly polarized signal compared to higher frequency signals (450 MHz). 4) Fluid content: Water filled layers display greater phase difference variability than air filled layers. High salinity water results in decreasing phase lead and an overall decrease in transmitted signal amplitudes. Results of this work were published in Geophysical Research Letters (Tsoflias and Hoch, 2006), and presented at the Society of Exploration Geophysicists 75th Annual Conference (Tsoflias and Hoch, 2005).
The research was expanded to include controlled experiments using concrete blocks to simulate vertical and horizontal fractures enclosed in homogeneous matrix. Fractures varied in aperture and fluid content, including saline solutions, and EM wavefields varied in frequency, polarization and angle of incidence. Three-dimensional numerical simulations and analytical models were used to simulate the concrete block experiments. All three methods yielded consistent and comparable results of GPR amplitude and phase response. These observations were also in agreement with the polycarbonate sheet experiments conducted earlier. This work was presented at the Spring 2006 AGU Conference (Jarvis and Tsoflias, 2006).
The methods developed by the controlled experiments enabled field testing of GPR to monitor the flow of saline tracers during hydraulic testing at a fractured bedrock test site. This study related in the field setting changes in GPR reflection amplitude and phase to fluid electrical conductivity, and investigated GPR frequency effects to reflected signals from a saline tracer filled fracture 7.6 m below surface. Findings of this work indicate that lower frequency GPR should be employed for monitoring saline tracer flow in discrete fractures. This result is in contrast to accepted wisdom that thin layer properties are better resolved by high frequency signals. In addition, these findings suggest that both phase and amplitude should be used to image bedrock fractures. Currently, signal phase is neglected in field investigations. This work was presented at the SEG 77th Annual Conference (Tsoflias and Becker, 2007), the Fall 2007 AGU Conference (Tsoflias et al., 2007), and is currently under review by the journal Geophysics.
