A Fractal Interpretation of Controlled-Source Helicopter Electromagnetic Survey Data Seco Creek, Edwards Aquifer, TX

Abstract

The Edwards aquifer lies in the structurally complex Balcones fault zone and supplies water to the growing city of San Antonio. To ensure that future demands for water are met, the hydrological and geophysical properties of the aquifer must be well-understood. Fractures often occur in a power-law distribution. Fracture distribution plays an important role in determining electrical and hydraulic current flowpaths. The thesis research presents an evaluation of the controlled-source electromagnetic (CSEM) response for layered models with a fractured layer at depth described by the roughness parameter, BV, such that 0</=BV, associated with the power-law length-scale dependence of electrical conductivity. A value of BV=0 represents homogeneous, continuous media, while a value of 0<BV shows that roughness exists. 1-D synthetic modeling shows that the existence of a fractured layer at depth is apparent in the CSEM time-domain response for models representing aquifers. The research also provides an analysis of the Seco Creek frequency-domain helicopter electromagnetic survey data set by introducing the similarly defined roughness parameter BH to detect lateral roughness along survey lines. Fourier transforming the apparent resistivity as a function of position along flight line into wavenumber domain using a 256-point sliding window gives the power spectral density (PSD) plot for each line. The value of ?H is the slope of the least squares regression for the PSD in each 256-point window. Changes in BH with distance along the flight line are plotted. Large values of BH are found near well-known large fractures and maps of BH produced by interpolating values of BH along survey lines suggest unmapped structure at depth.