GEOPHYSICAL INVESTIGATION OF SUBSIDENCE FISSURES NEAR WILLCOX, ARIZONA

Abstract

An interdisciplinary survey consisting of four geophysical methods was conducted on the western edge of the Apache Generating Station’s property in Willcox, Arizona. The aim of the survey was to apply various methods for the detection of earth fissures and desiccation cracks. The geophysical methods used were static magnetic field measurements, frequency domain electromagnetics (FEM), ground penetrating radar (GPR), and seismic. Two grids were delineated and surveyed by each method. Grid 1 was set up at a site containing a fissure with visible surface expression over some parts of the grid, and Grid 2 was set up at a site with little visible surface expression of the fissure, but was suspected to contain a fissure in the subsurface. At another location, northwest of the Apache Generating Station, three lines were surveyed in an area of known desiccation cracks. All of the methods showed an anomaly associated with the fissure in Grid 1. Furthermore, at locations where the fissure is not visible in Grid 1, there were still strong anomalies in line with the suspected location of the fissure extending below the surface. Magnetic data from Grid 2 suggests that magnetics may not be a useful method in subsurface earth fissure detection at this site, where we believe that the fissure is only a very small crack with small aperture at depth. The electromagnetic results from Grid 2 show anomalies extending from lines 1 through 5 where there is only minimal surface expression in lines 1 and 2 and no surface expression in lines 3-5. No anomaly was seen in the northern end of Grid 2. It was found that GPR in Grid 2 did not display conclusive results in distinguishing subsurface earth fissure anomalies from other anomalies, such as roots. Seismic lines in Grid 2 show anomalies in the profiles that could indicate the presence of earth fissures; however a thin high velocity horizon may appear as a subsurface fissure, and this made interpretations more challenging. At the desiccation crack site, there is evidence of a shallow feature, which we interpret to be a dessication crack and not an earth fissure. A dirt road was present at the desiccation crack site, and it is possible that the road may have produced some of the observed anomalies due to rain-water channeling effects. The locations of fissures were confirmed by trenches excavated at the locations where anomalies were visible in the geophysical data, but where the fissures were not exposed at the surface.