Identification of surface faults by horizontal resistivity profiles: Texas Coastal Zone

The land surface in the Texas Coastal Zone is interlaced with active and potentially active surface faults. They are subtle features which are difficult to identify until they have caused damage to manmade structures. To date (1978), significant damage has resulted. Faults intercept 2 airports, interstate highways at 11 different locations, and railroad tracks at 28 locations. Faults also pass through 11 residential communities. More than 200 houses in these communities in Harris and Galveston Counties show structural damage because of faulting. Fault movements of only a few inches per decade can cause significant structural damage, and no architectural style or structural design is immune. Foundations break, and cracks extend up the exterior and interior walls of the structure. Although breaks in buildings and highways are one of the best criteria for identifying surface faults, the damage has already occurred. Kreitler (1976) has listed four criteria that can be used to identify the presence of a surface fault before a structure is built. They are recognition of (1) topographic scarps; (2) shallow subsurface faults, using electrical logs or other geophysical data and subsequently extrapolating the fault to land surface; (3) shallow subsurface faults, by coring or trenching; and (4) the surface trace of lineations observed on a fault (lineation), using black-and-white, color, and color-infrared aerial photographs or other remote sensing techniques. This paper presents and evaluates another technique for the detection of surface faults--the use of horizontal electrical resistivity profiling. In the Texas Coastal Zone, similar surficial geology commonly occurs on both sides of a fault. Furthermore, shallow subsurface fault displacements are very small (less than 10 feet). It was anticipated that the small subsurface fault displacements might create slight lithologic or hydrologic variations on either side of the fault. Variations in electrical properties caused by lithologic or hydrologic differences, as well as fault gouge, should be marked by anomalous resistivity profiles. Consequently, resistivity profiles might provide another method for predicting and mapping faults.