MAPPING AND VISUALIZING FAULTS USING AEROMAGNETIC DATA AND A GIS IN NORTHERN BIG BEND NATIONAL PARK, TEXAS

High-resolution aeromagnetic data acquired over Big Bend National Park in 2002 are used to differentiate faults from topography and lithologic contacts in the northern portion of the park. Filtering techniques applied to the data accentuate the location of surficial and buried faults. The maximum horizontal gradient method, which requires a reduction-to-the-pole or a pseudogravity transformation of the magnetic data, helps map magnetization contrasts that can correspond to faults. Where magnetic surficial volcanic rocks crop out, faults cutting them will be expressed as ridge crests in the maximum horizontal gradient maps. Quaternary gravels that blanket much of the study area are invisible to the magnetic data, but faults cutting magnetic rocks underlying them may also produce gradient crests.

The GIS environment is ideal for data integration along with analysis and visualization. This study incorporates digital elevation models (DEM), digital orthoquads (DOQ), and preexisting mapped geology, with the filtered magnetic data. The 3-D environment of the GIS allows correlation of faults mapped at the surface, topographic scarps visible in the DEM and DOQ, with those believed mapped using the gradients from the aeromagnetic data. The GIS visualization could help establish the subsurface geometry of the structures mapped at the surface.

Crests of the horizontal gradients of the magnetic data representing magnetization contrasts can be converted to points, which can be integrated into the GIS. The GIS contains tools to transform the points to vectors that are compared to locations of mapped faults. Coincidence implies that the magnetic gradient crests represent faults and in these locations, are used to extend them into areas where they are not exposed. Some crests may reflect topographic effects in the data, illustrating the importance of spatial analysis when interpreting which gradients represent faults. Modeling of these faults with the magnetic data, constrained by dips at the surface can determine the subsurface extent and orientation of the faults. Faults buried beneath non-magnetic surficial rocks can be located by the magnetic gradient crests as well. Visualization of the faults by overlaying them on DOQ’s draped onto the DEM, will help direct field mapping studies.