IMPLEMENTATION OF A NEW APPROACH TO GRAVITY TERRAIN CORRECTION EMPLOYING DIGITAL IMAGE PROCESSING OF LANDSAT 7 DATA FOR THE VALLES CALDERA AREA IN NORTHERN NEW MEXICO

Digital image processing for Landsat 7 data of the Espanola Basin and Valles Caldera region of northern New Mexico was performed along with a study that tests an innovative gravity modeling approach. The innovative technique incorporates digital terrain data for an area of rugged topography and allows for this terrain to vary according to geologic and drilling constraints. The Pajarito Plateau along the flanks of the Valles Caldera was chosen as a test area because it represents a challenge for traditional gravity modeling techniques.

The objectives of this study were to: 1) test a approach toward gravity modeling that directly incorporates digital terrain data for an area of rugged topography and variable densities, 2) develop a 3D model of the subsurface based on forward gravity modeling, 3) determine the subsurface structure including the plateau to caldera transition, 4) understand the influence of subsurface structure on earthquake hazards, 5) incorporate digital image processing of remotely sensed data including classification schemes and, 6) relate classification results to the geologic constraints based on published geologic maps.

Previous studies have focused on forward modeling of the gravity field and only considered two dimensional models of the subsurface which incorporated average layer densities that did not account for the distribution of densities and thickness found in the Bandelier Tuff. By contrast, this study applied an innovative, geologically constrained method to develop a 3-D model of the subsurface. The geologic constraints were derived from published geologic maps, digital elevation data, Landsat 7 data, borehole data and limited seismic reflection data. Image sharpening and saturation stretch is applied to the remotely sensed data. In addition, classification and post-classification schemes are implemented along with principal component analysis. The resulting residual anomaly map resolves boundaries of the Velarde Graben in considerable detail and suggests the presence of an accommodation zone at its southern end.