Abstract: Remote Sensing for Geologic Mapping in Eastern Cordillera, Colombia

James M. Ellis, Wayne Narr

Geologic mapping in frontier areas can be done accurately and efficiently by integrating various remote sensing and photogrammetric methods with field mapping. Interpretations of Landsat, airborne radar, and aerial photographs were integrated with field mapping to geologically map part of a northwest-directed fold-thrust belt in the Eastern Cordillera, southwest of Santafe de Bogota, Colombia.

Each sensor provided particular advantages: (1) Landsat provided an accurate cartographic basemap, and the best source of cultural, physiographic, and regional formation mapping information; (2) airborne synthetic-aperture radar (SAR) provided the only cloud-free imagery and was optimal for regional mapping of structural geology because the radar beam's low illumination angle enhanced topographic relief. Qualitative structural information was obtained by stereoscopic viewing of overlapping flight strips. Map scale features recognized only on the SAR include the surface trace of a thrust fault, an overturned anticline, and a thrust structure produced by wedging or nappe emplacement; and (3) aerial photographs were used to estimate bedding attitudes quantitatively, to resolve questionab e geology, and to construct a 1:50,000 topographic map using standard photogrammetry.

Transfer of dip and strike data from aerial photographs to cartographic base was accomplished by digitizing corresponding control points on each aerial photo and the base, and then removing distortion using a transformation algorithm derived empirically for each aerial photo. These digital data, along with digitized field data, were projected into profile planes to help construct geological cross sections.

Field mapping showed that the interpretation made from remote sensing provided an accurate regional framework. Several iterations of field mapping and image re-interpretation resulted in a final 1:50,000 scale geologic map.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994