Digital Geologic Mapping of the Ferron Sandstone, Muddy Creek, Utah, with GPS and Reflectorless Laser Rangefinders XUEMING XU, JANOK B. BHATTACHARYA University of Texas at Dallas RUSSELL K. DAVIES AHCO Exploration ana Production Technology CARLOS L.V. AIKEN University of Texas at Dallas The traditional approach to geologic mapping consists of sketching, taking orientation and thickness measurements with compass and tape, and noting positions of features on topographic maps or photos. These methods are time consuming, often difficult to realize in rough terrain, and poorly constrain lateral variations in sedimentary fades in relatively flat lying strata. We describe a case study that captures the three-dimensional architecture of sandstone bodies and key geological surfaces such as stratigraphic boundaries and faults using digital capture techniques. The Perron sandstone in Utah is a superbly exposed ancient delta deposit that provides an important outcrop analog to fluvio-deltaic subsurface reservoirs. It has been the focus of many traditional outcrop studies, but here we use a methodology ("cybermapping") based on GPS with offsets from a continuous ranging mode reflectorless laser rangefinder ("laser sketch") for collection and analysis of basic stratigraphic and structural data in a relatively remote area. We also show how this data can be analyzed and visualized in three dimensions. The study area was mapped in two days, which included hiking several kilo- meters into the area. On-the-fly and rapid static post pro- cessing of GPS surveying was used for positioning the reflectorless laser rangefinders; 60,000 points were acquired mapping sedimentological and structural fea- GPS Solutions, Vol. 5, No. 1, pp. 15-23 (2001) © 2001 John Wiley & Sons, Inc. tures, terrain, and control points. The resultant quantita- tive 3D model of the geology and terrain allowed robust geometric visualization and analyses. © 2001 John Wiley & Sons, Inc. INTRODUCTION ecause of recent advances in GPS and reflector- less laser rangefinder (RLR) technology, it is now feasible to accurately and rapidly capture globally refer- enced geologic features remotely as shown by Xu and col- leagues (Xu, Aiken, & Nielsen, 1999; Xu et al., 2000) and Nielsen, Aiken, and Xu (1999). Specific geologic observations attached to geo- graphical positions are sometimes critical for evaluating spatial and temporal relationships. Traditionally, the field worker sketches the outcrop in the field to approxi- mate spatial relationships, measures geological features onto photos or topographic maps and measures dis- tances through taping, and pacing, and occasionally using conventional survey methods such as total sta- tions and levels. In areas like Muddy Creek, Utah, geological features are often delineated on photomosaics of steep cliff faces. Projections of complex topography onto a 2D plane (the film) introduces distortions, parallax effects, and other uncertainties that are difficult to remove using conven- tional mapping techniques. These methods are time con- suming, and sometimes impossible in rugged topogra- Digilai Geologic Mapping 15