AIRBORNE LASER ALTIMETER SURVEY OF LONG VALLEY, CALIFORNIA

Airborne laser altimetry has proved useful in recent years in examinin g volcanic landforms and glacial ice sheets. With the advent of accura te differential GPS aircraft tracking, we believe that airborne lasers will also prove useful in monitoring time-varying topographic uplift, with rates as low as several centimetres of vertical motion per year. The Long Valley, California, caldera provides an excellent testing gr ound for this new technology. The region has a history of extensive vo lcanism, and its central dome has recently been undergoing resurgent u plift of up to 4 cm per year. In September 1993 we conducted three air craft topographic surveys over the caldera and resurgent dome, utilizi ng a NASA T39 jet aircraft outfitted with a nadir-profiling altimetric laser (ATLAS), two P-code GPS receivers, a Litton LTN92 inertial unit for attitude, and aerial cameras. In addition, we operated two base-s tation GPS receivers for post-flight differential navigation and condu cted a kinematic automobile survey of roads crossing the dome. The air craft flew at a mean altitude of 500m above ground, and at speeds of 8 0-100 m s(-1). The laser had a divergence of 1.7 mrad, and output 50 p ulses per second, yielding footprints of 0.9 m diameter separated by a bout 2 m along track. Precision flying yielded multiple profiles along nearly identical paths, including crossing profiles over the resurgen t dome, off the dome, and along a nearby highway. The surveys included daily flights over Mono Lake for roll and pitch bias calibrations, an d over the well-surveyed Lake Crowley to provide an independent check of estimated elevations. Much progress has been made in developing ana lysis procedures for the laser timing and attitude corrections. Crosso ver points from repeated profiles over the dome and the lake indicate that although some profiles contain relative bias errors of up to 10 c m, most are substantially more accurate. The accuracy of the measureme nts will increase as the various sources of error are better defined a nd dealt with. Even with the present analysis level, the distribution of crossovers near the centre of the dome is roughly Gaussian, with a mean of 2.6 cm and a standard deviation of 11 cm. The standard error o f the mean is low (1 cm), due to the high number of crossover points, which holds promise for measuring systematic changes in the dome heigh t from year to year. Also, comparison of laser heights of Lake Crowley to tidal gauge heights yields only a 1-4 cm difference in absolute he ight. These encouraging results serve to confirm further the concept o f using aircraft laser surveys for geodetic tectonic monitoring.