Airborne laser altimeter survey of Long Valley, California

Abstract

SUMMARY Airborne laser altimetry has proved useful in recent years in examining volcanic landforms and glacial ice sheets. With the advent of accurate differential GPS aircraft tracking, we believe that airborne lasers will also prove useful in monitoring timevarying topographic uplift, with rates as low as several centimetres of vertical motion per year. The Long Valley, California, caldera provides an excellent testing ground for this new technology. The region has a history of extensive volcanism, and its central dome has recently been undergoing resurgent uplift of up to 4 cm per year. In September 1993 we conducted three aircraft topographic surveys over the caldera and resurgent dome, utilizing 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-station GPS receivers for post-flight differential navigation and conducted a kinematic automobile survey of roads crossing the dome. The aircraft flew at a mean altitude of 500m above ground, and at speeds of 80-100 m s-'. The laser had a divergence of 1.7 mrad, and output 50 pulses per second, yielding footprints of 0.9 m diameter separated by about 2 m along track. Precision flying yielded multiple profiles along nearly identical paths, including crossing profiles over the resurgent dome, off the dome, and along a nearby highway. The surveys included daily flights over Mono Lake for roll and pitch bias calibrations, and over the well-surveyed Lake Crowley to provide an independent check of estimated elevations. Much progress has been made in developing analysis procedures for the laser timing and attitude corrections. Crossover points from repeated profiles over the dome and the lake indicate that although some profiles contain relative bias errors of up to 10 cm, most are substantially more accurate. The accuracy of the measurements will increase as the various sources of error are better defined and 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 of the mean is low (1 cm), due to the high number of crossover points, which holds promise for measuring systematic changes in the dome height 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 height. These encouraging results serve to confirm further the concept of using aircraft laser surveys for geodetic tectonic monitoring.