HR: 0830h
AN: P51B-0446    [PDF]
TI: Inversion of Airborne Gravity Data over Subglacial Lakes in East Antarctica
AU: Blankenship, D
EM: blank@ig.utexas.edu
AF: Institute for Geophysics, John A. and Katherine G. Jackson School of Geoscience, University of Texas at Austin, 4412 Spicewood Springs Rd., Bldg. 600, Austin, TX 78759 United States
AB: The team of the University of Texas Institute for Geophysics (UTIG) has been performing airborne geophysical surveys in Antarctica since 1991. Over 260,000 line-km have been surveyed during nine field seasons. The UTIG airborne platform is a contracted DeHavilland Twin Otter instrumented with ice-penetrating radar, laser altimeter, magnetometer, and a gravimeter. The gravimeter utilized is a Bell Aerospace BGM-3 marine system, modified for airborne use, which provides measurements of vertical accelerations at 1 Hz, with verticality of the sensor maintained by a gyro-stabilized platform. The aerogeophysical surveys over subglacial Lake Concordia and Lake Vostok in East Antarctica were conducted by a team from UTIG over the course of the Antarctic field seasons. The region surrounding Lake Concordia was sampled by 6 profiles with a 10 km separation whereas the Lake Vostok survey block was 165 x 330 km with a line spacing of 7.5 km with 11.25 km and 22.5 km ties. 2D gravity inversion was performed for both lakes. The forward problem was solved using Talwani's algorithm for a 2D body of irregular shape. It is described by a non-linear equation between the body's shape and it's density contrast with surrounding rocks. The assumption was that the density contrast between ice/water and rock along the profile is constant. The densities of ice and water are close enough, so the ice and water of the lake can be considered as one body. For Lake Vostok the gravity data were inverted for 2-layered model, consisting of ice/water and sediment lying over dense bedrock. Inversion was performed by a conjugate gradient algorithm for several fixed values of density contrasts. The coordinates of layers' corners were chosen as model parameters. The model was constrained by the lake's boundaries and sub-ice topography, determined from radar sounding. Also, several pre-existing seismic soundings were used as `a priori' information incorporated into the model. The best agreement with seismic data was obtained for density contrast -1.6 g/cc between water and host rock and -0.6 g/cc between sediment and host rock. The differences in thickness of both water and sediment layers at the cross-points of the inverted profiles are within 50 m. The results of 2D inversion for several profiles over Lake Vostok are also used as constraints for 3D inversion. Lake Concordia is located at the very edge of a geophysical survey block. This creates uncertainty in obtaining a regional trend due to lack of data over one side of the lake. Also, there is no additional `a priori' seismic information. Inversion was performed for several values of density contrast between ice/water and surrounding rock. The obtained water thickness for all of density contrasts is not more than 200 m and a sediment layer can not be resolved.
DE: 0903 Computational methods, potential fields
SC: Planetary Sciences [P]
MN: 2003 Fall Meeting