PRELIMINARY RESULTS FROM A GROUND PENETRATING RADAR SURVEY OF MOUNT BAKER'S ACTIVE CRATER (SHERMAN CRATER), WASHINGTON

Mount Baker, in western Washington, is considered the second most active volcano in the Cascade arc. Glacier-filled Sherman Crater, the currently active center, is located 350 m lower than, and about 800 m south of, the summit. Sherman Crater has a history of sudden, significant increases in geothermal activity (e.g., 1975), and communities and reservoirs lie directly in the path of potential lahars from the crater. To assess the threat of lahars to these downstream communities, we used Ground Penetrating Radar to determine the volume of ice within the crater and image the crater’s subglacial morphology.

We used a GSSI SIR-3000 GPR system and a low frequency (80 MHz) antenna in common offset (reflection) collection mode to create subglacial profiles along several west-east and south-north transects within the crater. We also used the GPR in common midpoint (CMP) collection mode to determine the velocity and dielectric structure of the ice and bedrock beneath the ice. GSSI’s RADAN 6.0 was used to match the surface elevation to the surface topography of the ice, based on altimeter readings.

Our preliminary GPR profiles reveal a distinct basal reflector at the ice-bedrock interface as well as numerous internal reflectors likely related to zones of enhanced subglacial melting and to dust/debris layers formed during summer ablation. Along west-east transects, the crater’s bedrock topography largely follows the glacier’s surface, but thins dramatically at the lowest 85 m towards the eastern breach. Assuming a uniform EM wave velocity through the ice of 0.168m/ns (following a previous GPR survey of the Carmelo (summit) crater), we estimate the maximum ice thickness to be ~50 m. A strong sub-horizontal reflector near the deepest part of the crater, adjacent to the eastern breach, suggests the presence of a subglacial lake.

Ongoing GPR and CMP work in the crater this summer should further unlock mysteries held within the ice of Sherman Crater and help us assess the threat of lahars from potential melting of the crater glacier.