HR: 16:15h
AN: NS14A-04    [Abstracts]
TI: Ground Penetrating Radar Imaging of Tephra Fallout and Surge Deposits
AU: * Kruse, S
EM: skruse@cas.usf.edu
AF: Dept. of Geology, Univ. of South Florida, 4202 East Fowler Ave., Tampa, FL 33701 United States
AU: Martin, K
EM: ktmartin@helios.acomp.usf.edu
AF: Dept. of Geology, Univ. of South Florida, 4202 East Fowler Ave., Tampa, FL 33701 United States
AU: Connor, C
EM: cconnor@cas.usf.edu
AF: Dept. of Geology, Univ. of South Florida, 4202 East Fowler Ave., Tampa, FL 33701 United States
AU: Mora, R
EM: raulmora@hotmail.com
AF: Escuela Centroamericana de Geología, Universidad de Costa Rica y Red Sismológica Nacional, San José, Costa Rica
AU: Ramirez, C
EM: carlosjru@hotmail.com
AF: Escuela Centroamericana de Geología, Universidad de Costa Rica y Red Sismológica Nacional, San José, Costa Rica
AU: Alvarado, G
EM: galvaradoi@ice.go.cr
AF: Escuela Centroamericana de Geología, Universidad de Costa Rica y Red Sismológica Nacional, San José, Costa Rica
AU: Alvarado, G
EM: galvaradoi@ice.go.cr
AF: Area de Amenazas y Auscultación Sísmica y Volcánica, Instituto Costarricense de Electricidad, San José, Costa Rica
AB: GPR profiles on Cerro Negro volcano, Nicaragua, and Poás, Irazú, and Arenal volcanoes, Costa Rica, show this method has utility for mapping tephra blanket and surge deposit thicknesses, as well as ballistics distributions. These data are useful for estimating eruption volumes, particularly close to vents where deposits may be thicker than trenching depths. In the dry, highly resistive tephra of the Cerro Negro basaltic cinder cone, distinct deposits are clearly imaged between 2 and 20 m depth. The lowermost coherent reflection is presumed to be the contact with underlying pre-Cerro Negro lavas and weathered tephra deposits. Within the 2-20 m package, individual reflecting horizons are clearly resolved, and reflection attributes, particularly phase, may contain useful information on the nature of contacts, such as abrupt changes in granulometry. Because of the very high velocities at Cerro Negro (0.14 m/ns), even with 200 MHz antennas strata shallower than 2 m are difficult to resolve. In contrast, wetter ash, pumice, paleosol, and surge deposits on Irazú and Poás volcanoes show velocities as low as 0.045 m/ns. The corresponding shorter wavelengths permit strata as shallow as 40-70 cm to be imaged with 200 MHz antennas, with depth penetration typically 5 to 8 m. Comparison of trench observations and radar profiles indicates that strong radar reflections are produced by iron-rich zones at the water table and soil-ash contacts. Other features visible in the profiles are small (tens of cm) sub-vertical offsets of nearly horizontal units, and diffractions or disruptions in horizontal units presumed to reflect >30 cm blocks.
DE: 0689 Wave propagation (4275)
DE: 0994 Instruments and techniques
DE: 8404 Ash deposits
DE: 8494 Instruments and techniques
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly