HR: 1340h
AN: V43E-1464    [Abstracts]
TI: Imaging Structure, Stratigraphy and Groundwater with Ground-Penetrating Radar on the Big Island, Hawaii
AU: * Shapiro, S R
EM: srs52@cornell.edu
AF: Cornell University, Department of Earth and Atmospheric Sciences Snee Hall, Ithaca, NY 14853 United States
AU: Tchakirides, T F
EM: tft5@cornell.edu
AF: Cornell University, Department of Earth and Atmospheric Sciences Snee Hall, Ithaca, NY 14853 United States
AU: Brown, L D
EM: brown@geology.cornell.edu
AF: Cornell University, Department of Earth and Atmospheric Sciences Snee Hall, Ithaca, NY 14853 United States
AB: A series of exploratory ground-penetrating radar (GPR) surveys were carried out on the Big Island, Hawaii in March of 2004 to evaluate the efficacy of using GPR to address hydrological, volcanological, and tectonic issues in extrusive basaltic materials. Target sites included beach sands, nearshore lava flows, well-developed soil covers, lava tubes, and major fault zones. Surveys were carried out with a Sensors and Software T Pulse Ekko 100, which was equipped with 50, 100, and 200 MHz antennae. Both reflection profiles and CMP expanding spreads were collected at most sites to provide both structural detail and in situ velocity estimation. In general, the volcanic rocks exhibited propagation velocities of ca 0.09-0.10 m/ns, a value which we interpret to reflect the large air-filled porosity of the media. Penetration in the nearshore area was expectedly small (less than 1 m), which we attribute to seawater infiltration. However, surveys in the volcanics away from the coast routinely probed to depths of 10 m or greater, even at 100 MHz. While internal layering and lava tubes could be identified from individual profiles, the complexity of returns suggests that 3D imaging is required before detailed stratigraphy can be usefully interpreted. A pilot 3D survey over a lava tube complex supports this conclusion, although it was prematurely terminated by bad weather. Although analysis of the CMP data does not show a clear systematic variation in radar velocity with age of flow, the dataset is too limited to support any firm conclusions on this point. Unusually distinct, subhorizontal reflectors on several profiles seem to mark groundwater. In one case, the water seems to lie within a lava tube with an air-filled roof zone. Surveys over part of the controversial Hilana fault zone clearly image the fault as a steeply dipping feature in the subsurface, albeit only to depths of a few meters. The results suggest, however, that deeper extensions of the faults could be mapped by more aggressive surveying techniques (e.g. lower frequencies or high levels of signal stacking). In summary, GPR has proven to be a very effective technique in mapping stratigraphy, structure, and hydrology in the volcanic rocks of Hawaii. Future work is planned to evaluate more fully the merits of 3D imaging, the systematics of propagation velocity with composition and age of extrusion, the internal morphology of successive flows, and whether GPR can discriminate aa from pahoehoe flows at depth.
DE: 8494 Instruments and techniques
DE: 5109 Magnetic and electrical properties
DE: 1829 Groundwater hydrology
DE: 0694 Instrumentation and techniques
DE: 0925 Magnetic and electrical methods
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting