3-D IMAGING OF EARTHQUAKE-INDUCED LIQUEFACTION FEATURES WITH GROUND PENETRATING RADAR

MAHDI, H., Al-SHUKRI, H., University of Arkansas at Little Rock, Little Rock, Arkansas 72204, and TUTTLE, Martitia P., M. Tuttle & Associates, Georgetown, ME 04548, mptuttle@earthlink.net, mahdi@seismo.ualr.edu.

Ground Penetrating Radar (GPR) has been proven to be a powerful tool to study earthquake related features such as sand blows and faults. GPR was instrumental for citing trenches of liquefaction features and for defining the size and morphology of sand blows and the location and orientation of feeder dikes. Our primary goal in applying GPR for such studies was to optimize the location of trenches and possibly locate and map the contact between the sand blow and the buried paleo-surface. A secondary goal was to image sand blows in a 3-D setting in order to detail study these features. Trenching large sand blows is both costly and provides a limited three-dimensional view of the overall structure. GPR surveys will help us identify possible locations for the venting dikes and to visualize the subsurface features. In our GPR surveys of large elliptical sand deposits near Marianna, Arkansas, we imaged sharp contacts in near-surface sediments that were confirmed in trenches to represent boundaries between sand blows and buried soils. One survey in particular showed a sharp discontinuity in the boundary related to a large feeder dike. Because sand thickness was no more than 2 meters, we used a 400-MHz antenna in all of the surveys. This antenna is designed to provide the best resolution in the upper 5 meters of soil. Data acquisition was along parallel profiles oriented normal to the long axes of the sand deposits at all sites. Data reduction and analysis procedures included removal of the direct and the ground surface effects, frequency filtration, gaining control, profile migration, and three-dimensional visualization. For future work, we plan on refining the fieldwork procedures and optimizing the survey configuration to acquire the best result. We will also continue surveying other sites to help with the follow up trenching and sampling work.