HR: 0800h
AN: NS31B-0400    [Abstracts]
TI: Use of Seismic Reflection Data and Traveltime Tomography to Image the Near Surface Velocity Structure in the Mississippi Embayment
AU: * Ge, J
EM: jge1@memphis.com
AF: University of Memphis, 3876 Central Ave., Memphis, TN 38152, United States
AU: Magnani, M
EM: mmagnani@memphis.edu
AF: University of Memphis, 3876 Central Ave., Memphis, TN 38152, United States
AU: Waldron, B
EM: bwaldron@memphis.edu
AF: University of Memphis, 301 Engineering Admin. Bldg., Memphis, TN 38152, United States
AU: Powell, C
EM: capowell@memphis.edu
AF: University of Memphis, 3876 Central Ave., Memphis, TN 38152, United States
AB: The Memphis aquifer represents one of the highest quality reservoirs of drinking water in the nation and it is separated from the shallow unconfined aquifer by the Upper Claiborne clay. Recent studies show that the confining unit might be discontinuous over the greater Memphis area exposing the Memphis aquifer to potential contamination. We present the results of a seismic reflection profile collected near Memphis, TN with the goal of imaging the structures and potential breaches in the Upper Claiborne confining clay. The imaged area is characterized by a highly heterogeneous shallow velocity structure and low P wave velocities in the ultrashallow unconsolidated materials. The data were collected using a shotgun source and a 1 m source spacing, 0.25 m receiver spacing and a 168-geophone spread for a max offset of 42 m. Raw seismic data show several reflected arrivals in the first 200ms, widespread ground roll, and air wave energy as well as consistent refracted phases across the 1 km - long profile. In addition to the reflection profile we present the preliminary results of first arrival travel time tomography performed along the profile to constrain the velocity field in the shallow portion of the profile. The velocity was then used to remove the effect of the near surface velocity variations. The main data processing steps included elevation statics and frequency and FK filtering. First arrival travel time modeling started with an initial estimate of the 2-layer velocity model using the slope/intercept method. We then modeled first-arrival picks on 1095 shot gathers using the Geo TOMO+ package. The algorithm computes travel times by tracing turning rays and is also able to handle raypaths through low-velocity zones (blind zones). The final resolution is estimated through a ray-information density map, which shows the cumulative contribution of the ray segments traversing different areas of the model. Synthetic models were generated and tested for the tomography inversion, which indicates that the inversion is reliable.
DE: 1829 Groundwater hydrology
DE: 1871 Surface water quality
DE: 7203 Body waves
DE: 7290 Computational seismology
SC: Near-Surface Geophysics [NS]
MN: 2007 Fall Meeting