HR: 17:00h
AN: H14C-05    [Abstracts]
TI: Processing Ultra-Shallow Reflection Data in Areas with Laterally Varying Velocities
AU: * Vincent, P D
EM: pvincent@ku.edu
AF: Univ. of Kansas, Dept. of Geology 1475 Jayhawk Blvd., Lawrence, KS 66045 United States
AU: Tsoflias, G P
EM: tsoflias@ku.edu
AF: Univ. of Kansas, Dept. of Geology 1475 Jayhawk Blvd., Lawrence, KS 66045 United States
AU: Sloan, S D
EM: ssloan@ku.edu
AF: Univ. of Kansas, Dept. of Geology 1475 Jayhawk Blvd., Lawrence, KS 66045 United States
AU: Steeples, D W
EM: don@ku.edu
AF: Univ. of Kansas, Dept. of Geology 1475 Jayhawk Blvd., Lawrence, KS 66045 United States
AB: Ultra-shallow seismic reflection methods have proven effective as non-invasive subsurface characterization and monitoring tools, particularly in areas of high electrical conductivity where ground-penetrating radar is ineffective. Hydrologists often rely on geophysical methods to sense the properties of the near surface, at times in shallow layers that exhibit rapid lateral variation caused by saturated and unsaturated sand lenses, clay lenses, and unevenly weathered bedrock formations. Under these conditions, methods traditionally used to collect and process seismic reflection data must be examined for validity prior to application. In this study we present 2-D common-midpoint (CMP) ultra-shallow seismic reflection data collected in a flood plain near a tributary stream channel outside of Lawrence, Kansas. The 20-m long survey line intersected a visually identified abandoned stream channel roughly paralleling the current stream. Near-source first-arrival analysis shows that the P-wave velocities near the surface double over a horizontal span of 15 m, and a $\sim$5 m deep reflection that dominates shot gathers at one end of the survey is not evident in shot gathers at the other end. Such heterogeneity negates the equivalency of pseudo-walkaway and walkaway data, and makes normal-moveout (NMO) based CMP processing difficult. Walkaway surveys are often used in conjunction with CMP surveys to determine seismic velocity. We show that a drastic difference can be encountered when a pseudo-walkaway is substituted for a walkaway in the presence of significant lateral velocity variations. We also compare the final time-to-depth converted stacked sections created when a variety of source-to-receiver offset ranges are NMO corrected and stacked using several different velocity functions.
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting