Kansas Geological Survey, Open-file Report 2009-6
by
Richard D. Miller, Jianghai Xia, Shelby L. Walters, Joe M. Anderson, Brett A. Wedel, and Anthony R. Wedel
KGS Open-file Report 2009-6
March 2009
Water-bottom seismic data acquired immediately downstream of Gate #1 at Hardin Dam #3 located on the Arkansas River near Pine Bluff, Arkansas, proved simultaneously instructive, encouraging, and unsuccessful. A key goal of the survey was to delineate the suspected void located beneath the stilling basin around Gate 1 and to delineate the extent and origin of known dam-foundation degradation between Gate 1 and about Gate 7. The original research plan was to acquire and analyze surface waves from atop the concrete slab at the bottom of the stilling basin. Surface-wave seismic profiles were to be collected parallel to the axis of the dam, targeting areas with suspected sub-slab anomalous materials. Considering this was the first time such a survey had been attempted, several adaptations and developments were needed to efficiently generate and record surface-wave arrivals suitable for dispersion, backscatter, and inversion analysis.
Hydrophones were deployed along the bottom of the stilling basin with boat anchors and floats used to hold the hydrophone in place and locate the ends of the receiving cable. A two-boat configuration was used to attempt the acquisition--one was the source and receiver boat and the second was set up for recording data. The recording boat was secured to the side of the dam, while the source/receiver boat was free to maneuver parallel to the dam axis and directly over the concrete-lined stilling basin. Water flow measured at between 20k and 30k cfs inhibited the ability of the source/receiver boat to maintain a stable position long enough to detonate the source into a static cable position. Test data were finally acquired using the two-boat configuration as a barge, deploying the receiver spread on the bottom of the Gate 1 stilling basin and secured to the side of the two boats. The sources were detonated at different locations along the side of the boats, providing a data set that simulated a walkaway noise survey.
Shot gathers possessed arrival patterns characteristic of voids as observed at several sites with culverts, tunnels, and fault/fractures on land seismic data. This anomaly is interpreted on shot gathers to possess a curvature with apex near receiver 15, or approximately 10 stations from the source. Based on the interpreted shape of the seismic arrival returning from this anomaly, it appears to be around 8 to 10 ft below the top of the concrete slab and directly beneath the hydrophones' locations at a spot about midway along the hull of the rear boat in the two-boat configuration being used as a work platform. This seismic return could be from any one of several different types of anomalous features (anomalous to a normal layered earth). Included on that list are man-made foundation structures, voids, out-of-the-plane scatterers, boulders, construction fill, etc. It is however similar to model representations of voids and/or tunnels.
Improving interpretation using these data requires an improved spatial focusing of the source, which is unlikely with this data set considering the limited number of source locations that were occupied. With all the structures and abrupt changes in velocity/density likely present beneath the slab in this immediate area, the exact source of this "scattered seismic energy" cannot be determined uniquely. It is likely the interpretive confidence with these data could have been dramatically improved if we had collected a continuous line, where source and receivers moved from areas with known voids to areas known to be without voids. A data set of that type would have provided the necessary template for characterizing what these target voids look like on these kinds of seismic data.
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Kansas Geological Survey, Geophysics
Placed online June 15, 2009
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