Ultra-shallow imaging using 3D seismic-reflection methods

Three-dimensional ultra-shallow seismic reflection methods were used to image multiple reflectors less than 20 m deep, including the top of saturated zone, a paleo-channel feature and the top of bedrock at a field site located near Lawrence, Kansas. A small 3D demonstration survey was designed and acquired using source and receiver intervals of 0.5 m and source line and receiver line intervals of 2 m. The survey had a nominal fold of 48 and covered an area of similar to 15.5 m x 35.5 m. Large variations in velocity were present, ranging from similar to 300-600 m/s laterally in the shallowest layer and similar to 300-1600 m/s vertically, a degree of variability that is not uncommon in the ultra-shallow subsurface. Normal moveout corrections cannot account for intersecting reflection hyperbolae such as those caused by large vertical velocity gradients, so a novel processing scheme was applied by extracting offset-dependent subsets based on the optimum window for each reflection. The subsets were normal moveout corrected independently and then stacked together using conventional 3D processing techniques. Despite the large lateral and vertical velocity variations, we were successful in imaging the top of the saturated zone, paleo-channel features and the overburden-bedrock interface located at depths of similar to 5 m, 8 m and 14 m, respectively. Results of the 3D survey are in agreement with previous studies conducted at the site. The methods presented here could be applied to situations where 3D imaging of the ultra-shallow subsurface is necessary and may help to identify possible contaminant sinks or flowpaths.