Ultra‐shallow imaging using 3D seismic‐reflection methods

Abstract

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 ~15.5 m × 35.5 m. Large variations in velocity were present, ranging from ~300-600 m/s laterally in the shallowest layer and ~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 ~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. layers of shale and marine limestone characteristic of the

The 3D Autojuggie: automating acquisition of 3D near‐surface seismic reflection data

“…Because of this, they determined that their 3D data must be migrated to properly image the reflectors. Additional examples of reported near-surface 3D reflection surveys can be found in House, Boyd, and Haeni (1996) ;Spitzer, Green, and Nitsche (2001) ;Schmelzbach, Horstmeyer, and Juhlin (2007) ;Fradelizio, Levander, and Zelt (2008); Sloan, Steeples, and Tsoflias (2009); Bachrach and Reshef (2010); and Kaiser et al (2011).…”

“…Processing of the data consisted of geometry definition, CMP binning, trace editing, early, and surgical muting to remove the first arrivals and airwave, iterative velocity analysis and surface‐consistent statics corrections, normal moveout corrections, and CMP stacking. Data were normal moveout corrected using offset‐based subsets (Sloan et al ) due to intersecting reflections (Miller and Xia ) and merged prior to stacking. Processing of the 3D volume is described in detail in Sloan ().…”

“…Data were normal moveout corrected using offset‐based subsets (Sloan et al ) due to intersecting reflections (Miller and Xia ) and merged prior to stacking. Processing of the 3D volume is described in detail in Sloan ().…”

“…In total, the geophone/source spread was rolled 13 times occupying 1183 source locations, 2600 receiver locations, and collecting 236,600 traces in ~19 hours with a crew of four people, covering an area of ~1225 m 2 (Sloan ). In comparison, a previously acquired 3D survey at an adjacent test site with manually deployed geophones covered an area of ~550 m 2 in ~10 hours with a crew of 10–12 people and a total fold of 48 (Sloan et al ). This represents an 18% increase in square meters covered per hour, a 60%‐67% decrease in labour, and a 500% increase in fold coverage.…”

Seismic reflection surveys in glaciofluvial deposits in Finland

“…When such vibrators are not available, impulsive sources, such as a sledgehammer or explosives, have generally been used showing good results (e.g., Miller ad Steeples ). Field surveys are usually conducted in 2D, but 3D surveys are also becoming popular (Büker et al ; Sloan, Steeples, and Tsoflias ; Lundberg et al ). Land streamers have been developed to speed up field surveys and are very suitable on well‐built roads (van der Veen and Green ; Pugin et al ).…”