Martin Gutowski

Decimeter-resolution 3D seismic volume in shallow water: A case study in small-object detection

The 3D chirp subbottom profiler provides high-resolution imaging of coastal and inshore seabed and subseabed structure by combining the known, highly repeatable source waveformofchirpprofilerswiththecoherentprocessingand interpretation afforded by true 3D seismic volumes. Comprising 60 hydrophone groups arranged around a Maltese cross of four chirp transducers, 3D chirp permits acquisition of a true 3D volume with a horizontal resolution of 12.5 cm, providing an excellent base for shallow-water engineering, archaeological, military, and geologic studies. Here, we presentresultsfromsurveyinganatidalbasinonthesouthern coastofEnglandtomapbedrockprotrusionsandthesizeand distributionofburiedobjects.Thestudyareaof150250 m provided a series of unique challenges, including a large number of discrete objects ranging from tens of centimeters toseveralmetersinsize,buriedinathinveneer0.5to1.5 m of unconsolidated silt overlaying a flat bedrock surface that showed high acoustic contrast and short wavelength roughness. By comparing comprehensive postsurvey dredging of the entire site with a prestack time-migrated 3D volume, it is possibletoconfirma100%detectionrateforalldiscreteburied objects larger than 0.300.30 m in an illuminated area, although one acoustic anomaly could not be accounted for in thedredgingresults.

The geological Hubble: A reappraisal for shallow water

Traditional two-dimensional (2D) seismic acquisition techniques image the subsurface using a grid of orthogonal lines. Dips are recorded only in the along-track direction, limiting migration to a 2D along-track approximation of the inherently 3D wavefields. This produces profiles that are often complicated by out-of-plane reflections and with resolution (for structural interpretation) constrained by the line spacing rather than the source wavelength. The acquisition of true 3D seismic reflection data, in contrast, provides dip information for the reflected wavefields in both along- and across-track directions. This allows a full treatment of the 3D wavefields during migration, affording accurate 3D structural reconstruction, significantly improved resolution (theoretically 1/2 source wavelength), and increasing signal-to-noise ratio (SNR) through more effective noise cancellation.