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.