Seismic imaging of the water-column deep layer associated with the Deepwater Horizon oil spill

Abstract

The submerged hydrocarbon layer at a depth of approximately 1100–1200 m in the water column near the Deepwater Horizon well site was associated with a temperature anomaly that was imaged by seismic reflection data collected two days before and one day after the well shut-in. Conventional multichannel seismic reflection profiling data were processed to enhance water-column reflections by filtering out low-frequency energy and applying higher gain than typically used for imaging subsurface geology. The deep layer reflectivity, within a few km of the well site, has an impedance less than seawater by about a factor of approximately [Formula: see text], or equivalently, a negative sound-velocity step of about [Formula: see text]. Water-column profile data collected at the same time as the seismic data show a comparable sound velocity and temperature decrease associated with the deep layer. Mixing of less dense hydrocarbons entrains cold seawater, resulting in a temperature anomaly within the deep layer at the level of neutral buoyancy. This temperature contrast explains the negative sound-velocity anomaly associated with the deep reflector. Because of the small hydrocarbon concentration in the deep layer, their contribution to the seismic velocity anomaly is too small to be significant. The high horizontal resolution (3 m) of the seismic image reveals a layer with patchiness of several hundred meters lateral extent.