Acoustic Wipe-Out Zones - A Paradox for Interpreting Seafloor Geologic/Geotechnical Characteristics (An Example from Garden Banks 161)

Abstract

Abstract A fundamental problem associated with using high resolution seismic profiles for evaluations of seafloor geology is the "acoustic wipeout zone." This seismic response is commonly encountered on the northern and northwestern Gulf of Mexico continental slope. These zones of little or no data and are generally attributed to bubble-phase gas in surface and near-surface sediments by most geoscientists who use these data sets in support of geohazards assessments and seafloor engineering projects. However, studies over the last decade utilizing standard sampling techniques for geotechnical data collection, surface attribute data from 3D-seismic surveys, and direct observations/samplings from manned submersibles have revealed that "wipe-out zones" derive from a variety of processes which result from rapid flux of gases and fluids to the seafloor, seafloor lithification, elimination of internal structure within the sediment column, gas hydrate formation/decomposition, and slow seepage of gas to the modern seafloor. Each of these processes produces its own signature in the geology of the seafloor and shallow subsurface. This paper addresses one case in the feature variability spectrum from Garden Banks 161 where unusual geotechnical and geologic properties of the sediment and seafloor of the area are addressed and explained. Reliable faunal indicators suggest that this sediment originated from the deep subsurface, but is currently not charged with gas, is overconsolidated, is not laced with biodegraded crude oil, and does not support chemosynthetic organisms. However, the boring data, cores, and direct observations of the acoustic wipe-out area suggest an expulsion zone where large volumes of poorly sorted sediment have been transported to the seafloor. This study improves our understanding of the complex Gulf of Mexico slope floor and provides one condition that can result in an "acoustic wipeout zone" on high resolution as well as exploration scale seismic records. The conditions encountered in Garden Banks 161 are not commonly interpreted from standard geophysical data sets. Introduction As part of the development of every deepwater oil and gas prospect in the Gulf, the seafloor must be evaluated for potential geohazards to satisfy regulatory guidelines imposed by Federal agencies and to meet requirements for engineering purposes. Standard data sets acquired for these studies include high resolution acoustic data, both seismic and side-scan. Over the last few years new-generation 3D-seismic data, acquired primarily for exploration and reservoir characterization purposes, have been used to augment these higher resolution data sets. In the case of clearing tracks of seafloor with the Minerals Management Service regarding geohazards, sometimes 3D-seismic is used exclusively. One reason for the use of 3D-seismic for evaluating deepwater areas is that surface-tow high resolution seismic sources become less effective for producing an acoustic representation of the seafloor and shallow subsurface as water depth increases. Therefore, deeptow methologies and associated data sets as well as 3D-seismic have become the accepted methods. Regardless, there are certain bottom types and associated sections of the shallow subsurface that have an acoustic signature that is very difficult to interpret. One of these acoustic signatures is the "acoustic wipeoutzone." Many researchers have recognized this response on seismic records. The general interpretation is that bubble phase gas is contained in the shallow sediment column which causes scattering of the incident acoustic energy. Therefore, gas charged sediments are interpreted from seismic records by high amplitude reflecting horizons or an acoustically opaque or