Backscatter Characterization of Seep-Associated Seafloor Features in the Vicinity of Bush Hill, Northwest Green Canyon, Gulf of Mexico
Authors: Zhang Zijian , McConnell Daniel Russell 
Published: July 2010

Abstract
MultiBeam Echo Sounder (MBES) bathymetric and backscatter data are used to discriminate the backscatter response of four seafloor features: a carbonate mound, a hydrate mound, a small mud volcano, and pockmarks in the vicinity of the welldocumented Bush Hill seep site. Each of these features are associated with fluid migration though both the geologic structures and into and through the shallow sediments. The fluids migrate upward along fault, fractures, and/or salt from deeper sediments following natural dewatering and focused fluid flow. Some of these features reach to seafloor and have seafloor expressions that have been related to slow fluid flux environments, such as carbonate mounds. Others features are formed as fluids temporarily occupy and accumulate in some shallow porous sediments. Probably by means of trigger mechanisms, fluids also rapidly migrate to seafloor along vertical conduits. These rapid-flux areas are characterized by fluid expulsion features, such as mud volcanoes and pockmarks.
Physical property-based simulations and analyses are applied to the backscatter data associated with known features that have been documented in the vicinity of the Bush Hill site in the Northern Gulf of Mexico. The simulated backscatter responses are in good agreement with the observations from cores and visual surveys from manned-submersible platforms. Physical properties and seafloor roughness dominate the backscatter strength in seep associated sediments. In addition, quantitative information on the backscatter data gained from this modeling excersize and those done by others are expected to result in more refined interpretation of seep features when combined with seafloor morphology and the local geologic setting.

Introduction
MBES backscatter has been used for detailed seafloor characterization for many years. However, most studies have focused on types and variation of seafloor sediments (Loncke et al., 2002; Hellequin et al., 2003; Nitsche et al., 2004; Goff et al., 2004). Few examples document detailed seep features from MBES backscatter (Orange et al., 2002). Although we are aware of recent and ongoing efforts with this aim, the literature search made in advance of this study yielded no quantitative studies of backscatter strength related to the discrimination and interpretation of seep features.
Seafloor seeps are naturally occurring geological phenomena on the continental slope. They are caused by fluid migrating through the sediment column to seafloor. The chemistry, biology, and geology of the slope may be impacted at or near seafloor (Roberts et al., 2000). Roberts et al. (2006) presented a model that different seafloor expression of seeps and their associated features seen in 3-D seismic seafloor and shallow amplitude data. Roberts et al. propose that slow flux areas are mineral prone, which may be characterized by high seismic amplitude; and that rapid flux areas are mud prone within the seep feature, which may be characterized by low seismic amplitude. Paull and Ussler (2008), however, point out that seafloor erosion might expose the buried carbonate nodules might have formed along the sulfate-methane interface on the periphery of a high flux seep, instead of indicating a local low flux environment. Seafloor mounds are common seep-associated seafloor features. Sager et al. (2003) defined three types of seep mounds: predominantly authigenic carbonate, predominant gas hydrate, and predominantly mud. Another typical seep-associate seafloor feature is the pockmark. Pockmarks have been documented in many areas (Judd and Hovland, 2007).