Sidescan Sonar and Seismic Signatures of Mud Mounds at Petroleum Seeps on the Louisiana Continental Slope
Authors: Sager William W. , MacDonald Ian R. 
Published: October 2008

Mud mounds are common features at hydrocarbon seeps in the northern Gulf of Mexico and are thought to result primarily from the expulsion of formation fluids along deep-seated faults. Although such mounds are commonly observed in industry geophysical data, few studies have attempted to catalog their characteristics. In this study, we examined mounds in two areas of the upper Louisiana slope using several types of geophysical data: long-range sidescan sonar images, industry 3D multichannel seismic (MCS) data, and near bottom chirp sonar records, submersible observations, and cores for "ground truth." In the sidescan data, the mounds appear as ovoid spots showing high acoustic backscatter (BS). These are associated with larger high backscatter zones apparently related to less focused seepage. Three different sonar responses were observed:more-or-less uniformly high BS,high BS center surrounded by lesser BS halo ("bulls-eye"), andhigh BS with an acoustic dead spot near the center.

Multichannel seismic data over several mounds shows columnar wipe-out or chaotic zone signature below the features. Seismic seafloor amplitude extractions show several mound signatures:more-or-less uniform positive or negative,a ring of positive anomalies around the mound periphery, orno anomalous response.

Uniform and ringsignature mounds are thought to be active, with the ring feature caused by the preferential deposition of seep materials on the mound perimeter. Mounds that show no anomalous seafloor response are thought to be inactive. The main difference between the sidescan sonar and multichannel signatures is that the sonar backscatter anomalies are widespread whereas seismic anomalies only occur on the largest, apparently most active mounds. Evidently the sonar responds to small changes in sediment properties that do not significantly affect longer seismic wavelengths. Near bottom chirp sonar records and cores indicate high lateral variability in sediment characteristics, suggesting that both long-range data types integrate the response of seafloor with variable characteristics. Chirp sonar records display hard-bottoms, acoustic wipe-out zones (attenuation), and acoustic turbidity (prolonged bottom echo), all thought to result from hydrocarbon seepage. Cores recovered a variable mixture of seep materials including gas, oil, authigenic carbonate debris, indurated layers, and shells. All together, the data imply that mound signatures depend on activity, geologic structure, and possibly age.

Introduction

Seafloor mounds are common in the Gulf of Mexico and other petroleum-producing basins where seeps occur. Generally these fall into three categories:predominantly authigenic carbonate,predominantly gas hydrate, andpredominantly mud.

Authigenic carbonate mounds are formed as a byproduct of microbial oxidation of hydrocarbons and are generally small, less than tens of meters in diameter and meters in height1. Gas hydrate mounds form where hydrocarbon gas rapidly rises from the sea surface and is incorporated into a clathrate body at the seafloor2. Such mounds are also generally small. Larger mounds often appear to be made up mainly of sediment; although, many probably contain gas hydrate and carbonate bodies.