Using Seafloor Mapping (Bathymetry and Backscatter) and High Resolution Sub-Bottom Profiling for both Exploration and Production: Detecting Seeps, Mapping Geohazards, and Managing Data Overload with GIS
Authors: Orange D.L. , Angell M.M. , Lapp D. 
Published: October 2008

New advances in remote sensing allow for the simultaneous collection of high resolution seafloor bathymetry, backscatter and sub-bottom profiling. These data sets can be used to map large portions of the seafloor in detail for exploration targets (seeps) and hazard assessment To demonstrate, we discuss seafloor mapping, seep, geohazard, and GIS data from the Office of Naval Research's STRATAFORM study area in the Eel River basin, northern California. This study area includes active gas seeps as well as hazards such as mud volcanoes, slope failures, active faults, canyons, channels, hard pan, gas, and gas hydrate. We use a nested survey strategy that begins with compilation of available regional data, including bathymetry, seismicity, and multi-channel seismic lines. Regional data can constrain the geologic context in an area, and can be used to rapidly assess gross morphology and hazards. Within the STRATAFORM study area we combine high resolution multibeam bathymetry and backscatter with high resolution sub-bottom profiling data and side scan. Targets identified within this detailed georeferenced data set have been groundtruthed with ROV dives and cores.

All data are loaded into a GIS for rapid registration, visualization, database attribution and retrieval. The integration of high resolution and multi-channel seismic data with seafloor mapping allows one to link anomalous features on the seafloor to specific fluid, tectonic, or stratigraphic compartments in the subsurface. Anomalous features on the Eel River seafloor are associated with structural culminations that show evidence for fluid and gas migration in the subsurface; ROV dives to these seep targets documented active gas expulsion as well as biological and geological evidence of fluid flow.

In areas with existing 3D MCS coverage, multibeam may be used to refine hazards in areas where seismic imaging is problematic or where data quality for the upper 500 m, and especially the seafloor, is insufficient to characterize shallow geology. In areas without complete 3D MCS coverage, we recommend the collection of high resolution seafloor and sub-bottom data early in a the project cycle. Using an approach that combines seafloor mapping and sub-bottom profiling, the same surveys carried out to target sampling can be used to analyze geohazards. This dual-use approach decreases cycle time by providing information pertinent to both exploration and production early in a project's evolution.

Introduction

Seafloor mapping systems include multibeam as well as side scan. There are trade-offs between systems that require recognition of survey priorities: multibeam systems, if hull mounted, have a resolution that decreases with increasing water depth and collect data at speeds of ~10 knots; towed systems can provide higher resolution data in deeper water, but are towed at relatively slow speeds. New generation high-resolution multibeam systems may be appropriate for mapping complete offshore blocks for exploration and preliminary geohazard assessment, whereas towed side scan may be more appropriate for detailed seafloor studies or geohazard transects (pipelines, cables).