Research Projects

Seafloor gravity with ROVDOG

ROVDOG being deployed

Figure 1: Atlantis Massif core complex map

Figure2: ROVDOG on the seafloor

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Historically, geophysical studies of oceanic regions have lacked high precision (short wavelength) coverage of gravity. The primary available gravity data comes from shipboard measurements or remote sensing techniques (Smith and Sandwell - http://topex.ucsd.edu/marine_topo/mar_topo.html). Wavelength attenuation effects cause the small-scale gravity signature to disappear while at the same time the strength of the gravitational signal decreases as 1/r² away from the source mass. Other problems such as ship motion further affect the data quality. For many applications the small scale structure and/or an increased signal to noise ratio is needed. For this purpose, our group has constructed the seafloor relative gravimeter ROVDOG (ROV-Deployed Deep-Ocean Gravimeter). The work was done in collaboration with Statoil (www.statoil.com) for the purposes of offshore reservoir monitoring. The meters are Scintrex CG3M relative land gravimeters modified for remote operation on the seafloor. This is described in detail in Sasagawa et al., 2003.

We have done five field surveys at three sites using the ROVDOG gravimeters:

Troll Gas Field Reservoir Monitoring (time-lapse)
Atlantis Massif Core Complex
Sleipner CO₂ Sequestration (time-lapse)

Troll Gas Field Reservoir Monitoring

ROVDOG measurements over the Troll gas field were made in 1998, 2000, and 2002, in order to monitor water influx into the field.

Atlantis Massif core complex

Metamorphic core complexes are domal uplifts of deep origin metamorphic or plutonic rocks bounded by shear zones that separate them from cover rocks that are shallower in origin.

At the Atlantis Massif, spreading parallel corrugations on the domal surface have been interpreted at the exposed surface of a detachment fault that controls the evolution of the feature (Fig. 1). From November to December, 2000, seafloor gravity measurements were made using the ROVDOG system at depths ranging from 1500 to 3000 meters (Fig. 1). In place of an ROV, the DSV Alvin (http://www.whoi.edu/marops/vehicles/alvin/index.html) was the vehicle used to operate the gravimeters (Fig. 2). This was one part of a study focusing on this oceanic core complex (http://earthguide.ucsd.edu/mar/). More details can be found in Nooner et al., 2003.

Sleipner CO₂ Sequestration

The Sleipner field is a natural gas production area in the North Sea operated by Statoil, the Norwegian state owned oil company. The gas recovered there has an excess CO₂ content of about 10%, which is separated out and collected. CO₂ collected in this manner is typically released into the atmosphere, increasing the atmospheric content of this greenhouse gas. Sleipner is the world's first large scale concentrated CO₂ sequestration project. Here, the CO₂ is injected into a large, deep saline reservoir called the Utsira formation. This is a high porosity sandstone aquifer capped by low porosity shale at 720m below the seafloor.

A group known as SACS (Saline Aquifer CO₂ Storage, http://www.ieagreen.org.uk) began geologic analysis, reservoir simulations, seismic modeling, and geophysical monitoring of Sleipner in 1998. The primary monitoring technique is 4-D or time-lapse seismics. The results show a clearly defined CO₂ bubble. However, uncertainties exist such that the density, thus mass, of CO₂ within the Utsira sand is not precisely known. Preliminary gravity modeling shows that the density changes due to the implacement of CO₂ could be observable with a time lapse, high precision seafloor gravity survey. In August of 2002, we carried out the first phase of this survey, establishing a baseline for future surveys as well as obtaining a gravity data set for reservoir modeling. The repeatability of the survey based on repeated measurements at each site is ~2.5 μGal. A repeat survey is expected in 2005.

Media

Click here for a powerpoint presentation about the Sleipner CO₂ sequestration.