Gravity gradiometry is a well-established geophysical technique that is often used in the search for hydrocarbons. The technology measures small differences in the Earth’s gravity field associated with changes in subsurface geology.
Lockheed Martin is the only company to provide commercial moving-base gravity gradiometers and, until recently, broadly offered two types of gravity gradiometer to the exploration industry: the full-tensor gravity gradiometer (FTG) system, which is deployed in both airborne and marine modes, and the partial tensor system, typically deployed in airborne mode only. By their intrinsic design, these systems have a greatly improved resolution when compared to conventional (scalar) gravimeters and thus provide a natural advantage when used in exploration.
In addition, geoscientists can benefit from having multidirectional gravity gradients as they yield extra information on geometrical and density changes in subsurface geology that give rise to gravity anomalies. Historically, these gravity gradiometers have been widely used in a variety of geological settings to rapidly screen basins and assist with the mapping of basin architecture, basement depth, faults and intra-sedimentary structure.
A recent significant advance in technology means that the existing generation of gravity gradiometers has been surpassed by Lockheed Martin’s next-generation instrument called the enhanced FTG (eFTG). The eFTG is the world’s most advanced moving-base gravity gradiometer, possessing a noise floor about three times lower than the FTG and providing data with higher bandwidth. These improvements mean eFTG data have increased accuracy and higher spatial resolution, therefore widening the range of geological targets that can be mapped with gravity gradiometry.
Given that the eFTG incorporates similar design principles from pre-existing FTG and partial-tensor gravity gradiometers, it is worth summarizing these designs before describing the eFTG.
The FTG system contains three gravity gradiometry instruments (GGIs), each consisting of two opposing pairs of accelerometers arranged on a spinning disc with measurement directions tangential to the disc. The opposing accelerometers are separated by a prescribed distance, which is known as the measurement baseline. The GGIs are orientated orthogonally so that all components of the gravity gradient tensor are measured, allowing the system to fully describe the gradient field at every point in the survey. Although there are only five independent tensor components, the arrangement of the 12 accelerometers in the FTG system provides six independent and orthogonal measurements of the gravity gradient tensor.
Partial tensor design
With a single horizontally mounted disc containing eight accelerometers, the partial tensor system measures only the two horizontal curvature components of the gravity gradient tensor. Signal power in the horizontal components is weaker (about half that of the vertical components), so to offset a reduction in the signal-to-noise ratio (S/N), the disc measurement baseline is about double that of an FTG’s GGI.
From measurements of the horizontal components, vertical tensor components are derived by mathematical transformations performed after the survey is completed. The vertical gravity gradient (Gzz) is considered to be the most useful for interpretation purposes by the geoscientist, primarily since this component is a more intuitive representation of the subsurface geology. Despite the differences in design between the two systems and provided that the line spacing is not too large (relative to the target depth), the resulting derived Gzz component from the partial tensor system can be comparable to that from the FTG.
Increased bandwidth and S/N
The eFTG system combines the best design elements of both gradiometers, essentially comprising three digital partial tensor discs/GGIs mounted in an FTG configuration. This means the eFTG GGIs have eight accelerometers per disc with a measurement baseline roughly double that of the FTG accelerometer separation. The increase in accelerometer count and larger baseline means the eFTG has a threefold improvement in S/N over the entire bandwidth. With 24 accelerometers the eFTG provides 12 gravity gradient outputs per measurement location (the eFTG essentially measures the full tensor twice and with double the accuracy in each case).
With its increased capability and performance, the eFTG benefits apply throughout the entire geological section, helping to deliver a more accurate Earth model. The greater sensitivity of the instrument allows the detection of smaller geological features with subtler density contrasts and improves the application of mapping structure in deeper basins. Surveys can be done more cost-effectively since the increased S/N means that the line spacing can be increased in some geological scenarios. The higher bandwidth and increased spatial resolution of data means these data can be more tightly integrated with seismic data than before; applications include joint gravity-seismic inversions and the ability to quality-control and refine seismic velocities. eFTG data also can be used ahead of a seismic survey, providing a highly detailed map on which to precisely locate seismic lines in the optimal locations.
FTG and eFTG data comparison
To visually demonstrate the differences between the FTG and eFTG instruments, a feasibility study was carried out to test the expected responses to a geological model that would generate both short and long wavelength signals. The following workflow was adopted:
- Construct a 3-D geological model;
- Assign densities to the key lithological units;
- Forward-calculate the model response;
- Add typical instrumentation noise to the model response (this simulates realistic survey conditions); and
- Analyze final filtered grids.
AustinBridgeporth is offering the eFTG to the exploration industry on an exclusive basis. The system is survey- ready and can be deployed in airborne or on shipborne platforms. The eFTG will allow geoscientists to image subsurface structure and complexity in unprecedented detail, facilitating cost-effective exploration even in a “lower for longer” oil price environment.