Overview of Full Tensor Gravity Gradiometry
Gravity gradiometry is the study and measurement of spatial variations in the acceleration due to gravity. The gravity gradient is the spatial rate of change of gravitational acceleration.
Gravity gradiometry data is used by oil, gas and mining companies to measure the density of the subsurface, effectively the rate of change of rock properties. It offers a step change in resolution and bandwidth from that of conventional airborne gravity data. The acquired gravity gradiometry data assists in the building of sub-surface geological models to aid exploration.
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What is it?
Gravity gradiometry measures the variations in the acceleration due to gravity between two or more points. The gravity gradient is the spatial rate of change of gravitational acceleration. It can be deduced by differencing the value of gravity at two points separated by a small distance and dividing by this distance. The two gravity measurements are provided by accelerometers which are matched and aligned to a high level of accuracy.
An accelerometer is basically a mass on a spring. A gravimeter measures the acceleration of the mass due to gravity. In Figure 1 below, the gravity gradiometer measures the acceleration of Mass A and B. The difference in acceleration is then calculated and divided by Distance C. That figure is the gravity gradient.
Figure 1. - Simplified view of Gravity and Gravity Gradiometry
While a conventional gravity survey records a single component of the three-component gravitational force, usually in the vertical plane, Full Tensor Gravity Gradiometry uses multiple pairs of accelerometers to measure the rate of change of the gravity field in all three directions. The end result is a more accurate representation of the gravity field being surveyed. This is shown in Figure 2.
Fig. 2. Conventional gravity measures ONE component of the gravity field in the vertical direction Gz (LHS), Full tensor gravity gradiometry measures ALL components of the gravity field (RHS)
Gravity v Gravity Gradiometry
In addition to measuring the entire gravity field about any given measurement point, Gravity gradiometry has two other major advantages over conventional scalar gravimetry which results in a significant increase in resolution and accuracy.
Firstly being the derivatives of gravity, the spectral power of gravity gradient signals is pushed to higher frequencies. This generally makes the gravity gradient anomaly more localised to the source than the gravity anomaly. The graph (Figure 3) compares the gz and Gzz responses from a point source.
Figure 3. – Vertical gravity and gravity gradient signals from a point source buried at 1 km depth
Secondly, and perhaps more importantly, the effects induced by platform motion (i.e. air turbulence or heavy sea state) are strongly suppressed. On a moving platform, the acceleration disturbance measured by the two accelerometers is the same, so that when forming the difference, it cancels in the gravity gradient measurement. This is the principal reason for deploying gravity gradiometers in airborne/marine surveys where the acceleration levels are orders of magnitude greater than the signals of interest.
Due to these factors gravity gradiometry offers a significant increase in resolution and accuracy over conventional scalar gravimetry.
East Africa: Reducing Exploration Timelines with Full Tensor Gravity Gradiometry
East Africa has received its fair share of exploration attention over the past few years. The discoveries in Uganda in the Albertine basin have instigated significant exploration enthusiasm in this vast region, as have the licensing of vast tracts of land (and lakes) in Ethiopia, Kenya, Malawi and Tanzania. Tullow Oil and its partner Africa Oil have already been reporting encouraging results with their first well in Kenya’s Block 10BB demonstrating a working petroleum system in the region.
The challenges in these frontier areas are enormous, however - vast tracts of remote exploration acreage with limited or no data coverage to explore and against an ever challenging time line. Against this backdrop, Full Tensor Gravity Gradiometry (FTG) is fast becoming a recognized technology addressing many of these challenges.
The East African geology, essentially comprising relatively young sediments juxtaposed against a much denser Achaean basement, is ideally suited to the use of this gravity exploration technique. FTG measures the variations of the Earth’s gravity field with such a high degree of resolution and bandwidth that detailed basement structure maps can be derived which, in turn, allows for the optimal positioning of the seismic campaign.
Contribution by David Jackson
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