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Insight: The increasing use of Gravity Gradiometry in the Exploration Workflow

Sunday, March 10, 2013

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.

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.
Simplified Explanation
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

Full Tensor
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.

Case Studies
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.

The challenge in positioning seismic blindly in say a 10,000 square kilometre block, however, is fraught with difficulties. Poorly positioned lines may not image the geology optimally and potentially condemn a vast area as being non-prospective. Being an airborne technique, an FTG survey can be acquired efficiently and rapidly over large areas and thereby focus the seismic budget. The environmental footprint is also negligible.

Other advantages of deploying FTG in the unique exploration setting of East Africa include an improved definition of the sedimentary basin and internal architecture; and the identification of structural leads which can become a focus for seismic acquisition.

The early seismic in the basin can also be calibrated to the FTG and if necessary, the seismic programme can be altered in places of shallow basement or insufficient depth of burial of potential source rocks.

Modelling the East African Rift

In order to show how gravity gradiometry can be used in a Rift system, a 2D cross section was created from existing seismic data and then extruded into the Y-direction to generate a pseudo 3D model (Figure 4). The model was then offset to simulate strike-slip faults and the conventional airborne gravity and gravity gradiometry response calculated (Figures 5 & 6).


Figure 4. Pseudo 3D model with simulated strike-slip offsets

Broad basement geometry is imaged with conventional airborne gravity, but detail is poor. Strike-slip movement is only imaged where the basement is very shallow.



Figure 5 (top): Conventional gravity
Figure 6 (bottom): Gravity Gradiometry


With the gravity gradiometry data deeper basement blocks are imaged as well as strike-slip motion at depth. The FTG generates such positive and clear results from the rift due to the inherent way the technology works. It can accurately map the contrast between the basement and sediment cover and pick up the architecture of the rift in considerable detail.

Full tensor gravity gradiometry has been proven to be an important tool in gaining a clear picture of the East African Rift’s geology, decreasing exploration disk and increasing exploration success.

NE Greenland: Integration of regional 2D seismic and full tensor gravity gradiometry
Three phases of 2D seismic acquisition and PSDM processing were completed by ION GeoVentures from 2008 to 2012. The objective was to provide a regional evaluation of the untested NE Greenland Atlantic passive margin (Helwig et al, 2012). In 2012 ARKeX completed an airborne Full Tensor Gravity (FTG) survey for Ion over the area for both the pre-round blocks open to the KANUMAS Group and the blocks for the 2013 Greenland Licensing Round. (Figure 7)

The FTG survey is the largest ever acquired offshore and covers 50,000 sq.km. The high resolution gravity gradiometry and magnetics have been integrated with the Northeast GreenlandSPAN seismic data to enhance the understanding of frontier basin architecture, and in reconciling the linkages of the inherent tectonic fabrics. From these data a higher resolution 3D structural model of the Northeast Greenland margin can be built in order to guide structural interpretations and the implications for petroleum systems on this margin.


Figure 7: Tectonic elements of the NE Greenland Margin and locations of the Ion GreenlandSPAN seismic and the full tensor gravity gradiometry survey

The prospectivity of the northern part of the Danmarkshavn Basin is likely to be heavily influenced by the presence and shape of salt bodies. Prior to the acquisition of the FTG survey over 25 individual diapirs had been mapped using Ion’s high quality PSDM seismic volume. However, even though the seismic quality is excellent, it is classed as a regional frontier survey with distances of over 50km between some lines. Modelling of the gravity gradiometry, and its integration with the seismic, has enabled the building of a 3D architecture of the salt and associated sedimentary packages in areas away from seismic control. In addition to salt geometries, fault linkages have also been extrapolated away from seismic control. Magnetic data analysis has also led to a better understanding of the basement composition, which along with fault linkages, allows models for reservoir fairway analysis to be progressed.

Benefits for regional exploration programs

  • Improved definition of sedimentary and internal architecture
  • Identification of structural leads which can become the focus for further seismic acquisition
  • early seismic in the basin can be calibrated to the FTG and if necessary, the seismic program can be altered in areas of shallow basement or insufficient depth of burial of potential source rocks
  • a fast and efficient way of exploring vast exploration acreage


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References:
James Helwig J.H., Bird, D.; Emmet, P.; Dinkelman, M.G.; and Whittaker, R. (2012) Interpretation of Tectonics of Passive Margin of NE Greenland from new seismic reflection data and geological-geophysical constraints. Third Conjugate Margins Conference, Dublin, 2012.

Jackson et al (2013) The Sky Above, the Ice Floes, and the Earth Below. Geo ExPro February 2013
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Author: David Jackson, Chief Geologist
Company: ARKeX Ltd


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