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October 2024
In regions where well and seismic data are scarce, such as is the case for large areas in Africa, one of the best ways to do a quick initial mapping exercise is airborne geophysics.
At Earthline Consulting, we specialise in subsurface exploration by plane. This enables us to gain an understanding of what lies beneath the earth’s surface without the time constraints of a ground survey.
Data, particularly information about subsurface geology, is key to successfully exploiting natural resources, be they mineral, hydrocarbon or renewables such as geothermal and natural hydrogen. Especially in Africa, where large swaths of the continent have not been mapped in detail, conducting subsurface exploration by plane means we are able to survey large areas of land to produce nationwide subsurface resource maps. This would be a significant step forward and key to achieving a unified, cohesive understanding of subsurface geology and resource potential.
There have been some notable sub-surface exploration initiatives by organisations such as the National Geoscience Data Centre (NGDC), Operated by the British Geological Survey (BGS) and the U.S. GeoFramework Initiative (USGI). However, national-scale subsurface mapping projects in Africa remain elusive.
The African Magnetic Mapping Project (AMMP) is one example which includes 1:2,000,000 and 1:5,000,000 shaded relief total field magnetic maps, as well as country digital grids with 1 km x 1 km resolution draped on topography.
Other commercial subsurface geological mapping projects, in some cases funded by African governments or via agencies such as the World Bank, have contributed to geological mapping across the continent. These subsurface exploration projects help the understanding of Africa’s geology and natural resources but are often at insufficient resolution, sometimes based on satellite-derived data or older geophysical airborne techniques.
Innovative technologies can enable accelerated development of natural resources, especially when the technology in question can be deployed quickly and efficiently and with minimal environmental footprint. This is one of the key advantages of the deployment of the latest version of Full Tensor Gravity Gradiometry when conducting subsurface exploration by plane.
FTG is a way of measuring changes in the density of the subsurface caused by subsurface geology. It is used by natural resource and mineral prospectors to measure and map changes in the density of the subsurface.
Just as seismic geophysics has evolved over recent decades, the development of gravity gradiometry when compared to conventional scalar gravity, can be likened to the jump in resolution from 2D to 3D seismic, and the latest advanced Enhanced Full Tensor Gravity Gradiometry (eFTG) can be further compared to the development of ultra high-resolution 3D seismic.
This information is used to construct a picture of subsurface faults, structures and lineaments, which can then be used to accurately identify a variety of oil, gas, groundwater and mineral targets.
Because FTG and other geophysical instruments such as magnetometers and LiDAR can be deployed at the same time in an aircraft, large areas can be surveyed rapidly with different and complementary techniques.
Subsurface exploration by plane allows us to cover large areas very rapidly. For example, in late August 2024, the airborne acquisition of eFTG, gravity, magnetic, and LiDAR data began. Covering over ~28,000 square kilometres of the Kwanza Basin, onshore Angola, 38,000+ line kilometres of data will be flown by the end of September. Coverage includes 23 onshore exploration blocks with limited legacy geophysical data coverage.
This survey was conducted in conjunction with the Agencia Nacional de Petroleo, Gas e Biocombustiveis, (ANPG) to provide data on the geological structures, sedimentary basins and salt tectonics in the Kwanza Basin to enable the participating operators to fast-track their exploration effort and to provide a basin-wide dataset to launch new international onshore bid rounds for the region.
The eFTG datasets will allow detailed information to be gleaned in terms of the structural composition, subsurface trends and lineaments and impact of the broader regional geology over the survey area, as well as highly-detailed information achieved by infill over individually licensed blocks. The magnetic data acquired at the same time is crucial to understanding the distribution and variation of volcanic rocks. Of particular interest is the identification of sedimentary basins and basement structure and specifically, the new eFTG data prepares the groundwork for challenging seismic imaging of the sub-salt structure.
These larger-scale ‘National’ surveys conducted using subsurface exploration by plane allow governments and agencies to have a country-wide geophysical dataset that can be used for multiple purposes. These purposes include mineral, hydrocarbon and renewables exploration, as well as providing data that can be used for environmental and infrastructure planning purposes. The data also allows unification of well data and numerous geophysical data sets such as 2D and 3D seismic data, which is usually present over quite limited areas.
In many African countries where data coverage is sparse, such as Angola where only 10 percent of the country’s land has been geophysical and geochemically surveyed, these larger airborne surveys can be an extremely cost-effective way of ‘kick starting’ or rejuvenating much needed resource exploration.
It is quite common, especially in oil and gas exploration, for companies to abandon subsurface exploration in a sedimentary basin after just a couple of unsuccessful wells and temporarily, at least, assign that area to ‘barren status’. Early exploration in Namibia provides such an example, with that country now a thriving production and exploration hotspot.
In such cases where there is a lack of early success, host Governments often require new data to encourage entrants with a fresh approach to the area. Or if a new licensing round is initiated, an up to date, unifying geophysical dataset can be the catalyst to encourage new participants and fresh thinking to a basin with great potential but limited sub-surface information.
Image showing eFTG dataset and 2D seismic line. Taken from 2023 Namibia survey.
The hot colours (red and white) show structural highs, cool colours (blue and purple) show structural lows. Note the insets showing three stratigraphic wells drilled to date, and the strong correlation of the 2D seismic line to the eFTG data. Image courtesy of ReconAfrica.
For more information on subsurface exploration by plane and the advantages it can bring, please contact the team at Earthline Consulting today.
