Total pioneers ‘cable-less 3D seismic’ surveys in Uganda
By Jean-Michel Enjolras
What is a ‘seismic survey?’ Total E & P Uganda’s Director of Geosciences explains all—and says his company is doing all it can to minimise the environmental impacts.
Hi-tech ‘cable-less, three dimensional seismic’ surveying is being used for the very first time in onshore Africa in order to develop an accurate, underground picture of the Albertine Graben oilfields. This technique, deployed by Total E&P Uganda, has the added advantage of lower environmental impact on environmentally sensitive areas.
Seismic surveying is a commonly used technology in petroleum exploration and production. It is based on the same principle as the ultrasound scanners used in medical clinics: sending out sound waves and developing a picture from the waves that bounce back.
In the oil industry, sound waves are used to define and study the shape and characteristics of rock formations below the surface of the earth. The sound waves are emitted by vibrator trucks or explosives. A series of geophone receivers and a recording truck then register the returning waves and measure the travel time between the source and the receiver.
At the exploration stage, the most commonly used technology is the ‘2D’ seismic. This involves laying telemetry cables, over land that has first been cleared of vegetation, along parallel lines separated by distances of between one and five kilometres. This gives a broad picture of the subsurface and helps identify the most likely places to find trapped hydrocarbons.
‘3D’ surveys are mostly used for more detailed appraisal once hydrocarbons have been discovered in an area. In this process, the data set is acquired along a grid, and so provides a three dimensional picture of the subsurface. This more accurate image of the size and characteristics of the discovered oil field enables optimisation of development plans—for example the number, location and trajectories of the future production wells.
For its 3D survey in the Murchison Falls National Park, Total chose very recent and specific equipment in order to reduce the impact on an ecosystem that is both highly sensitive and also very important for tourism.
On the recording level, for instance, the 3D seismic is acquired with the latest, cable-less technologies available in the industry—the Fairfield ZLand system.
The field recording units, or ‘nodes’, consist of cylindrical boxes measuring 15 x 12.7 centimetres and weighing 2.2 kilos. They have an internal power supply (Li-Ion battery), internal geophone receiver, GPS positioning and inbuilt memory.
These nodes are positioned at 25 metre intervals along lines 100 metres apart. They are placed below the ground level and covered with a few centimetres of grass or soil. They are easy to put in place, allowing a minimum cutting of the vegetation and a minimum disturbance to the wildlife. After a few days of recording, the nodes are picked up and the data are downloaded in the laboratory at the base camp.
The energy sources are deployed on lines perpendicular to the receiver lines, and are of two different types. Mini vibrators (see picture) send a signal through a vibrating steel plate pressed on the ground. In areas where the vibrators cannot access, light explosives charges, are buried six metres deep in a slim hole.
This cable-less seismic system is a high performance, low impact surveying method ideally suited to work in sensitive environments. It produces quick results with less manpower and fewer supporting vehicles. Optimum coverage and quality is achieved without deploying kilometeres of cables. Thus it has a much more limited impact on wildlife and vegetation compared to other techniques.
This shows the commitment of Total and its partners to always look for optimal technical solutions, and take every mitigation measures to minimize the environmental and social impacts of oil activities.
In line with its biodiversity commitment, Total E & P Uganda has also started what will be a growing series of biological surveys in collaboration with Ugandan Wildlife Authority (UWA). We have employed a biodiversity team to lead this work, monitoring animal and plant populations, defining avoidance maps ahead of activities, and compliance procedures to be followed during all operations.
We are investing in research, planning and equipment across all our departments to avoid the impact of activities, and are engaging national and international expertise to ensure the best possible standards of implementation, as well as complying with national regulatory requirements and internationally recognized best practices in all our operations.
Jean-Michel Enjolras is Director of Geosciences for Total Exploration & Production Uganda