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Deep magnetotelluric soundings in the East African Rift of Ethiopia reveal geothermal potential
By Graham Chandler
A major geological event that happened in continental Africa somewhere between 15 and 20 million years ago today pleases and fascinates at least two professions: paleoanthropologists and geologists. The African plate and the Somali sub-plate began to spread apart and form a massive rift that stretches some 3,000 kilometres running from the Red Sea in the north clear to the Zambezi River in the south. Known as the East African Rift System, most of it is graben-like. Over the millennia, volcanic and erosion activity have left it with a lot of volcanic rocks along with sedimentary succession. It's what the process has exposed that interests these professionals.
For paleoanthropologists, it's a delight. The rift has uncovered sites like Olduvai Gorge, Laetoli and Hadar which have revealed early man finds that go back millions of years. Laetoli has human footprints that have shown humans were walking upright as long as 3.5 million years ago. And the well-known 3 million year old "Lucy" skeleton was found at Hadar in the Afar region of northeastern Ethiopia.
It is that region of the East African Rift System that interests geothermal specialist Mohammednur Desissa: the Main Ethiopian Rift, or MER, which covers about 150,000 square kilometres or nearly 15% of his country. The geophysicist from the Geological Survey of Ethiopia is concentrating in particular on the Dabaho rift segment of the MER for his MSc thesis at the University of Edinburgh.
Desissa explains. "It's situated very close to the Afar triple junction within the Afar Regional State, 600 kilometres northeast of Addis Ababa," he says. "The Afar region is an area of active extensional tectonics and volcanism where the Gulf of Aden, the Red Sea and the Ethiopian Rift Systems join." Of these three, he's investigating in the Red Sea arm, which is the southern part of this volcanic segment, around the Tendaho graben; around Lake Afar near the main highway to Djibouti. Desissa explains that each of the three rift arms have different rates of extension and that, coupled with weakening of the lithosphere from the Palaeogene mantle plume beneath Afar may have contributed to making the location of the triple junction unstable.
The instability Desissa speaks of holds great promise for exploration and development of geothermal energy for Ethiopia. While geothermal is not a new field for the country, progress has been slow. The potential has attracted the attention of geologists for a century or so, but serious geothermal exploration has been undertaken only over the past 40 years.
These geological, geochemical and infrared studies have revealed several active hot springs and fumaroles of around 177 degrees C. Desissa says strong thermal indications have been evident from gravity, magnetics and DC resistivity measurements too. Sixteen potential sites have been identified, that could provide 1,000 MW of power.
There's even one in production. "Aluto-Langano is the only locality where electric power is generated using geothermal energy in Ethiopia within the MER," notes Desissa. "It generates about 9MWe and is connected to the existing hydro power grid."
But he thinks there's so much more potential, especially the MER's southern sector around Lake Abaya whose fault centres are expected to be active heat sources. "MER is where the promising geothermal areas are situated," he says. "It's low lying and one of the most accessible areas of the country. The shallow geothermal source has been mapped at depths around 500 metres, while the deepest are below 1500 metres as proved by the magnetotelluric (MT) method and drilling." And the high measured temperatures add to the exciting potential: 250 to 300º C at 500 metres, he says.
It's Desissa's pursuit of the MT method that has upped the ante here—it tends to be more revealing in the MER's geology. "The MT method differs from other electromagnetic (EM) geophysical techniques that require artificial sources of EM energy to probe the earth," he says, and is therefore well adapted to deeper crustal investigations. Around the southern part of the MER in the Lake Abaya prospect area where several faults and volcanic activity are prevalent, a DC resistivity survey had previously been conducted but due to local topology and geography, sufficient information could not be obtained. Long electrode separations are impossible due to the rough terrain, high power generation is needed, and depth penetration is limited due to the upper resistive and conductive layers. "These problems are alleviated or at least minimized by going to the MT method," says Desissa.
He says the MT method became particularly useful in the Lake Abaya area and other parts of the MER because it's where highly resistive volcanic rocks overlay the conductive sedimentary strata. "The depth penetration of conventional DC resistivity methods and shallow seismic waves are not successful compared to the MT sounding due to the consolidated volcanic rocks." So in his project with the Geological Survey of Ethiopia he undertook to use the method to map and delineate the geo-electrical and geologic structures in the area, particularly the lateral and vertical extent of the active hydrothermal zones. His team set up electrodes 50 metres apart in several sections and recorded MT measurements. Electric and magnetic time series were first transformed into frequencies and the data processed to deduce which of the structures control the major thermal activities.
The data correlated well with volcanic remains and currently observed hot springs and smoking fumaroles in the surrounding areas. Building the geo-electrical models for 500 metres and 2000 metre depths, some tantalizing finds were revealed: geological conduits that could flow heat to the surface in more than sufficient amounts for geothermal energy sourcing. Particularly exciting was indication of a hot magma chamber at a shallow depth - less than two kilometres. The project's next step is to carry out further testing: closely spaced long period MT measurements that would further confirm the structure.Meanwhile, the rift keeps spreading; but at a snail's pace. Geologists have speculated that the East African Rift system will eventually become the boundary of a new detached lithospheric plate and move northeasterly, forming a new ocean basin. That won't happen any time soon - by the time it does, paleoanthropologists and geologists will have entirely new species and structures to study.