Geothermal energy development
Initiating a challenging geophysical field program:
Dr G Morrel Fonseka
Sri Lanka has 10 known hot springs
located on a stretch between Mahapelessa in the South and Mulaitivu on
the island's North East. The temperatures of these springs at the
earth's surface range from 42 to 62C
The temperatures of hot springs are known to be much higher at
greater depths. The reason for the occurrence of hot springs can be due
to normal heat flow
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Seven hot springs in Trincomalee attract many visitors. File
photo |
within the earth or due to the presence of Hot Dry Rock (HDR). A hot
dry rock is created when hot magama from the interior of the earth
intrudes into the earth's outer layer, also known as the crust. If the
event is generally less than 500 million years old, it will still be
anomalously hot due to the decay of radioactive material within.
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MT program |
* Advancement
of knowledge in geothermal energy
* Training of young geologists and geophysicists
* Use of hot water for industrial use
* Clean power generation |
Deep fault zones in Sri Lanka could well be harbouring large
quantities of thermal waters at elevated temperatures. A geophysical
field campaign that began on July 19, 2010 will pave the way to locate
such extensive deep HDR and hot water reservoirs that could be used for
power generation.
Enhanced Geothermal Energy refers to an emerging technology that
works efficiently for energy development, even at a low temperature
differential and will be an appropriate application in Sri Lanka.
One physical property of a rock is its conductivity, i.e. how well it
conducts electricity. Hot rocks and those permeated with water,
generally have higher conductivity than their surroundings. Natural
variations in the earth's magnetic field, arising from worldwide
thunderstorms and currents in the ionosphere and magnetosphere induce
currents in conducting material and therefore in hot and wet rocks.
Induced currents will be stronger where the conductivity is higher.
The ratio of these induced electric currents to the inducing magentic
field is termed the impedance and a simple formula relates impedance to
the subsurface conductivity. Magnetotellurics is a technique in which
these natural variations of the earth's magnetic and resulting electric
fields are measured over some time period. Short period record relate to
near surface conductivities, in the first few tens of metres, whereas
long period measurements can be used to determine earth conductivities
to depths of many hundreds of kilometres.
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Dr G Morrel Fonseka |
MT is thus the preferred method for geothermal exploration. For
economical extraction geothermal sources need to be located within a few
kilometres of the earth's surface. At most hot spring sites MT stations
were conducted over 7 km long traverses with a station separation of
around 1 km and a recording time of several hours. From these we expect
to obtain 10 km deep cross sections of the earth's conductivity for
geothermal studies.
Longer recordings were taken at one point along each traverse for
later academic investigations of the MOHO discontinuity at around 30 km
depth. At the Mahapelessa hot spring the traverse spans 27 km to cross a
controversial section of the Highland Vijayan geologic boundary. The hot
springs have an association with this boundary.
Preliminary processing was done within the field instruments, to
yield the earth's impedance and 1-dimensional conductivity independently
at each MT site. These
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Prof Bruce Alan Hobbs
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| Prof Bruce
Hobbs has been an academic at the University of Edinburgh, UK
for 38 years. He received his Ph.D in mathematics (1968),
specializing in Electromagnetic Induction in the Earth and
Oceans, which is an area of theoretical Geophysics. Since 1975,
he has chaired and co-chaired several international
organizations in the field of geophysics. In 2007/08 he was a
distinguished lecturer with the European Association of Geo
scientists and Engineers (EAGE). He is co-inventor of the
original MTEM patent which enables surface measurements to
discriminate between hydrocarbons and water in a potential
reservoir, thus saving drilling costs. The invention led to a
spinout from the University of Edinburgh. MTEM Ltd was the
largest spinout in Scotland and second largest in the UK. In
2007, his company was acquired by PGS and Bruce became Head of
EM Research and later Chief EM Scientist of the organization.
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may be collated to form preliminary subsurface conductivity
maps. These and the vast amount of professionally acquired digital data
were made available to all the participating institutes and will also be
lodged with the National Science Foundation through which researchers
can obtain access. The cost of acquiring these data if done on a
commercial basis would be over Rs 22 million.
This program brings vast benefits to Sri Lanka, including advancement
of knowledge in the area of geothermal energy development, training of
young geologists and geophysicists in geophysical field procedures and
transfer of new geophysical technology. When data is analyzed later by
rigorous mathematical processing and if prospective geothermal sources
are discovered, there will of course be hope for clean power generation
and the use of hot water for industrial use. The results may also show
deep fractures containing water suitable for irrigation or human
consumption.
The writer is a former University teacher at the Open University,
Nawala and a President of the Geological Society of Sri Lanka. He
initiated the MT program with the help of his colleague Prof Bruce Alan
Hobbs |
