Release Date: June 2003
SONARDYNE TRANSPONDERS ASSIST CUTTING EDGE ELECTROMAGNETIC
SURVEY TECHNOLOGY
EMGS (ElectroMagnetic GeoServices) of Trondheim, Norway has purchased over 50 �PGT� acoustic transponders from Sonardyne International Ltd to play a key role in the application of its cutting edge survey technology.
EMGS is a newly established company specialising in the use of SeaBed Logging (SBL), an advanced technique for detecting hydrocarbon layers in deep-water offshore areas. Whereas seismic surveying uses sound waves to define structures and locate potential reservoirs, SBL complements the technology by employing electromagnetic waves to establish whether the reservoirs contain hydrocarbons or water. EMGS consequently believes that its technology can dramatically increase the productivity of exploration projects with corresponding cost benefits.
The patented SBL remote sensing technology uses a mobile, horizontal electric dipole source and an array of seafloor electric field receivers that are each connected to a Sonardyne transponder. The dipole source transmits a low frequency electromagnetic signal (0.05 to 10 Hz) into the overlying water column and downwards into the underlying sediments. Electromagnetic (EM) energy is rapidly attenuated in the conductive seafloor sediments but in more resistant layers, such as hydrocarbon filled reservoirs, attenuation is lower and the EM energy is efficiently propagated along the layer. The energy constantly leaks from the layer back to the seafloor, and when the source receiver offset is comparable to, or greater than, the depth of burial the energy from the layer will dominate over directly transmitted energy. The detection of this guided energy is the basis of SBL.
The Sonardyne transponders were delivered by Aquadyne AS in Asker, Norway after
agreement between EMGS and their advisers, Multiwave Geophysical Company in
Bergen. Aquadyne AS is partly owned by Sonardyne International and is
responsible for Sonardyne�s business in Norway.
The PGT transponders were chosen because of their industry-leading power output of 212dB @1m ref 1uPa, their incorporated acoustic release mechanism and data telemetry capability. Two transponders mounted on the head and tail of the EM source antenna and towed from the vessel some 50 meters off the seabed, are used to accurately position the source. The transponders are incorporated into the package and are linked via RS232 to the receiver electronics.
As each receiver package is deployed, a Sonardyne Ultra-Short BaseLine (USBL) positioning system is used to track its decent and position it accurately on the seabed in up to 4,000 metres of water. During interrogation from the USBL system, status information from the receiver is passed to the transponder for imbedding in the reply signals sent to the surface. This enables the health status of each receiver to be remotely monitored from the surface for the duration of the EM survey.
For recovery, a uniquely coded signal is sent to the selected transponder to activate the release of the anchor weight enabling the receiver to float to the surface. To ensure efficient operations during ascent, the transponder and receiver package are tracked to the surface via the USBL system. This also enables the dynamic positioning of the vessel to manoeuvre directly adjacent to the receiver once on the surface. Each transponder is also equipped with an acoustically commanded electrical output to activate a back-up �burnt-wire� release mechanism for redundancy.
In November 2000, the first SBL survey was run over known oil fields offshore West Africa with funding from Statoil and support from the Scripps Institution of Oceanography and Southampton Oceanographic Centre. The technology successfully demonstrated the benefits available and, the following year, resulted in the first commercial SBL survey being conducted in the Norwegian Sea for Statoil and Shell Enterprise.
EMGS
was established in February 2002 by Svein Ellingsrud, Vice President Technology
and Manufacturing and Terje Eidesmo, President. They began developing the
technology five years earlier whilst working for Statoil which had been studying
the use of EM waves in down-hole tools for investigating reservoir properties
and water front movement. The commercial availability of a strong
electromagnetic source inspired the idea of a possible direct hydrocarbon
indicator. The Norwegian Geotechnical Institute also participated in discussions
which ultimately identified the possibility of "logging from the
seabed" by scaling-up the distance, wavelength and power outputs. As well
as their in-house technical experts, EMGS have several partners including
Multiwave Geophysical who have responsibility for data acquisition.
The successful application of SBL technology now makes it possible to map
subsurface resistivity remotely from the seafloor. Because a hydrocarbon filled
reservoir will typically have a higher resistivity than shale or water-filled
reservoirs the EMGS technique has the unique ability of distinguishing between a
hydrocarbon filled and a water-filled reservoir.
Sonardyne PGT transponders are manufactured at the Company�s headquarters in Yateley, England. Sonardyne also has offices in Aberdeen, Houston, Singapore and Brazil. The Company is recognised as a world leader in the design and manufacture of advanced acoustic instruments and systems used for accurate survey, positioning and tracking in shallow and full ocean depth operations.
-ENDS-
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