Velocity

2013

NovAtel's Annual Journal of GNSS Technology Solutions and Innovation

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ACES/geophones A customized dual-antenna design enables the NovAtel GNSS receiver in the geophones to not only provide position and time but also orientation — a crucial element for successful seismic surveys. Image credit: Sercel photo. Building a Better geophone S orting out reflected signals was also a challenge for Sercel SA, a French manufacturer of geophysical instruments, as they worked on the next generation of their geophones— seismic sensors that map underground formations by sensing reflected sound waves. As used in geophysical exploration for natural resources, these waves—created by "thumper trucks" or underground explosions—bounce off of the various types of subterranean materials and structures, enabling researchers to generate a three-dimensional map that can span geologic formations miles wide. Geophones are remarkable for their coverage. A line of the devices—that is, a set of sensors each connected to the next by a single cable— can stretch for five miles, says Daniel Boucard, Sercel's Product Development Manager. Such a line can have as many as 1,000 geophones connected together with an in-line amplifier added every 20 to 24 sensors to boost the signal. Multiple lines of sensors, spaced between 100 and 3,000 metres apart, are placed along side each other so that, when complete, a spread of sensors can cover an area that may stretch five miles long and two miles wide. 12 velocity 2013 Once the geophones have recorded a set of readings, lines on one end are taken up and reconnected to the other end so that the sensor net moves in a rolling fashion across the area being surveyed—typically to map out potential oil and gas deposits. The real challenge comes in setting up the sensor "spread" or network. Crews can take a week to 10 days with 100 people working on laying out the lines, says Boucard. Moreover, the geophones have to be placed precisely—so precisely that most spreads require a surveyor to establish the precise location for each geophone. The labor expense is significant, especially in high-cost parts of the world like Europe and North America. But work doesn't end there. Each geophone must be aligned before the test, in most cases oriented toward magnetic north. The standard geophone still being used by most people is a single-axis device, according to Gordon Ryley, NovAtel Product Manager, measuring the seismic waves that reach the sensors from different directions. "Basically it is capturing the Z component," he says. "You have to have a geophone oriented in the right direction to catch the different reflections of the waves." For more Solutions visit http:/ /www.novatel.com

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