Velocity

2013

NovAtel's Annual Journal of GNSS Technology Solutions and Innovation

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Aces/geophones To set up a traditional geophone survey, crews would go through the survey area, flagging each spot that needed a geophone. Then crews laying out the sensors would place them at the flagged locations and adjust their orientation using a compass or similar tool. Such measurements are neither especially easy nor precise, he adds. Moreover, once the manual measurement was done, surveyors could not recheck the location and orientation of the sensors without returning to the geophone locations. Stand-Alone geophone To simplify the survey process NovAtel and Sercel began working together to develop a geophone that would use a built-in GNSS receiver/ dual-antenna combination to determine its own location and orientation. "Back in 2006 we had a customer who approached us," says Ryley. "Sercel had developed the DSU3, a new revolutionary 3 component (3C) digital geophone design that could capture all components.  They could actually grab all the seismic waves with one geophone. The old way you'd have different types of geophones or adjust the orientation to capture the waves from different directions. But in order to use the data from the 3C geophones, you need to know the exact orientation of the geophone or you have to accurately align the geophone to a reference direction such as magnetic North." Initially the firm was only interested in using GPS to derive accurate timing once the geophones were deployed. "They would either use a thumper truck or an explosive device to generate the seismic waves," said Ryley. "They had to have accurate timing so that, when they post-processed the data, they would know precisely—based on the exact time the seismic waves were generated and the time of the received reflections—what the structure of the ground looked like below the geophones." The customer brought the problem of needing precise position, time, and orientation to NovAtel— and NovAtel was able to create a unique solution. "Timing is easy—anybody can do that," Ryley says. "We came up with a solution that says For more Solutions visit http:/ /www.novatel.com 'What if you can actually identify the position and orientation of the geophones?'" Attitude Adjustment Working with Sercel, NovAtel developed a dual GNSS receiver/antenna that fit on Sercel's existing geophone platform. The GNSS card used in the geophones is a custom  design from NovAtel that incorporates a 14-channel GNSS MINOS6Lite ASIC that can track GPS, GLONASS, and satellite-based augmentation signals. The MINOS6Lite is a reduced lighter subset of the MINOS6 ASIC used on the NovAtel OEM6 cards. Using two antennas enabled the sensor to determine its own location and, using interferometric techniques, orientation. With that information, and the benefit of post-processing, the new geophone design—dubbed DSUGPS for digital sensor unit with GPS—eliminated the need for a separate survey of the instrument spreads. "Instead of having people to measure exactly the location of each point," says Boucard, "with DSUGPS the measurement is made by the sensor itself." The dual antenna also makes it possible to determine the geophone's orientation using interferometric techniques, says Ryley. "We can then tell from the angle of each of the geophones which direction they are pointing." Two antennas enable the DSUGPS units to obtain an azimuth accurate to within three degrees, which was good enough for the customer. With the exact position and the exact orientation of the geophones, the firm's post-processing program could create a 3D map of the area beneath the test area to a very high accuracy. The new technology not only cuts labor costs and setup time, it increases the chances that a survey campaign will be successful. Poor orientation "has been the cause of failure on other systems," says Boucard. "Usually people have been using a compass or something like that [to determine the orientation of the geophones]. Firstly, the accuracy is not very good. Secondly, sometimes between the time you come and measure [a geophone location] and the time the wave is recorded, the sensor have moved a little bit—and you don't know what was going on in between. Having the " The new technology not only cuts labor costs and setup time, it increases the chances that a survey campaign will be successful. " 2013 velocity 13

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