Posted by Iga Pawelec

Nodal land seismic acquisition systems are increasingly popular for acquiring active seismic data. Such systems offer many advantages, such as reduced system weight, smaller crew sizes, increased operational safety, and more freedom in survey geometry design. To bring this state-of-the-art acquisition technology to new generations of geophysicists, Colorado School of Mines partners with node manufacturers and source providers as a part of a geophysical field camp – considered by many to be the signature experience in our undergraduate curriculum. For two weeks, students can learn how to use the geophysical equipment and, more importantly, how to design an acquisition to answer specific geophysical questions.

With the heavy emphasis on student learning, our seismic field campaigns often considerably differ from standard industry practices. In 2022, we deployed two 1C nodal systems and a few 3C units along Colorado state highway 131 near Steamboat Springs. Given the scarcity of geophysical data on the deeper structure in this area, our goal was reconnaissance: nodes were laid out along a 6km stretch of the highway with variable spacing (5-10m on one system, 50m on another) for about a week. The primary seismic source was the mini-vibe shaking a low-dwell sweep in a 2-102Hz band every 10m. Although data quality was in question from the very beginning due to the proximity of heavy traffic and the source only being able to operate with limited power, the student effort paid off. After offset binning and stacking to create a super gather, surface waves are visible with stunning clarity. Thanks to the availability of longer offsets, the computed phase velocity spectrum is very high resolution. The fundamental mode of the dispersion curve can be picked down to 2.25Hz, very close to the source starting frequency. This allows for 1D inversion for the shear wave velocity profile, with a penetration depth well beyond 100m. Due to a strong reflector in the shallow subsurface, surface waves may offer an attractive imaging alternative to the conventional reflection approach. With the proper processing, the continuous recordings available from this site can potentially unravel even greater depths. Stay tuned to hear about updates on that front!

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