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Geology & Geophysics

In seismic exploration, what is transducer.” This vibrator type has weak and don’t generate much force. important is the total amount of energy distinct advantages over previous Because of these limitations, output by the source. For example, if marine vibrator designs, which mimic Geokinetics used something radically the airgun has a sound pressure level traditional, land vibrators, using large different: Terfenol-D. Terfenol-D is a of 100 and lasts for 0.1 seconds, the plates or diaphragms to induce pres- highly magnetostrictive metal. When a total acoustic energy would be 10. If a sure waves into the water. This idea piece of Terfenol is placed in a strong marine vibrator has a sound pressure works well for higher frequencies, but magnetic feld, it changes its shape, or level of 1 and lasts for 10 seconds, the fails at lower frequencies. The reason size, or both. And it does this with a total acoustic energy would also be this approach fails is because a vibrat- lot of force. Terfenol-D is an ideal way 10. For seismic data acquisition, these ing plate or diaphragm is only good at to power the marine vibrator with a two sources provide the same energy generating frequencies proportional to strong, reliable, driving force that will for seismic imaging, but the vibrator is the diameter of the plate. As frequen- stand up to the rigors of round-the- 100 times quieter. Intuitively, this has a cies go lower, the wavelength gets clock seismic acquisition.

much smaller environmental impact. longer, and the ability of a vibrating The new marine vibrator is a broad- plate to generate pressure waves at that band, marine seismic source that can lower frequency decreases. generate acoustic energy for seismic

Geokinetics’ Marine Vibrator

The Marine Vibrator project dates back A solution is to effectively change exploration from 5Hz to over 150Hz. 20 years, to 1994 when the initial con- the vibrating plate into a vibrating Because of its confguration, it can cept was proposed. A set of prototype balloon. Now, instead of moving the effciently operate in water depths as vibrators was built and tested in 1999 plate back and forth to make the acous- shallow as 1m without suffering the to provide a “proof-of-concept.” From tic wave, we infate and defate the energy losses that airguns experience a geophysical perspective, these tests balloon very quickly. As the balloon in shallow water. The instantaneous were very successful, collecting excel- changes volume, the water near it can- sound pressure levels generated by the lent quality seismic data. However, the not fow around it because the balloon vibrator are a small fraction of those prototype vibrators were fnicky and is expanding in all directions. A vibra- generated by airguns and, because of broke down often. There was a hiatus tor that changes volume will improve this, the vibrator is a more “environ- in vibrator development until 2007, low frequency acoustic energy genera- mentally sensitive” acoustic source. when a renewed interest came from tion. This is one of the principles of Marine vibrators will not replace recognizing their potential benefts for the new vibrator’s fextensional design. airguns for marine seismic acquisi- shallow-water operations. Because of this design, the new vibra- tion, but they were never intended to.

An improved driver for the vibra- tors are much more effcient at generat- They are another tool for the seismic tors was developed and another set of ing lower frequencies for seismic data. industry as it continues to adapt to the vibrators was built. These new vibra- The vibrator’s drivers have also been changing environmental standards. tors have been undergoing testing and improved. Previous marine vibrator continued development since that designs have used either conventional Bill Pramik is the time. hydraulics (like land vibrators) or vice president of

Along with drivers, many other magnetic voice coils (like home stereo Acquisition vibrator components and functions speakers). Hydraulic systems are lim- Technology at have been improved. Changes to the ited, because the servo-valves that con- Geokinetics, Inc. internal springs’ confguration have trol motion have reaction-time limita- where he oversees increased energy output. Modifcations tions. Magnetic coils don’t suffer these research and to cooling and pressure compensation same limitations, but are inherently development of new systems have improved reliability. The seismic acquisition hardware and

Three marine vibrators can be addition of a very sophisticated, feed- methods including recording technol- combined into a multi-source back-control system ensures that ogy, seismic sources, and seismic array to cover a broad the vibrator’s output is receivers. Pramik was employed by spectrum of seismic the preferred sig- PGS for 15 years, and served 3 years as frequencies.

nal. These and other their Vice President of Geophysics and refnements ensure that Quality before moving to Geokinetics the vibrator will be a 2010. Prior to that, he worked for robust and effcient Amoco Production Company for 16 seismic acquisition years, including a 6 year assignment to tool. their Research Center. Pramik received his Geophysics degree with a minor in

Mathematics from Virginia Polytechnic

Design

So, how does the vibrator actu- Institute, where he was awarded a 2-yr ally work, and why is it better than Amoco Foundation Scholarship.

previous marine vibrator designs?

The vibrator uses a “fextensional oedigital.com October 2013 | OE 35 034_OE1013_G&G2_SEG.indd 35 9/29/13 9:08 PM