New marine vibrator offers improved performance

Bill Pramik

October 11, 2013

Houston-based Geokinetics showed its latest source advance in their booth at the SEG Convention this September. Bill Pramik, Geokinetics’ vice president explains the technology.

More and more, the oil and gas industry is being asked to reduce its impact on the environments in which it works. In one sense, this is ironic, because the industry recognized a long time ago that reducing its environmental footprint has a positive effect on profitability. In simple terms, if the environment is minimally impacted, there is no need to spend time, resources, and money correcting problems. Nonetheless, the industry also recognizes that it has a responsibility, as good corporate citizens, to perform work in a manner consistent with environmental best-practices and guidelines.

Geokinetics has been proactive when it comes to protecting the environment. In many areas, the company’s internal policies are more stringent than prevailing regulations and it constantly monitors performance looking for improvement opportunities. One such opportunity being developed by the Technology Research and Development team in Houston is the Geokinetics Marine Vibrator. It is a major step forward to the goal of being good environmental stewards.

Seismic acquisition

The marine source is set in a frame for installation on a vessel.So exactly what is a marine vibrator? As the name implies, it is a seismic vibration source that’s used in the water. In much the same way that Vibroseis trucks are used to acquire land seismic data, marine vibrators can be used to collect seismic data in the marine environment. That, of course, raises the question: Why would we want to do this?

Image: The marine source is set in a frame for installation on a vessel. 

There can be little doubt that humans have an impact on the environment. These impacts can be divided into three basic categories: good, bad, or inconsequential. Which category an impact falls into depends on the nature of the impact and somewhat from the perspective it is examined. In the seismic industry, “inconsequential” environmental impact is the goal. There is some environmental impact from operations; the goal is for that impact to have no consequences or, at least, the smallest consequences practical.

Offshore, where seismic companies acquire ocean bottom and transition zone seismic data, the source of choice for over 50 years has been the airgun. Before that, dynamite and other explosive charges were detonated in the water as the energy source. The airgun was hailed as a safer and “friendlier” seismic source and was quickly adopted by the industry. Now, 50 years later, airguns are beginning to lose their status as an environmentallyfriendly source.

The relevant literature is full of statements regarding the harm that seismic airguns might do to the environment. Many of these statements are based on incomplete, incorrect, or anecdotal reports about environmental damages caused by airguns. The truth is that there are very few scientific studies about the impact of airguns on the environment, and those that exist generally focus on one or two specific species (e.g. cetaceans), rather than the environment as a whole. Nonetheless, it is prudent that we begin looking at alternate seismic energy sources for the marine environment.

A marine vibrator will probably never replace airguns as the energy source for marine seismic acquisition, just as land vibrators have not replaced explosives for land acquisition. However, there are a number of situations where a marine vibrator can be advantageous. Some of these advantages relate to environmental concerns, others involve seismic data quality and acquisition efficiency.

Differences

One of the major differences between airguns and vibrators is the sound pressure level that is transmitted into the water. It is like comparing a shotgun blast to the hum of a room air conditioner. The shotgun, like the airgun, releases all of its acoustic energy in a single pulse with a very high sound-pressure level, but with a very short duration. In contrast, a room air conditioner, like a marine vibrator, has a very low sound-pressure level, but it runs for a relatively long period of time.

In seismic exploration, what is important is the total amount of energy output by the source. For example, if the airgun has a sound pressure level of 100 and lasts for 0.1 seconds, the total acoustic energy would be 10. If a marine vibrator has a sound pressure level of 1 and lasts for 10 seconds, the total acoustic energy would also be 10. For seismic data acquisition, these two sources provide the same energy for seismic imaging, but the vibrator is 100 times quieter. Intuitively, this has a much smaller environmental impact.

Geokinetics’ Marine Vibrator

The Marine Vibrator project dates back 20 years, to 1994, when the initial concept was proposed. A set of prototype vibrators was built and tested in 1999, to provide a “proof-of-concept.” From a geophysical perspective, these tests were very successful, collecting excellent quality seismic data. However, the prototype vibrators were finicky and broke down often. There was a hiatus in vibrator development until 2007, when a renewed interest came from recognizing their potential benefits for shallow-water operations.

An improved driver for the vibrators was developed and another set of vibrators was built. These new vibrators have been undergoing testing and continued development since that time.

Along with drivers, many other vibrator components and functions have been improved. Changes to the internal springs’ configuration have increased energy output. Modifications to cooling and pressure compensation systems have improved reliability. The addition of a very sophisticated, feedback- control system ensures that the vibrator’s output is the preferred signal. These and other refinements ensure that the vibrator will be a robust and efficient seismic acquisition tool.

Design

Three marine vibrators can be combined into a multi-source array to cover a broad spectrum of seismic frequencies.So, how does the vibrator actually work, and why is it better than previous marine vibrator designs? The vibrator uses a “flextensional transducer.” This vibrator type has distinct advantages over previous marine vibrator designs, which mimic traditional, land vibrators, using large plates or diaphragms to induce pressure waves into the water.

Image: Three marine vibrators can be combined into a multi-source array to cover a broad spectrum of seismic frequencies. 

This idea works well for higher frequencies, but fails at lower frequencies. The reason this approach fails is because a vibrating plate or diaphragm is only good at generating frequencies proportional to the diameter of the plate. As frequencies go lower, the wavelength getslonger, and the ability of a vibratingplate to generate pressure waves at that lower frequency decreases.

A solution is to effectively change the vibrating plate into a vibrating balloon. Now, instead of moving the plate back and forth to make the acoustic wave, we inflate and deflate the balloon very quickly. As the balloon changes volume, the water near it cannot flow around it because the balloon is expanding in all directions. A vibrator that changes volume will improve low frequency acoustic energy generation. This is one of the principles of the new vibrator’s flextensional design. Because of this design, the new vibrators are much more efficient at generating lower frequencies for seismic data.

The vibrator’s drivers have also been improved. Previous marine vibrator designs have used either conventional hydraulics (like land vibrators) or magnetic voice coils (like home stereo speakers). Hydraulic systems are limited, because the servo-valves that control motion have reaction-time limitations. Magnetic coils don’t suffer these same limitations, but are inherently weak and don’t generate much force.

Because of these limitations, Geokinetics used something radically different: Terfenol-D. Terfenol-D is a highly magnetostrictive metal. When a piece of Terfenol is placed in a strong magnetic field, it changes its shape, or size, or both. And it does this with a lot of force. Terfenol-D is an ideal way to power the marine vibrator with a strong, reliable, driving force that will stand up to the rigors of round-theclock seismic acquisition.

The new marine vibrator is a broadband, marine seismic source that can generate acoustic energy for seismic exploration from 5Hz to over 150Hz. Because of its configuration, it can efficiently operate in water depths as shallow as 1m without suffering the energy losses that airguns experience in shallow water. The instantaneous sound pressure levels generated by the vibrator are a small fraction of those generated by airguns and, because of this, the vibrator is a more “environmentally sensitive” acoustic source. Marine vibrators will not replace airguns for marine seismic acquisition, but they were never intended to. They are another tool for the seismic industry as it continues to adapt to the changing environmental standards. OE

Bill PramicBill Pramik is the vice president of Acquisition Technology at Geokinetics, Inc. where he oversees research and development of new seismic acquisition hardware and methods including recording technology, seismic sources, and seismic receivers. Pramik was employed by PGS for 15 years, and served 3 years as their Vice President of Geophysics and Quality before moving to Geokinetics 2010. Prior to that, he worked for Amoco Production Company for 16 years, including a 6 year assignment to their Research Center. Pramik received his Geophysics degree with a minor in Mathematics from Virginia Polytechnic Institute, where he was awarded a 2-yr Amoco Foundation Scholarship.