Robot Farts: Debunking the Myth of 'Seismic Blasting'

Seismic blasting? Explosion bombs? Or something far more mundane? I explain how the injection of compressed air into the ocean from remotely-controlled devices to produce acoustic pressure waves is a relatively benign process. Farts from several small mechanical robots. Seismic exploration can in fact be directly compared to ultrasound investigations of the human body: incredibly sensitive instruments record weak vibrations reflected from within the target medium and are used to produce remarkably informative images.

The Perils of Misplaced Language

A prominent Australian mining executive recently referred to the compressed air released during marine seismic surveys as "explosion bombs". Although his statement was gamesmanship being used to attack an industry rival, the comment nevertheless had some impact in the mainstream media.

In a typical 'marine seismic survey' a small ship tows a vast amount of sensitive listening devices close to the surface of the ocean. In the photo below, each 'seismic streamer' is a polyurethane cable about 8 kilometers long with passive listening devices ('hydrophones', 'velocity sensors', 'accelerometers'; all miniaturized forms of underwater microphone) every 3 to 12 meters. Such equipment represents thousands of 'marine seismometers' capable of detecting infinitesimally weak vibrations emanating from many kilometers below the ocean floor, way below the hearing threshold of any living creature.

Seismic vessel operations. In the upper photo, each white turbulence corresponds to a surface float being towed through the water. The cartoon in the lower figure illustrates that below the surface, many  seismic streamers, each several kilometers in length, contain sensitive listening devices used to detect weak vibrations reflected from the geological strata several kilometers below the ocean floor. The six white objects towed closer to the vessel in the upper photo are the floats above each array of pneumatic acoustic sources. Long air hoses connect these devices to the vessel. The entire array equipment is towed at a brisk walking pace of about 8 kilometers per hour. Image courtesy of PGS.

What is all this equipment listening to? According to our mining friend, some form of 'explosion' is involved. Let's look closer. The reality is that the precisely controlled release of compressed air from several devices is used to generate sound waves that illuminate geology below the surface. In a manner analogous to how an ultrasound technician can provide remarkable images of human organs, blood vessels, and even blood flowing through our veins and arteries, geophysicists use these controlled sound waves to form 'seismic images' extending many kilometers below the ocean floor. Experienced geologists can identify where strategic hydrocarbon or mineral deposits might be located, just as an experienced physician can make remarkable diagnoses without having to pursue any surgical intervention.

Explosions, Bombs, or Simply Lots of Bubbles?

Are the devices used to inject compressed air into the ocean a few meters below the survey really a form of 'explosion' or 'bomb'? Such descriptors are quite dramatic but the mundane reality is that the devices, originally known in the 1960s as 'pneumatic acoustic repeaters', are more accurately described as 'robot farts'.

Machinery within a small ship send compressed air down a long hose known as an 'umbilical' to a series of canisters suspended below a floatation device at the surface. A valve within each canister is remotely opened at intervals of several seconds and the air inside is released through a small opening (or 'port') into the surrounding water. This release of air is swift, 99% of the energy is typically released into the water in only about 30 milliseconds (0.03 seconds), and analogous to the sound of someone farting in their bathtub, the collective sound waves from an 'array' of canisters are detected by extraordinarily sensitive listening devices towed behind the same ship that tows the acoustic devices.

Two underwater photos taken in rapid succession that capture the compressed air released from several 'acoustic devices suspended about 5 meters below a surface float. In the upper photo, the initial bubble expands to about 0.5 meters in diameter and then collapses due to the hydrostatic pressure of the surrounding water. As the air rapidly forms clouds of much smaller bubbles and rises to the surface due to buoyancy forces, the vast array of listening devices towed behind these acoustic pressure 'sources' record extremely weak vibrations reflected from the geology deep below the ocean floor--analogous to how ultrasound records vibrations reflected from within the human body.

The video below shows this process in operation. As a small boat skims across the ocean surface towards the air bubbles emerging at the surface, a camera operator places a waterproof handheld video camera below the surface so that the equipment is clearly visible.

Video recorded by placing a video camera below the surface where pneumatic acoustic sources are being operated. Compressed air is released into the surrounding water and a large cloud of bubbles rapidly rises to the surface. The sound emitted by this action is reflected by geological layers below the ocean floor and detectable by thousands of extremely sensitive listening devices towed further behind the vessel.

Note that the camera operator is only about 10 meters from the action.

As any high school physics student will recall, the buoyancy of an underwater object is equal to the (mass of displaced water) minus (the mass of the object). If the mass of the object is larger than water per unit volume (about 1 kilogram per liter), the object will be 'negatively buoyant' and will sink. If the mass of the object is less than water it will be 'positively buoyant' and a buoyancy force will act upwards upon the object forcing it to rise to the surface.

The mass of air within a bubble is negligible, so it follows that air bubbles have greater buoyancy than anything else, hence the quite 'energetic' manner in which the compressed air being released rises to the surface in both the photos and the video.

Whilst the frothy surface of the ocean may appear to fizz quite energetically, there are no 'explosions', 'plumes' of water, or anything gravely hazardous to living creatures in close proximity. I draw your attention to the movement of the underwater camera during each 'seismic source activation' (the triggering of the signal that simultaneously releases the compressed air from each air gun). A slight camera shudder is visible but no one gets hurt.

Making Sense of Underwater Sounds

As the physical properties of water are relatively uniform throughout the oceans from the perspective of how sounds travel, the decay of the acoustic energy from seismic surveys follow the laws of physics in highly predictable ways. In locations where the (temporary) disturbance from seismic surveys need to be managed within specified 'sound level versus distance' thresholds, there are established methods and practices followed and accompanied by strict monitoring and reporting obligations.

The images below compare similarities in acoustic imaging of the human body and the geological strata in the outer crust of the earth. Scale is the most important difference: ultrasound works on scales of several centimeters and seismic imaging works on scales of several kilometers.

Both ultrasound (upper) and seismic (lower) use similar principles. Ultrasound provides images within the human body at scales of several centimeters. This example shows a cross-section of a kidney. Seismic provides images of geological strata at scales of several kilometers. In this example, the uppermost white line represents the ocean floor, above which the block of grey represents water. Each deeper line represents geological layers with strong acoustic contrasts--typically caused by a fundamentally different style of geological deposition. The oldest geological layers are observable at the lowest part of this image and have been successively overlain by younger sediments and deformed by forces such as those related to collisions between tectonic plates. The age of each geological layer is determined by carbon dating of minute organic lifeforms and plant matter trapped within the rock fabric as it formed from a slurry of mud and organic matter into lithified rocks as we recognize them today.

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