Seismic reflection, NZ’s failure to consider the impact on marine life, more
Greenpeace and the iwi Te Whanau a Apanui argued last week in the High Court at Wellington that the Minister of Energy failed to consider the impact on marine life of seismic surveying in giving the Brazilian oil company Petrobras’ deep sea oil exploration licence. More here
Seismic testing is an exploration technique used by oil and gas companies to explore the ocean for oil and gas sediments. In order to measure these sediments, large ships fire high-intensity air guns deep into the ocean. The sound energy from seismic testing is potentially damaging to many species of marine life, including whales, dolphins and seals.
The blasts from seismic air gun can reach volumes of 260 decibels (anything above 180 decibels is believed to be harmful to marine mammals). The injuries that may be caused by sounds at this level include permanent hearing loss, disorientation, brain hemorrhaging and death.
The ocean is an acoustic environment, not a visual one and marine mammals rely heavily on sound for their survival. Without their heightened sense of hearing, marine mammals cannot find food, avoid predators or communicate with each other.
The loud noises drown everything out, including the animals’ own voices, and could injure or even kill animals too close to the airgun blasts. Some dolphins and whales may even beach themselves to escape the noise.
Reflection seismology is a method of exploration geophysics that uses the principles of seismology to estimate the properties of the Earth’s subsurface from reflected seismic waves. The method requires a controlled seismic source of energy, such as dynamite/Tovex, a specialized air gun or a seismic vibrator, commonly known by the trademark name Vibroseis.
The harder stuff, over the jump…
Seismic waves are mechanical perturbations that travel in the Earth at a speed governed by the acoustic impedance of the medium in which they are traveling. The acoustic (or seismic) impedance, Z, is defined by the equation:
where V is the seismic wave velocity and ρ (Greek rho) is the density of the rock.
When a seismic wave traveling through the Earth encounters an interface between two materials with different acoustic impedances, some of the wave energy will reflect off the interface and some will refract through the interface. At its most basic, the seismic reflection technique consists of generating seismic waves and measuring the time taken for the waves to travel from the source, reflect off an interface and be detected by an array of receivers (or geophones) at the surface. Knowing the travel times from the source to various receivers, and the velocity of the seismic waves, a geophysicist then attempts to reconstruct the pathways of the waves in order to build up an image of the subsurface.
The US Department of the Interior is currently proposing seismic testing for the Middle and South Atlantic. Tens of thousands of whales and dolphins will be at risk, and habitats exposed to the sound include the only known calving ground for endangered right whales. There are only about 400 right whales left in the wild—disrupting the lives of mothers and babies could push them closer to extinction.
Whales use their voices to find mates. Dolphin mothers communicate with their babies with clicks and whistles. These sounds will be silenced by the airguns. If they can’t talk to one another, these social animals are lost—and if they get too close to the airguns, they may not survive at all.
The main environmental concern for marine seismic surveys is the potential of noise associated with the high-energy seismic source to disturb animal life, especially cetaceans such as whales, porpoises, and dolphins, as these mammals use sound as their primary method of communication with one another.High-level and long-duration sound can cause physical damage, such as hearing loss, whereas lower-level noise could cause temporary threshold shifts in hearing, obscuring sounds that are vital to marine life.
A study has shown that migrating humpback whales will leave a minimum 3 km gap between themselves and an operating seismic vessel, with resting humpback whale pods containing cows exhibiting increased sensitivity and leaving an increased gap of 7–12 km. Conversely, the study found that male humpback whales were attracted to a single operating airgun as they were believed to have confused the low-frequency sound with that of whale breaching behaviour. In addition to whales, sea turtles, fish and squid all showed alarm and avoidance behaviour in the presence of an approaching seismic source. It is difficult to compare reports on the effects of seismic survey noise on marine life because methods and units are often inadequately documented.
Dr. Bernd Würsig, a professor for marine biology at Texas A&M University in Galveston, Texas states that the Gray whale will avoid its regular migratory and feeding grounds by >30 km in areas of seismic testing. Similarly the breathing of gray whales was shown to be more rapid, indicating discomfort and panic in the whale. It is circumstantial evidence such as this that has led researchers to believe that avoidance and panic might be responsible for increased whale beachings although research is ongoing into these questions.
Offering another point of view, a joint paper from the International Association of Geophysical Contractors (IAGC) and the International Association of Oil and Gas Producers (OGP) argue that the noise created by marine seismic surveys is comparable to natural sources of seismic noise, stating:
“The sound produced during seismic surveys is comparable in magnitude to many naturally occurring and other man-made sound sources. Furthermore, the specific characteristics of seismic sounds and the operational procedures employed during seismic surveys are such that the resulting risks to marine mammals are expected to be exceptionally low. In fact, three decades of world-wide seismic surveying activity and a variety of research projects have shown no evidence which would suggest that sound from E&P seismic activities has resulted in any physical or auditory injury to any marine mammal species.”