Cape Nelson acoustic logger
Contrary to the claims of many environmental groups (eNGOs), the sound levels generated in the vicinity of seismic surveys are not audible “24 hours per day, 7 days per week for weeks or months on end” nor can they be described as “deafening blasts”. Firstly, a seismic vessel cannot operate its seismic source 24 hours per day due to line turns and standby as a result of weather, third party users, technical problems, etc. Secondly, the claims made by many eNGOs ignore the science, distort the facts and disregard the significant level of monitoring carried out by the oil and gas industry over the years.
One way of monitoring the soundscape in the vicinity of a seismic survey is to deploy seabed acoustic loggers to cover the period before, during and after such surveys.
The following is an example of a 5 day playback of the sound levels recorded by an acoustic recorder (logger) deployed in 28m of water off Cape Nelson, near Portland, Victoria, when two seismic surveys were recorded in the vicinity. One seismic survey was recorded immediately adjacent to the logger, with one traverse coming within 144m of it, whereas the other survey was recorded further East along the continental shelf 38-81km from the logger.
The two plots in the above figure are identical but the top plot has been annotated with traverse designation, the distance of each end of the relevant seismic traverse from the logger and the start/finish times of the traverse . For example, line OS03-05, the first line annotated on the left hand side of the plot, was recorded between 4km and 23.9km from the seabed logger between 21.50 and 00.05hrs. Key parameters for this plot are:
- Horizontal scale is time in hours/days, with every 6 hours annotated on both displays and the dates annotated on the top display.
- Vertical scale is frequency.
- Colour coding is instantaneous sound energy level at each 1Hz of frequency
Key observations that can be derived from this plot are:
- The plot is overwhelmingly blue, or certainly between blue and yellow. These two colours represent the ambient noise levels of a very calm sea (blue – 80) through to a stormy sea (yellow – 120). Thus, contrary to some claims, even if a particular marine animal stayed at the same location throughout the 5 days represented by these playbacks, it would not hear sounds 24 hours per day.
- Only on very few occasions (ie 5) does the sound level at the logger become red, which represents an instantaneous sound level of approx. 140 dB re 1 µPa2/Hz on this plot. This sound level is still well down on the levels considered to elicit behavioural impacts, let alone physiological impacts. Thus, assuming the sounds get up to approx. 140 dB re 1 µPa2/Hz for approx. 0.5 hours on each of these 5 traverses, this is only about 2.5 hours during a period of 120 hours or approximately 2% of the time – well short of the “24 hours/day over a period of one or two months” claimed by many eNGOs and others.
- Only for one short period of less than half an hour during this 120hr period (<0.4% of the time), when the source array gets to approx 144m of the sea-bed logger (or a marine animal stationary at this location if it did not take evasive action!) would the instantaneous sound level reach about 150 dB re 1 µPa2/Hz (which is equivalent to approx190 dB re 1 µPa (Peak) and 175 dB re 1 µPa (RMS).
- The above sound levels are no more intense (indeed, often less intense) than the sound levels at which many species of whales vocalise. For example, sperm whales and blue whales vocalise at sound levels up to 235 and 188 dB re 1 µPa @1m respectively.
- Note how the sea-state went up on 16 November (to approx. 3-4) but the vessel was able to continue recording despite the sea-noise on the receivers. Sound arrivals from seismic traverses as close to the recorder as 8.3km could not be detected above the natural ambient noise in the area.
- Note there are no sound arrivals at the logger from the survey 38-81km away.
In conclusion, empirical measurements like the above totally contradict eNGO claims, so one wonders why they persist with such claims and disregard the science, facts and observations that are readily available.