Cape Nelson acoustic logger
Definitely not!! This is invariably claimed by most environmental groups (eNGOs) but it is another way in which they distort the facts and spread misinformation. This “issue” was briefly mentioned in a previous article entitled “What seismic sound levels are generated in the vicinity of a seismic survey?” and this article will expand on the “24/7” claim by many eNGOs.
It would be virtually impossible for a seismic vessel to emit seismic pulses 24 hours per day for days on end. This is because:
1. seismic surveys are nearly always acquired in a grid pattern which require the vessel to conduct a “line change” from one seismic traverse to the next; and
2. varying (and sometimes significant) amounts of standby is experienced as a result of weather, third party users (fishing, shipping and even whales) and technical issues.
Thus, seismic data (ie when the source is active) is never acquired on a 24 hr basis. If seismic data were gathered 24hrs/day, seismic surveys would often be completed in half the time. The only way in which a seismic vessel could acquire data for 24 hours per day would be:
A. on one very long regional traverse, which, at a vessel speed of approx 9km/hr, would mean that the vessel would move approx 216km in 24 hours. Thus, it could not possibly be considered as operating “in the same area” for a full day; and
B. if it were to acquire overlapping circular traverses. This method of acquisition is very rarely used, as a significant level of noise is generated on the sensors due to the flow noise in the turns. It would also only be used in areas of extremely complex geology, such as the Gulf of Mexico, where it has been used to image very steeply dipping horizons such as the edges of salt intrusions.
The playback of the sound levels logged by the Cape Nelson acoustic recorder deployed offshore Portland, Victoria, Australia displayed below and described in more detail in a previous article, shows a series of seismic traverses being recorded. The two plots in the figure are identical but each traverse in the top plot has been annotated with traverse designation, the respective distance of each end from the logger and the respective start/finish times. In addition, the dates covered by the display are annotated along the horizontal axis.
The first traverse, OS03-05, is recorded at a distance of 23.9km to 4.0km from the logger between 21:50 and 00:05hrs (ie it took 2hrs 15min to record). The next traverse, OS03-01, is recorded between 03:31 and 04:32 hrs (1hr 1min to record). Note that the line change from traverse 05 to 01 took from 00:05hrs to 03:31hrs, a total time of 3hrs 26 minutes. By adding up total recording time and total line change time during this 5 day period it can be seen that recording time is only about 51hrs whereas line change time is 62hrs. Adding in 20 minutes for the soft-start (or ramp-up) while the source is increased from lowest possible volume to full operating volume before the beginning of each traverse still gives us less time that the source is operational (approx 58 hrs) compared to when it is not operational. In this case, the source was therefore operating just under 12hrs/day (ie just under 50% of the time) and that was with NO standby. It would be very unusual to have a source fully operational during a seismic survey more than about 70% of the time, yet again demonstrating that many eNGOs’ claims are totally misinformed.
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 in dB re 1 μPa2 at each 1Hz of frequency.