Ben Chichester
It’s a bit awkward arriving anywhere in open water earlier than scheduled. I guess you’d just have to float around in circles for a while, so yesterday we spent cruising south at a leisurely pace of 8 knots towards the start of our first seismic survey line! In the afternoon we bobbed over the Puerto Rico trench, just north of the top of the Lesser Antilles island arc. The ship’s multi-beam swath bathymetry and single-beam bathymetry told us that the trench was nearly 8 km deep in places! That’s 80 James Cooks.
Tests are required of the airguns and the 3 kilometre streamer, which hosts hydrophones that ‘hear’ the airguns, before the survey begins. So over the last few days we’ve been doing just that. The airguns are deployed 40 metres behind the stern, 8 metres below the sea surface. When the airguns fire, acoustic waves travel almost directly downwards, penetrating the seafloor and reflecting from layers of rock tens of kilometres below. The reflected, refracted and reverberated acoustic waves are then detected by the hydrophones, but more importantly by island and seafloor seismometers in the surrounding region.
This testing gave us newbies a taster of the sound we’re going to have to live with 24 hours a day. When firing, on every minute each of the four airgun beams is supplied with compressed air at 2000 psi. The four compressors are found below deck at the back of the ship, but they can be heard throughout. A fellow crewmember described their sound inside the ship alike to when the ship belly-flops into the water after the front of the ship catches air in rough seas. Mildly disconcerting… Remarkably though I had phased out the regular, omnipresent thud by bedtime the day of the first tests.
With shallow waters to navigate around the islands, as well as yachts cavorting about unawares, a 3 kilometre streamer was deemed a tad too long for the obstacle course. So, as well as testing the equipment, cap’n wanted to test the manoeuvrability of the ship while trailing the streamer with a decided length of 300 metres. This actually did involve sailing round in circles. The gathering boobies probably think we’re nuts.
Airguns honking, streamer deployed, dolphins absent – today, it’s squeaky bum time!