Eavesdropping

We have been steaming for 48 hours now and are nearing our final operational area. The winds were a bit lighter for part of the day today as we moved further away from the north African coastline, but as got into quite deep water we did not see a great deal of marine life. Several sperm whales, dolphin groups, and a sea turtle were spotted throughout the day, however, as we came through these poorly-studied waters. As we did during our transit yesterday, we have used this time to process the large amount of biological, oceanographic, and acoustic data that have been obtained thusfar.

One of our primary tools in eavesdropping on the underwater acoustic scene to locate, identify, and track marine mammals are arrays of hydrophones. With these listening sensors towed behind the vessel (you can see one being deployed here), and with the expert acoustic observers we have to fine-tune measurements and interpret what we are hearing (and also seeing on special displays), different species can be discriminated and estimates of signal strength (to give a sense of near or far) and bearing to calling animals can (sometimes) be made.

There are two arrays of hydrophones towed in parallel behind the ship, one on the right side, one on the left side. Each array is composed by a 150m long towing cable (which can be extended to 300m) ending with a 12m long oil filled tube that holds two wideband acoustic sensors each and their preamplifiers. The electronics onboard amplify the signal to be digitized with a 4 channels AD converter at 192kHz sampling rate connected to a powerful computer with dedicated, custom software designed for both research and mitigation purposes.

This software allows us to acquire, record to disk, analyze and display up to 8 channels in real time. A high resolution spectrographic display (a picture of the sound showing frequency, or "pitch", over time) shows all the features of the incoming sounds, even those we can't hear, because they are too high in frequency or too brief for our ears to detect. Several examples of the kinds of recordings made with these specialized listening sensors and processing software are given here; these are of long-finned pilot whales and striped dolphins recorded recently in our research.

» Listen to Striped dolphin clicks shown on three channels (note 24kHz zoom on top channel)

» Listen to Pilot whale bursts and whistles with harmonics

A special directional display shows the signals coming from the two arrays (4 hydrophones) to provide intuitive cues as to the direction of incoming sound. By using these two displays, researchers can recognize sounds coming from different species and estimate the direction from which they came. This information is especially important in the current research project to position the tag boat as close as possible to the position where the diving animals are supposed to emerge at the surface.

Tomorrow morning we will be back on effort in our new operating area. The weather forecast is quite favorable for the next several days, while it remains quite poor for the Alboran Sea area we recently left. So, we embark on the last week of our collective efforts here with renewed hope and optimism for favorable seas and accommodating whales.