Undersea Recording: Pop-Ups
The Bioacoustics Research Program has developed an autonomous acoustic recording device for deployment on the ocean floor at depths up to 6,000 m. The device, known as a "pop-up," includes a microprocessor, hard disk for data storage, acoustic communications circuitry, and batteries, all sealed in a single 17-inch glass sphere. An external hydrophone is connected to the internal electronics through a waterproof connector. At the conclusion of a mission, the positively buoyant sphere separates itself from its anchor and "pops up" to the surface for retrieval. As of the end of 2008, BRP pop-ups have been successfully deployed in hundreds of applications in over twenty countries worldwide.
Figure 1. Waveform and sound spectrogram of a two-part blue whale call recorded off of southern California by a pop-up. The dark spots at 20-second intervals at 19 Hz are calls from a fin whale. The original data were recorded at a sampling rate of 2000 Hz, then filtered and decimated to 250 Hz.
Figures 2 illustrates the relationships among the major components of the pop-up. Figure 3 shows a fully assembled pop-up in its shipping case. The analog acoustic signal from the hydrophone is routed, via a low-pass (anti-aliasing) filter, to two separate microprocessors that control data acquisition and communications, respectively. The data acquisition (DA) processor performs analog-to digital conversion and saves the digitized acoustic data to hard disk according to a programmed acquisition schedule. The DA processor can also send a signal to the communications processor to initiate release of the anchor at a predetermined time. The DA processor can be reprogrammed or tested after the pop-up is sealed (e.g., on deck before deployment) via a laptop computer connected to the pop-up's waterproof external connector.
The communications processor monitors the acoustic signal from the hydrophone, and controls the burn wire release mechanism and the piezoelectric pinger that is used to acknowledge receipt of an acoustic signal from a surface vessel. It also can receive an "internal release" signal from the data acquisition computer. When an acoustic or internal release signal is detected, the communications processor applies a voltage to the burn wire, which then corrodes within 5 minutes, releasing the pop-up from its anchor. When an acoustic query signal is detected, the computer transmits an acoustic response via the pinger.
Figure 2. The major functional components Figure 3. Pop-up recorder in
of a pop-up. shipping case.
Deployment and retrieval
The device can be deployed from a surface vessel by two people without using mechanical aides. Each pop-up weighs approximately 20 kg (without an anchor). The unit is retrieved by a surface vessel equipped with an acoustic transponder unit that communicates with the deployed pop-up. Prior to deployment, the sphere is encased in a plastic "hardhat" case.
Figure 4, below, shows a pop-up being deployed over the side of a sailboat off the coast of Massachsetts.
Figure 4. Pop-up recorder being lowered into the ocean from the deck of the vessel "The Song of the Whale" during a deployment sponsored by IFAW, May 2000. The glass sphere is enclosed in the yellow protective "hardhat" case.
The case is connected to an anchor by a stainless steel burn wire that corrodes when a voltage is applied to it. At the completion of the mission, the burn wire is electrified to +27 volts, causing it to corrode and break within approximately 5 minutes. The sphere and its hardhat case then separate from the anchor, and the sphere rises to the surface. The release can be activated by sending a coded acoustic signal from the retrieval vessel on the surface. Additionally, the microprocessor can also be preprogrammed to initiate a release at a specified time. Finally, a separate"fail-safe" timer guarantees eventual release.
When the retrieval vessel is near a deployed pop-up (as determined by GPS), the vessel can communicate with the pop-up via an acoustic transponder. When the pop-up receives the appropriate signal from a vessel on the surface, it acknowledges by emitting its own acoustic response signal. Depending on what signal is sent from the surface vessel, the pop-up responds either with its acoustic response alone, or by triggering the burn wire to release the anchor.
To facilitate retrieval, the pop-up is equipped with a VHF beacon transmitter and a high-intensity strobe light which are automatically turned on when the device reaches the surface. The retrieval vessel can then locate the floating pop-up either visually or by use of a directional receiver tuned to the same frequency as the pop-up's beacon transmitter.
Once the pop-up has been retrieved, the hard disks are removed from the sphere and the acoustic data are downloaded for analysis or storage. After the data are downloaded, the disks are erased and returned to the pop-up, new batteries are installed, and the pop-up can be re-deployed.
Programmable recording schedules
Version 1 pop-ups, circa 1997, recorded continuously for 10 days, with a pre-programmed start and stop time. Newer versions can record according to a predetermined sampling schedule that is downloaded to the unit's microprocessor prior to deployment.
Recording life and data capacity
Version 1 pop-ups had a recording life of approximately 21 days, recording continuously at a sampling rate of 2000 Hz. The total amount of data collected by these units was limited by battery life rather than data storage capacity. Version 2 pop-ups (first deployed in 1998) have a maximum deployment life of approximately 90 days (assuming continuous recording). This increase results from power-saving improvements in circuit design and use of lithium batteries or an extra battery pack, rather than the alkaline cells used in version 1. Still longer lifetimes have been achieved by a reduced (meaning: less than continuous) recording schedule or by recording at a lower sampling rate.
Currently, each pop-up carries one hard drive, capacity up to 120GB. The actual amount of data that can be stored depends on the recording schedule and the sampling rate.
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