The acoustic energy (sound) that is imparted to the sea floor by some of our collection of sources requires an initial input of energy that must be generated external to the device.
The air and water guns require an air compressor onboard the ship to provide the compressed air. The shipboard air compressor is a large unit requiring a considerable amount of deck space and ship power (see photograph below, left). The pressure of air output (in pounds per square inch, psi) is regulated by a technical operator.
Boomer and sparker systems require a power source (power supply; see photograph below, right). Though smaller than the air compressor, it is quite heavy and has high ship-power requirements. The output voltage is variable and set by the operator who also sets the automatic triggering parameters for the energy pulse.
The topside controller acts as a data-acquisition station and controller interface for the chirp vehicles. A Windows-based data acquisition program communicates with the controller during field operations and allows the operator to set all survey acquisition parameters on the controller. Collected digital data are saved on disk or onto a Network Attached Storage device in the SEG-Y format. All signal processing of the returning chirp data is done in the controller unit utilizing DSP boards, and these digital data are then passed off to the acquisition software for display and written to disk. The chirp controller also powers the vehicles, and sends down preset waveforms to the vehicles upon startup that allow the operator to record narrower frequency spectrums within the wide band pulse. For example, using the 512 system (.5 - 12 kHz), the operator could select the 2-7 kHz spectrum to see how the returning data looks on screen. This helps to optimize the data quality for a variety of sediment types, and allows the operator to switch spectrums "on the fly" during the initial stages of a survey to determine which spectrum is best suited for the survey environment and bottom type. Phase information contained in the chirp signal is also optionally saved with the data, which can be used in bottom classification software.
This topside controller operates the 3.5 and 200 kHz transducers bolted onto a sidemount for our small boat operations. The unit has built-in digitizers to transform the analog data into a digital form by means of correlation processing of the chirp signal. The unit can also operate very large arrays, giving the possibility for full ocean depth operation. The controller communicates with either a laptop or desktop via a SCSI connector, which operates the controller via a Windows-based data acquisition program. All digital data are saved in the SEG-Y format, and can optionally be saved in a compressed binary format proprietary to the controller's manufacturer.