Whereas the multibeam echo sounder returns information about the depth and shape of the seafloor, the TOPAS system is designed to provide information about the density of the material below the water-seafloor boundary, hence its designation as the “sub-bottom” profiler.
The TOPAS system provides observations penetrating up to 100 meters into the ocean bottom. It accomplishes this by using a “parametric” array which combines two independent 15-21 kHz signals whose interaction creates a frequency-modulated sweep of a much lower frequency than for example the multibeam or other echo-sounders. These resulting lower-frequency sound waves are not interfered with as much by the bubbles surrounding the ship’s hull or other density fluctuations in the water column, and interact primarily with the much denser material constituting the bottom and sub-bottom. The technique of creating a low frequency pulse by combining two higher frequency signals is used in order to allow a physically smaller array transceiver array, requiring less power, than would be required to generate the same signal from a single transmitter. This allows a the construction of a more compact instrument which can deployed on more ships.
Just as the sound waves emitted from the multibeam bounce partially off density gradients in the water column and the bottom, those sent out from the TOPAS bounce mostly off the seafloor and then some of them partially penetrate deeper into the ocean crust before bouncing back off density boundaries in the underlying sediment and rock. These return pings are timed and the return time of each of these signals makes up the main green and blue plot in the TOPAS interface.
An important distinction when comparing the multibeam output to that of the TOPAS: because the speed of sound through water is fairly predictable (especially when you have the data from an XBT and the World Ocean Atlas), it’s possible to read the depth-to-bottom directly from the multibeam output. By contrast, since the TOPAS is dealing with sound that has spent time travelling through a sub-bottom medium of unknown density, it is not possible to read the depth of the various sub-bottom features observed by the TOPAS directly in its readout. All TOPAS reports are the time delays between sending and receiving its modulated sonic pulses.
These time delays depend on the density of the sub-bottom material, but also on the distance the sound must travel through water before and after interacting with the bottom. This is the reason that when standing watch it is important to make sure the “trigger delay” — the time between emitting a sweep and beginning to listen for echoes — is set so that the TOPAS will be listening when its own signals are returning to it.
Even though the sub-bottom density data gathered by TOPAS is mostly unrelated to the magnetic survey we are conducting of the JQZ, it is still a very valuable resource for a number of reasons. This sub-bottom profiling technology is relatively new, and demonstrating its success, particularly in a region of the Pacific for which even ordinary depth mapping data is sparse or nonexistent, is good progress for the oceanographic community as a whole. It also gives us a chance to help NSF show off the state of the art capabilities of the Sikuliaq, and hopefully inspire and encourage other researches to use this ship as a platform for explorations of their own.