Single Beam Bathymetry systems
Schematic of an echosounder
Single-beam bathymetry systems are generally configured with a transceiver (transducer/receiver) mounted to the hull, or sidemount, to the ship. These systems measure the water depth directly beneath the research vessel. The hull-mounted transceiver transmits a high-frequency acoustic pulse in a beam directly downward into the water column. Acoustic energy is reflected off the sea floor beneath the vessel and received at the transceiver. The transceiver contains a transmitter, which controls pulse length and provides electrical power at a given frequency.
This transmit-receive cycle repeats at a fast rate, on the order of milliseconds. The continuous recording of water depth below the vessel yields high-resolution depth measurements along the survey track. Additional information such as heave, pitch, roll of the vessel can be measured with a Motion Reference Unit (MRU), and used to “correct” the position of the depth measurements during processing.
Initially, the two-way travel time was converted to depth using an assumed sound velocity in water, usually 1500 m/sec. Currently, the WHSC uses a sound velocity profiler to acquire data about the precise sound velocity in the ambient water mass and these velocity measurements are used in the conversion from travel time to depth.
|Schematic showing the transducer as a red box, either hull-mounted or sidemounted to the ship. The acoustic energy pathway is shown by the dotted red line, with a two-way arrow, representing the direct path taken by the outgoing and returning energy.|
The frequency ranges of the systems the WHSC Sea-floor Mapping Group operates range from 3.5 kHz to 200 kHz. The 3.5 kHz system also functions as a subbottom profiler, as the energy can penetrate the sea floor somewhat and produce a very detailed map of the surficial sediment thickness.
|3.5 kHz profiler attached to base of sidemount pole
in the onshore laboratory.
|Sidemount in underway tow position secured to the side of the research vessel.|
Single-beam echosounders are relatively easy to use, but only provide depth information along the ship's trackline. Thus, features lying between tracklines, which are often spaced several 10's to 100's of meters apart, are not visible to the system.
|Topside display units for two different echosounder systems. The system on the left is depicted with the paper copy printer that accompanies the system. Both systems display real-time water depth values.|
We usually place the echosounder on a sidemount that swings from a stowed placement alongside the inside ship's rail to a vertical position amidships directly over the rail.
Sidemount fitted with echosounder and ranging transducers.
Sidemount deployed on the port side of the research vessel.
Sidemount with echosounder transducer
|These photographs were taken during USGS research cruises in the late 1990's to early 2000.|
The 3.5 kHz and 12 kHz echosounders are used almost universally in our studies to measure water depth. The 3.5 kHz system generally achieves 10 - 50 meters of penetration beneath the sea floor, depending upon sediment type, as well as recording water depth. The data are stored digitally and subsequently integrated with other data types.
The following three examples are bathymetric data obtained during a USGS research cruise in Lake Michigan aboard the R/V Laurentian, 1989. Horizontal time lines are 25 msec. spacing. (from Foster, D.S. and S.M. Colman)
|Relatively transparent section is composed of post-glacial/glacial lacustrine deposits. These overlay banded glacial units.|
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