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Single Beam Bathymetry systems

schematic of an echosounder
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.

diagram of the singlebeam systems
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.

Knudsen transducers 3.5 kHz profiler
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.

Odom echosounder display  Odom echosounder Display
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.

System Operation

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.

Odom sidemount
Sidemount fitted with echosounder and ranging transducers.
Sidemount with Odom echosounder
Sidemount deployed on the port side of the research vessel.
Sidemount with Odom echosounder
Sidemount with echosounder transducer
These photographs were taken during USGS research cruises in the late 1990's to early 2000.

System Usage

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.

Example of bathymetric data obtained using the 3.5 kHz> system
Example of bathymetric data obtained using the 3.5 kHz system during a USGS gas-hydrates research cruise aboard the R/V Gyre in the Keathley Canyon region, Gulf of Mexico, 2003. The sediments (the dark banded layers) are hemi-pelagic muds in a mini-basin setting juxtaposed between highly-faulted structural highs. Each sediment packet is bounded by unconformities. Horizontal tic marks are 2-minute time markers. Vertical scale is two-way travel time with tic lines shown every 10 ms. Data are highly vertically exaggerated: horizontal distance shown is about 5 km; vertical distance shown is about 75 m. Vertical noise bursts randomly across the record are interference from the GI gun shots. (from Hutchinson, D.R. and P.E. Hart)

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)

>Example of bathymetric data obtained using the 3.5 kHz system
Relatively transparent section is composed of post-glacial/glacial lacustrine deposits. These overlay banded glacial units.
Example of bathymetric data obtained using the 3.5 kHz system
Example of bathymetric data obtained using the 3.5 kHz system
Two views of a glacial till “tongue”, Lake Michigan. The upper image shows more detail in the overlying units. Note there is a 62.5 msec. delay time prior to the beginning of the vertical display. The lower image shows an interpreted section on a different survey trackline. The tic marks on the left-hand side of the bottom image denote approximate depth in meters, from 100 to 200 meters; each tic marks about 25 m. depth.

Below is a 3.5 kHz record acquired on the north slope of Puerto Rico, roughly parallel to the shoreline from a USGS research cruise aboard the R/V Farnella, 1985. It crosses the upper parts of the submarine canyons that lie offshore, oriented perpendicular to the coastline. In this example, there is little subbottom penetration.

3.5 kHz record acquired on the north slope of Puerto Rico
Echosounder profile along the upper continental slope of the US Atlantic margin

Echosounder profile along the upper continental slope of the US Atlantic margin southwest of Hudson Canyon, 1987 from a USGS research cruise aboard the R/V Farnella. The echosounder used a 10-kHz sound source since the survey focused on the deep-water parts of the US Atlantic margin. The profile shows a landslide scarp near the northeast edge of the profile, and two shallow valleys farther to the southwest.

References

Foster, D.S. and S.M. Colman, 1991, Preliminary interpretation of the high-resolution seismic stratigraphy beneath Lake Michigan, U.S.G.S. Open File Report 91-21, 42 pp., 2 plates.

Hutchinson, D.R. and P.E. Hart, 2004, Cruise Report for G1-03-GM, USGS Gas Hydrates Cruise, R/V Gyre, 1-14 May 2003, Northern Gulf of Mexico, USGS Open-File Report 03-474, online.


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