Topic: Bathymetric Survey Techniques

(a)  Single-beam echo-sounders (SBES) (b)  Multi-beam echo-sounders (MBES)

•  Bathymetry is the measurement of water depths - bathymetry is the underwater equivalent of terrestrial topography.

•  A transect of water depths along a straight line is termed a bathymetric profile.

•  A contour map of water depths is a bathymetric chart.

•  The production of a detailed bathymetric base-map is a critical method for marine, freshwater, environmental and coastal investigations.

•  2-dimensional and 3-dimensional bathymetric models can aid our understanding and interpretation of the landscape and individual sites, providing morphological maps of the seabed, lakebed and riverbed.

•  Conventionally, water depths are indicated by shades of blue - dark blue corresponding to deep water and light blue to shallow areas. However, any colour scale can be (and is) used.

•  This survey technique provides very little detail on the material properties (e.g sediment type, roughness etc.) of the seabed or sub-surface (see the following lectures on side-scan sonar and sub-bottom profilers). However, when combined with other geophysical techniques, bathymetric charts can act as a template for site-specific and reconnaissance scale surveys.

History of Bathymetric Survey

•  The simplest and oldest means of bathymetric survey is the sounding pole. This technique involves using of a long straight pole to measure water depths - effective, but limited in use to shallow water.

•  The sounding pole was replaced as a survey technique by the traditional lead-weighted line. Mariners conventionally took soundings in shallow water with a lead sinker on a fishing line primarily to locate navigational hazards and safe anchorages in near-shore zones and estuaries.

• "Un%l"the"20th"Century,"the"lead"line"was"the"only"effec%ve"tool"for"deeper"water"bathymetric"measurement.""

• "The"line"was"tradi%onally"marked"with"knots"or"coloured"%es"at"regular"intervals.""

• "In"the"mid"1880's"piano"wire"was"used"as"replacement"for"the"fishing"line"G"this"allowed"for"greater"strength"of"line."

• "The"main"disadvantage"of"the"leadGweighted"line"was"that"the"survey"vessel"had"to"be"at"a"stands%ll"when"a"depth"measurement"was"taken."Also"the"X"and"Y"(la%tudeGlongitude)"posi%on"of"the"survey"vessel"had"to"be"taken"manually"G"this"was"also"%meGconsuming.""""

•  Sometimes the lead weight was hollowed to allow for packing with tallow or wax, and so a bottom sediment sample could be taken with the depth measurement.

•  Seabed sediment samples are important to mariners to identify spawning ground, habitats, suitable anchorages etc..

•  Early sonar systems, developed during World War I by the American, British, and French were used to find both submarines and icebergs.

•  They were called ASDICs (named for the AntiSubmarine Detection Investigation Committee). These early units were crude if not effective.

Revolution of SONAR (SOund Navigation And Ranging)

• "During"World"War"II,"underwater"acous%cs"made"great"leaps"and"enemy"submarines"could"be"detected"more"easily"by"surface"ships"as"they"sent"a"stronger"and"beOer"formed"sound"pulses"into"the"water.""

• "The"pulse"would"bounce"off"the"submarine's"hull"and"give"away"their"distance"from"the"surface"ship.""

• "Sonar"(an"acronym"for"SOund"NAviga%on"and"Ranging)"became"very"important"for"the"detec%on"of"submarines.""

Modern Bathymetric Systems

•  The accuracy of bathymetric surveys is dependent on both the acoustic (sonar) technique used and the effective integration of an accurate navigation system (GPS).

•  Two systems are currently used for the acquisition of bathymetric data:

a)  Single Beam Echo Sounders (SBES)

b)   Multi Beam Echo Sounders (MBES)

MBES SBES

MBES SBES

Area insonified

MBES SBES

(a) Single Beam Echo Sounders (SBES)

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5 STEPS TO ACQUIRING SBES DATA STEP 1 Single-beam echo-sounders comprise a single hull-mounted transducer that acts as both an acoustic transmitter and receiver (transceiver).

STEP%2%

The"echoGsounder"system"produces"a"ver%cally"transmiOed"narrow"(typically"5G15o)"acous%c"pulse"with"a"single"frequency"(typically"within"the"50"G"300"kHz"range).""

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STEP%3""

The"SBES"records"the"%me"taken"for"an"acous%c"pulse"to"travel"from"the"transducer"to"the"seaGboOom"and"back"again"(twoGwayG%me).""""

two-way-time

STEP%4%

The"water"depth"(d)"is"calculated"as"half"the"product"of"the"twoGwayG%me"(TWT)"and"the"mean"ver%cal"sounding"velocity"(V).""

d = TWT x V

2

The average velocity of an acoustic pulse travelling through seawater is 1480 ms-1, a figure that varies from place to place due to changes in seawater temperatue, pressure and salinity.

STEP%5%

The"returns"(or"echoes)"are"detected"by"the"transceiver"and"recorded"and"displayed"in"hardcopy"or"digital"format."""

•  Typically, the bathymetric survey suite comprises a DGPS, a topside recorder/viewer (usually a laptop PC), a transducer and a cable.

•  The resultant data from a bathymetric survey is usually a 3-column ASCII data file containing X, Y and Z values, where:

X = Longitude Y = Latitude Z = Depth (m)

(b) Multi Beam Echo Sounders (MBES)

•  A ‘recent’ development from the echo-sounder is the swath (or multibeam) bathymetric system.

•  By comparison to the focused beam of the echo-sounder, swath systems record continuous depth measurements along a swath that extends perpendicularly either side of the survey vessel, where a multi-element transducer provides many (30-150) individual soundings of the water depth and echo strength for each ping.

•  A wide swath (up to 7 times the water depth) can be surveyed in a single pass through an area at survey speeds up to 20 knots. Survey lines are spaced to provide overlapping coverage of the seafloor.

•  These techniques have detected features that are often missed by ordinary bathymetric soundings as they inherently acquire much higher resolution data.

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Roll The rhythmic movement of a ship or towfish about its longitudinal axis.

Yaw An instability characterized by the side-to-side movement of a ship or towfish about its vertical axis.

Pitch An instability in the ship or towfish expressed by the alternate rise and fall of the nose and tail about a horizontal axis.

roll

yaw

pitch

heave

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Inertial Measurement Unit (IMU) or Motion Referencing Unit (MRU)

•  An Inertial Measurement Unit is a closed system that is used to detect altitude, location, and motion.

•  Typically installed on survey vessels, it normally uses a combination of accelerometers and angular rate sensors (gyroscopes) to track how the vessel is moving.

•  Typically, an IMU detects the current acceleration and rate of change in attitude (i.e. pitch, roll and yaw rates) and then sums them to find the total change from the initial position. These corrections are then applied to the geophysical data to correct for the effects of pitch, roll and yaw.

•  An IMU stands in contrast to the GPS system, which uses external satellites to detect the current position.

EXAMPLE SYSTEM

Case Study Mapping Crater Lake, Oregon State, USA

•  Crater Lake, the deepest lake in the United States, occupies a caldera in Mount Mazama, a Cascade Range volcano that once stood about 3,700m.

•  About 7,700 years ago in a catastrophic eruption that lasted only a few days, Mount Mazama ejected about 50 cubic km of magma in the form of pumice and ash.

•  Near the end of the eruption, the mountain collapsed upon itself to form a large caldera.

•  After this climactic event, volcanic activity resumed within the caldera, creating Wizard Island and other new landforms. All but the uppermost portion of the Wizard Island volcano is hidden from view below the surface of Crater Lake.

•  Until recently, nobody knew what the bottom of Crater Lake looked like in detail.

•  The first indications of the lake depths came from an 1886 survey by a joint USGS/U.S. Army expedition led by William G. Steel and under the direction Maj. Clarence E. Dutton (Dutton, 1889).

•  This mapping survey collected 186 soundings using a Millers lead-line sounding machine.

1886 Survey – 186 soundings

1959 Survey This survey used an acoustic echo sounder and radar navigation to collect 4000 soundings. The data were contoured by Williams (1961) and Byrne (1962) and resulted in a fairly detailed map of the large-scale features within Crater Lake. By the late 1990`s however, maps generated from this survey were proving inadequate for the scientific research being conducted in and around the lake.

2000 Survey U.S. Geological Survey, Pacific Seafloor Mapping Project in cooperation with the National Park Service and the Center for Coastal and Ocean Mapping, University of New Hampshire used a state-of-the-art multibeam sonar system to collect high-resolution bathymetry and calibrated, co-registered acoustic backscatter to support both biological and geological research in the area (aquatic biology, geochemistry, volcanic processes, etc).

•  This survey collected over 16 million soundings and the resulting data portrays the bottom of Crater Lake at a spatial resolution of 2m.

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