Seafloor Mapping

1
Seafloor Mapping

• Measuring water depth is a fundamental
  observation at sea
• Primarily by acoustic methods
• Satellite altimetry (radar) can be used for
  regional reconnaissance
• Electromagnetic methods restricted to shallow
  waters due to severe attenuation.
• Sound was first used to measure water depth in
  the early part of the 19th century, but it was
  not until the 1920s that acoustic measurements
  became routine
• Prior to this, a weighted lead line was used.

2
Basics

• A conventional single-beam echo sounder records
  the time takes for a sound pulse to travel from a
  hull-mounted or towed transducer to the sea
  bottom and back again.
• Water depth is half the product of the two-way
  transit time and the mean vertical velocity, with
  a small correction for transducer depth.
• Measurements require a sharp leading edge to the
  transmitted pulse, precise timing of the bottom
  reflection and an accurate value of seawater
  velocity.
• Attenuation of acoustic waves over distances of
  20 km is small for frequencies below 50 kHz.
• Typically high audio frequency or ultrasonic
  transducers are used.

3
Basics

• The average velocity of sound in seawater is
  approximately 1500 m s-1, but varies as a
  function of water temperature, pressure, and
  salinity.

From Jones, E.J.W., 1999

• Velocity is routinely measured at sea using
  expendable bathy-thermographs (XBTs). These
  measure temperature variation with depth, which
  is then converted to velocity using a measured
  value of salinity (hull mounted sensor) and known
  pressure variations with depth.

4
Basics

• Temperature at a typical deep ocean site varies
  rapidly in top part of the water column this
  region is know as the thermocline
• In the same region, salinity changes are small.
• To the base of the thermocline, sound velocity is
  largely governed by temperature the velocity
  decreasing with the fall in temperature.
• At greater depths pressure dominates, causing
  velocity to increase.
• This results in a velocity minimum near 500 m.

From Jones, E.J.W., 1999
5
Single-Beam Echo Sounding

• The output pulse of an echo-sounder is produced
  by energizing a bank of piezoelectric of
  magnetostrictive transducers with a trigger
  signal from an inboard transducer.

• The main beam of the transducer bank is usually
  cone shaped with a half-angle at the 6 dB power
  point of 1o-40o.
• In 5 km of water a cone with an angle of 40o
  intersects the sea bottom with a footprint that
  is 3.6 km in diameter. The acoustic returns from
  this point are integrated to a single point.
• The corresponding footprint for a 1o beam is
  0.087 km.
• The narrower the beam, the higher the resolution.

From Jones, E.J.W., 1999
From SeaBeam operators manual
6
Single-Beam Echo Sounding

• Echoes detected by the transducers are typically
  amplified and displayed as marks of varying
  darkness on electrosensitive or thermosensitive
  paper, or more recently, computer monitors.
• The output is sometimes recorded digitally.
  However, data volumes are huge much greater
  than most forms of seismic data WHY??
• Dynamic range is increased with automatic gain
  control that suppresses strong signals and
  enhances weaker ones.

From Jones, E.J.W., 1999
7
Single-Beam Echo Sounding

• In the example to the right the received signal
  (and bottom multiple) are plotted as a function
  of depth (assuming a constant water velocity).
• When plotting the data, the paper advances at a
  constant rate. Therfore, the vertical exageration
  of the plot may vary as a function of the ship
  speed.
• When the seabed is inclined, the dip derived from
  the sounder record is less than the true dip
  the bottom echo travels along a path normal to
  the prevailing slip, but we assume it travels
  normal to the water surface.

From Jones, E.J.W., 1999
8
Single-Beam Echo Sounding

• If gradients are so steep that the ray paths fall
  outside of the main transducer beam then the
  bottom may be weakly recorded on side lobes, or
  not at all.
• Diffuse echoes from steep slopes are returned as
  a result of backscatter and specular reflection
  from limited areas with surfaces normal to the
  transducer.
• A steep slope may produce a series of incomplete
  hyperbolae.

From Jones, E.J.W., 1999
9
Single-Beam Echo Sounding

• Side echoes can be reduced with the use of
  narrow-beam echo-sounders.
• Wide beam (30o) echo sounders typically work at
  frequencies less than 10 kHz.
• Narrow beam (3o) echo sounders echo sounders
  typically work at frequencies up to 200 kHz.
• The narrower the beam, the higher the resolution
  due to the smaller footprint.
• In the example to the right (Red Sea) the narrow
  beam system (d) also images a brine layer (R) in
  the water column.
• High and low frequency systems can be used
  simultaneously. The harmonic (frequency
  difference) can also be used to image the
  seafloor.

From Jones, E.J.W., 1999
10
Swath Mapping

• Much of our knowledge of the seafloor comes from
  conventional echo sounders.
• Many areas of the seafloor are covered with
  sounding lines that are so widely spaced (tens to
  hundreds of kilometers) that we have to rely on
  low-resolution satellite altimetry to derive
  bathymetry.
• We know less about the seafloor than we do about
  the surfaces of Mars or Venus.
• Locally sounding lines may be dense enough to
  derive 3-D images of the seafloor, but these
  conventional acoustical surveying methods are not
  practical (time and money) for creating detailed
  images of the seafloor.
• New acoustic systems profile not only the seabed
  lying directly beneath a vessel but regions to
  the side. There are three principal methods
• Sidecan sonar an image of sound reflected from
  targets to the side of the ship.
• Multibeam systems measure bathymetry of the
  off-track region
• Hybrid swath systems combine sidescan and swath
  bathymetry.

11
Sidescan

• Sidescan sonar is based on systems built during
  WWII to detect submarines.
• Emitted sound pulses from sideways looking
  transducers to reveal underwater objects.
• In the 1950s it was realized that the same
  systems could be used for examining seafloor
  irregularities.
• In contrast to the cone shaped transmission
  pattern of a conventional echo-sounder, the main
  beam is narrow fore and aft (1o-2o) and wide in
  the transverse direction (20o-40o).

From Jones, E.J.W., 1999
12
Sidescan

• The transducers consist of a linear array of
  piezoelectric elements operating in the range of
  9-500 kHz. Magnetostrictive transducers
  sometimes cover the lower frequencies.
• A short pulse is emitted and is returned from the
  seabed directly beneath the ship and by
  backscatter and specular reflection from the
  seafloor to the side.
• Pulse lengths vary from tens to hundreds of
  microseconds depending on the range and
  resolution needed.

From Jones, E.J.W., 1999
13
Sidescan

• Intensity of backscatter from the seafloor is
  governed by the backscattering coefficient, Sb,
  which is the ratio of the intensity of sound
  scattered by unit area of the seabed and the
  intensity of the incident plane wave.

• Where Pb is the power scattered per unit solid
  angle, Ii is the incident power and A is the
  insonified area of the seafloor. Sb is closely
  related to seabed roughness and the difference in
  sound velocity and density across the seafloor.
• The roughness of the seafloor can be on scales
  ranging from over 10 km to microtopography with
  wavelengths less than the incident sound (15 mm
  for a 100 kHz transducer). Examples include
  sediment waves to coarse bedding.
• Transducers are typically hull mounted or towed
  in a stable housing with sonars looking to both
  port and starboard.

14
Sidescan

• Sonar range is determined by the main beam angle
  and the height of the transducer above the
  bottom.
• Beam widths are typically 20o-40o, giving ranges
  of 0.35-0.75 km when the transducer is towed
  200 m above the seafloor.
• The transducer fish is towed close to the sea
  surface when long range is required.
• Angle varying gain is applied to compensate for
  amplitude loss in more distant parts of the scan.
• Some sidescan images are displayed as a function
  of slant range this can lead to image
  distortion near the ship track.

From Jones, E.J.W., 1999
15
Sidescan

• Applications
• Mapping bedrock geology possible where
  sediments are sparse.
• Seabed mineral resources placer deposits may
  have different acoustic properties.
• Geologic hazards areas of gas release may be
  identified by pock marks on the seafloor.
  Unstable slopes, canyons, etc.

From Jones, E.J.W., 1999
16
Sidescan

• Early sidescan sonar recording of the ocean floor
  were made with a deep-towed transducer operating
  at 120 kHz or 240 kHz which was towed a few
  meters above the sea bottom on several thousand
  meters of cable.
• Large drag forces meant that tow speed was low
  (1-2 knots).
• Height above the seafloor was monitored so action
  could be taken to avoid collisions (by increasing
  ship speed).
• The next generation, GLORIA (Geological Long
  Range Inclined Asdic) was built in the 1960s.
• Rectangular arrays of transducers on both the
  port and starboard sides.
• Each array is made up of 2x30 piezoelectric
  transducers 0.17 m in diameter and 0.17 m apart.
• Transducers separately wired to form six
  horizontal sections of 2x5 transducers.
• Usually only three sections per set are used for
  transmission.
• Output signal is a 100 Hz linear frequency
  modulated pulse about 2 seconds long and repeated
  every 30 seconds.
• Carriers frequencies are 6.7625 kHz (port) and
  6.2875 kHz (starboard). The frequency separation
  minimizes cross-talk.

17
GLORIA

• Horizontal beam width is 2.7o at the half power
  point vertical beam is 35o wide with its axis at
  a fixed inclination of 20o below the horizontal.
• The sensitivity of the array to sound coming from
  directions close to the vertical is low, reducing
  the effects of multiple reflections.
• The fish is towed 400 m astern and 50 m below
  the sea surface at speeds of 5-8 knots.
• Maximum target distance is typically 4 times the
  water depth.
• Can be operated in heavy seas.
• 1100 km2 h-1.

From Jones, E.J.W., 1999
18
GLORIA
From Jones, E.J.W., 1999
19
TOBI

• A variety of deep towed sidescan systems have
  been developed to give higher acoustic definition
  than the long range systems.
• TOBI contains a 30 kHz sidescan, a magnetometer,
  and options for other sensors.
• Towed 400 m above the seafloor at 1-2 knots,
  creating a swath about 3 km wide to port and
  starbord.
• Resolution varies from 4x7 m close to the
  vehicle to 42x2 m at a range of 3 km.

From Jones, E.J.W., 1999
20
Multibeam

• Below is an early SeaBeam system.
• Twenty 12 kHz acoustic projectors, each
  consisting of four magnetostrictive elements, are
  set in a 6 m mounting along the keel. The array
  transmits 7 ms pulses, insonifying an area of the
  seafloor 54o x 2.67o normal to the ship track.
  The plane of emission is kept vertical by an
  electronic stabilizing system.
• The receiving system, consisting of 40 line
  hydrophone arrays with axes set in the
  fore-and-aft direction, is mounted across the
  keel.

From Jones, E.J.W., 1999
Courtesy Mineral Mining Agency of Japan
21
Multi-Beam

• The incoming echoes are formed into 16 beams by
  vector summation.
• Each beam is related to returns from zones of
  seafloor subtending an angle of 2.67o normal to
  the ships track and 20o in the fore-and-aft
  direction.
• The processed acoustic signals come from those
  areas of the seafloor where the transmitting and
  receiving beams overlap. In practice, these areas
  are elliptical.

From Jones, E.J.W., 1999
22
Multi-Beam

• In this example, 16 acoustic signals are recorded
  after each transmission.
• Each is sampled every 3.33 seconds, corresponding
  to a slant range of 2.5 m at 1500 m s-1. This
  limits the resolution.
• Corrections are made for receiver gain, water
  layer refraction, and ship movement.
• Most systems also measure the strength of the
  backscattered signal so that sonograms can be
  produced.
• There are many newer systems
• Simrad EM120 12 kHz, 191 beam, bathmetric sonar
  system capable of hydrographic charting and
  seafloor acoustic backscatter imaging in water
  depth up to 11,000 m. Angular coverage is up to
  150 degrees depending on depth. Width of coverage
  is generally six times water depth up to 2000 m.
  In deep water, a width of 20 km is achievable
  depending on bottom composition. The sonar
  transducers are mounted in the port hull.
• Simrad EM1002 A 95 kHz, 111 beam system designed
  to operate from shoreline down to a depth of
  about 1000 m. Angular coverage is up to 150
  degrees. Width of coverage is about 1500 m in
  deeper waters and up to 7.4 times water depth in
  shallower water.

23
Multibeam

• Ship hull design is important a modern SWATH
  design can minimize noise.

http//www.soest.hawaii.edu/agor26/BowD.JPG
24
EM120
Courtesy Hawaii Happing Research Group
25
Mutibeam
From http//www.soest.hawaii.edu/agor26
26
System Integration
27
EM120
28
EM120
29
EM120 Bathymetry
30
EM120 Sidescan
31
Hybrid Systems

• By recording the full waveform of backscatter a
  multibeam echo-sounder can provide a sidescan
  sonar image as well as bathymetry. This facility
  is now incorporated into multibeam systems
  including EM120 and Hydrosweep DS-2.
• Several towed systems provide both swath
  bathymetry and a sidescan sonar image for
  example HAWAII-MR1 (Hawaii Actoustic Wide Angle
  Imaging Instruement Mapping Research 1).

• H-MR1 is towed approx. 100 m below the sea
  surface. Each side of the vehicle contains two
  linear transducer arrays (11 kHz port 12 kHz
  starboard).
• On each side, the phase angle between the signals
  arriving at each array is related to the
  direction of the reflecting target.
• Range is determined from reflection time. When
  combined with angle, the depth can be calculated.
• Sidescan is derived conventionally.

Courtesy of Mark Rognstad, University of Hawaii.
32
From Jones, E.J.W., 1999

• Towed behind a depressor weight which is in turn
  attached to the ship -- decouples the system from
  the ships motion.
• Operates better in heavy seas than most scientists

33
H-MR1 Bathymetry
34
H-MR1 Sidescan
35
References Used

• Kearey, P., M. Brooks, and I. Hill, An
  Introduction to Geophysical Exploration, 2002.
• Jones, E.J.W., Marine Geophysics, 1999.