backcatter intensity sonograph similar to SSS but also provides highly accurate colocated bathymetry. Under optimal conditions SSS is capable of producing fullcoverage high-resolution two-dimensional (2D) imagery of the seabed. The analog
grayscale image can be visually interpreted for characterizing sediment grain size
and bedforms, delineating hardbottom features, or detecting objects on the order of
10s of cm in size. The analog grayscale image can also be digitized and quantitatively classified using first order (tonal) and second order (textural) statistics
(Blondel and Gomez-Sichi 2009). Tonal statistics use a variety of indices (e.g.,
extrema or median values) to describe the amount of acoustic energy returned as
backscatter. Textural statistics contain the most useful information for classification and relate to the relative spatial variation of backscatter intensities, providing
quantitative measures of roughness, patchiness, randomness, etc.
Multi-beam echo sounders (MBES): MBES transmit between 100 and 500
narrow beams at varying angles to cover a fan-shaped swath of seabed 3–79 the
water depth. The complicated beam geometry necessitates the integration of
precise vessel attitude and positioning into data processing in order to produce an
accurate measurement of bathymetry. This is accomplished using the output of a
global positioning system (GPS) and an Inertial Motion Unit (IMU), which
together records the location and orientation of the vessel (and hence the beam) at
the time of echo generation. Early MBES transducers were exclusively hullmounted, but recent reductions in the size of the various components have produced systems that can be pole-mounted (albeit with compromised signal quality)
or mounted on Autonomous and Remotely Operated Underwater Vehicles. MBES
can be operated over a very wide range of depths (0.5 m to full ocean depth), but
are most efficient in deeper water where the spreading beams cover a wider swath.
MBES simultaneously acquire accurate high density bathymetry and backscatter
intensity information, which can be interpolated into high-resolution seafloor
topography and morphometrics. The current generation of MBES also support the
capture of water column information to elucidate abiotic (e.g., oil spill) or biotic
(e.g., fish, plankton, diatoms, marine mammals) features. In a typical early MBES
application, the topographic surface would be visually interpreted as classes of
geomorphological structure. Recent applications have also utilized bathymetryderived indices (e.g., slope, rugosity) and backscatter intensity (related to seabed
roughness and hardness) either as additional layers for subjective classification, or
for quantitative multi-dimensional clustering techniques.
9.1.4 Selecting an Acoustic System
Choosing between acoustic systems involves considerations such as budget (e.g.,
deployment costs, hardware, software), survey objectives (e.g., resolution, coverage), output (e.g., bathymetry, backscatter, thematic classification, object detection), survey area (e.g., extent, range of depth, heterogeneity), and post-processing
(e.g., expertise, data storage, computing requirements).
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225
grayscale image can be visually interpreted for characterizing sediment grain size
and bedforms, delineating hardbottom features, or detecting objects on the order of
10s of cm in size. The analog grayscale image can also be digitized and quantitatively classified using first order (tonal) and second order (textural) statistics
(Blondel and Gomez-Sichi 2009). Tonal statistics use a variety of indices (e.g.,
extrema or median values) to describe the amount of acoustic energy returned as
backscatter. Textural statistics contain the most useful information for classification and relate to the relative spatial variation of backscatter intensities, providing
quantitative measures of roughness, patchiness, randomness, etc.
Multi-beam echo sounders (MBES): MBES transmit between 100 and 500
narrow beams at varying angles to cover a fan-shaped swath of seabed 3–79 the
water depth. The complicated beam geometry necessitates the integration of
precise vessel attitude and positioning into data processing in order to produce an
accurate measurement of bathymetry. This is accomplished using the output of a
global positioning system (GPS) and an Inertial Motion Unit (IMU), which
together records the location and orientation of the vessel (and hence the beam) at
the time of echo generation. Early MBES transducers were exclusively hullmounted, but recent reductions in the size of the various components have produced systems that can be pole-mounted (albeit with compromised signal quality)
or mounted on Autonomous and Remotely Operated Underwater Vehicles. MBES
can be operated over a very wide range of depths (0.5 m to full ocean depth), but
are most efficient in deeper water where the spreading beams cover a wider swath.
MBES simultaneously acquire accurate high density bathymetry and backscatter
intensity information, which can be interpolated into high-resolution seafloor
topography and morphometrics. The current generation of MBES also support the
capture of water column information to elucidate abiotic (e.g., oil spill) or biotic
(e.g., fish, plankton, diatoms, marine mammals) features. In a typical early MBES
application, the topographic surface would be visually interpreted as classes of
geomorphological structure. Recent applications have also utilized bathymetryderived indices (e.g., slope, rugosity) and backscatter intensity (related to seabed
roughness and hardness) either as additional layers for subjective classification, or
for quantitative multi-dimensional clustering techniques.
9.1.4 Selecting an Acoustic System
Choosing between acoustic systems involves considerations such as budget (e.g.,
deployment costs, hardware, software), survey objectives (e.g., resolution, coverage), output (e.g., bathymetry, backscatter, thematic classification, object detection), survey area (e.g., extent, range of depth, heterogeneity), and post-processing
(e.g., expertise, data storage, computing requirements).
9 Acoustic Applications
225