lobe; destructive and constructive interference at its edges create so-called side
lobes, which need to be suppressed as much as possible; see Fig. 8.4), and beamsteering (the beam-former can be steered, which is the capability to modify the
refracted angle of the beam). Beam-forming also requires a careful choice of
frequency (the beam-former pattern is frequency dependent, where the higher the
frequency, the narrower the lobes) and spacing of hydrophones if an array of
discrete transducers are used (to avoid spatial aliasing).
Good positional accuracy of SONAR postings demands precise information on
the vessel’s location at all times during operation. Vessel location is commonly
obtained using GPS. For more accurate locations, Differential GPS (DGPS) can be
used for sub-meter positioning, or Real Time Kinematics (RTK) GPS can be used for
centimetre level positioning, both of which utilize a base station in addition to the
rover onboard the vessel. Vessel heading is measured with a gyrocompass located at
the centreline of the vessel. Knowledge of the vessel attitude (heave, pitch and roll)
also greatly improves the acoustic travel time measurement accuracy. Heave is the
vessel going up or down, pitch is the movement of the bow going up or down and roll
is the movement of the port and starboard side going up and down. These movements
are measured with a motion sensor and reported to the data collection computer,
along with the location and heading measurements, to make the appropriate corrections to the acoustic distance travelled. Any movement of the boat changes the
orientation of the transducer to the substrate which increases or decreases distance.
In terms of acquisition, the geometry for the lines to be acquired is defined in
such a manner as to completely cover the area, inclusive of the necessary overlap,
at the desired spatial resolution. Line geometry is planned based on the charts,
maps and environmental conditions of the area before the acoustic survey. The
easiest geometry, known as a ‘lawnmower’ pattern, is to define a series of parallel
lines crossed by either perpendicular or tie lines, which are used to validate the
quality of the data and ensure consistency. During the survey, following these lines
is called ‘‘mowing the lawn’’. While planning a survey with an acoustic system
that uses a large footprint like side scan sonar or multi-beam, the amount of
coverage overlap between the lines also has to be defined.
8.2.4 Processing Requirements
Once the data are collected, they have to be processed. The amount and sophistication of the signal processing depends on the system used and the intended
objectives of the survey. Here, a signal is defined as a change or a disturbance in the
normal ‘‘background’’ environment, where an acoustic signal is a disturbance in the
background pressure of the seawater. The signal conveys information about both
the nature of the disturbance and also of the environment. However, the recorded
data include measurement of both signal and noise; hence, thresholds need to be set
for the declaration of what is signal and what is noise (Fig. 8.7). This is usually
accomplished based on models of signal and noise. Since noise creates the
8 Acoustic Methods Overview
205
lobes, which need to be suppressed as much as possible; see Fig. 8.4), and beamsteering (the beam-former can be steered, which is the capability to modify the
refracted angle of the beam). Beam-forming also requires a careful choice of
frequency (the beam-former pattern is frequency dependent, where the higher the
frequency, the narrower the lobes) and spacing of hydrophones if an array of
discrete transducers are used (to avoid spatial aliasing).
Good positional accuracy of SONAR postings demands precise information on
the vessel’s location at all times during operation. Vessel location is commonly
obtained using GPS. For more accurate locations, Differential GPS (DGPS) can be
used for sub-meter positioning, or Real Time Kinematics (RTK) GPS can be used for
centimetre level positioning, both of which utilize a base station in addition to the
rover onboard the vessel. Vessel heading is measured with a gyrocompass located at
the centreline of the vessel. Knowledge of the vessel attitude (heave, pitch and roll)
also greatly improves the acoustic travel time measurement accuracy. Heave is the
vessel going up or down, pitch is the movement of the bow going up or down and roll
is the movement of the port and starboard side going up and down. These movements
are measured with a motion sensor and reported to the data collection computer,
along with the location and heading measurements, to make the appropriate corrections to the acoustic distance travelled. Any movement of the boat changes the
orientation of the transducer to the substrate which increases or decreases distance.
In terms of acquisition, the geometry for the lines to be acquired is defined in
such a manner as to completely cover the area, inclusive of the necessary overlap,
at the desired spatial resolution. Line geometry is planned based on the charts,
maps and environmental conditions of the area before the acoustic survey. The
easiest geometry, known as a ‘lawnmower’ pattern, is to define a series of parallel
lines crossed by either perpendicular or tie lines, which are used to validate the
quality of the data and ensure consistency. During the survey, following these lines
is called ‘‘mowing the lawn’’. While planning a survey with an acoustic system
that uses a large footprint like side scan sonar or multi-beam, the amount of
coverage overlap between the lines also has to be defined.
8.2.4 Processing Requirements
Once the data are collected, they have to be processed. The amount and sophistication of the signal processing depends on the system used and the intended
objectives of the survey. Here, a signal is defined as a change or a disturbance in the
normal ‘‘background’’ environment, where an acoustic signal is a disturbance in the
background pressure of the seawater. The signal conveys information about both
the nature of the disturbance and also of the environment. However, the recorded
data include measurement of both signal and noise; hence, thresholds need to be set
for the declaration of what is signal and what is noise (Fig. 8.7). This is usually
accomplished based on models of signal and noise. Since noise creates the
8 Acoustic Methods Overview
205