The what, the why, the how of Multibeam survey planning
When planning the survey lines islands, coastlines, shoals and other obstacles within the survey area that may have influence
on safety or efficiency of the survey must be taken into account. Nautical charts in digital format and/or aerial photos are extremely
important as background data. The achievable coverage of the multibeam echo sounder and the overlap required between neighbouring
lines is usually used to determine the line spacing.
A survey plan will normally define the following factors:
- The survey area (use background charts and/or aerial photos)
- Bottom conditions
- The survey lines
- The direction and order of the survey lines
- The survey lines required for system calibration
- The location and timing for sound speed profiles.
A fully comprehensive survey plan is most useful in areas of deep waters or where the depth and hence coverage is fairly constant.
In shallow waters where the depth changes rapidly and may not even be known, a comprehensive plan may not be as useful, especially
if the survey is to be run with a small and agile vessel. A defined survey area boundary plus a few pre-planned lines for calibration may
then be enough. Actual coverage is obtained on the spot instead of being used to determine the survey lines.
Factors to consider
- A survey is normally planned taking the following into account:
- The geography and extension of the survey area
- Suitable areas for calibration patch tests
- Echo sounder coverage
- Seafloor topography
- Sound speed variations
- Weather conditions
- The requirements for calibration of the positions (time delay), heading, roll and pitch sensors must be considered, and how and
where to gather sound speed profiles.
- Coverage capability determines line spacing, and as it varies with bottom reflectivity, this must be estimated. Usually 10% overlap
between lines is sufficient, but if large variations in bottom reflectivity is expected, or reflectivity is unknown, it may be necessary to
increase the overlap. The overlap must also be increased if the vessel’s roll is excessive.
- If there are steep slopes on the bottom, it is strongly advised to run along these slopes, not up or down them. This is beneficial
for keeping coverage reasonably constant along the survey lines, thus making survey planning easier. However, the main reason for
this advice is that the echo sounder performance will usually be poorer when running up or down a slope rather than along. This is
because less acoustic energy is reflected towards the transducer from steep slopes, causing poorer detections and the possibility of false
detections in sidelobes. Sidelobe detections is however very rare in the Kongsberg multibeam echo sounders due to the advanced signal
processing implemented. Note that if circumstances require that survey lines are run up or down slope, reduction of vessel speed may
be required to allow the echo sounder to track the bottom continuously. During the survey, planned lines may be activated to generate
steering information for the bridge and helmsman’s display. The purpose of the Planning module is thus to provide help before and
during the survey.
- Coverage capability is also affected by weather conditions and possibly also by vessel speed. Heavy seas and possibly vessel speed
lead to increased noise level, and may also cause aeration on the Sonar Head or the transducer array.
- Aeration is a function of sea state, but also of the heading with respect to the wave direction and the vessel speed. It is strongly
advised that one builds a record of coverage and aeration problems versus sea state, heading with respect to wave direction, and
vessel speed. This record may be helpful in ensuring that surveys can be performed efficiently with a minimum of line rejections and
corresponding reruns and infills.
- Any drift rates of roll, pitch and heading should also be recorded to enable efficient planning of calibration intervals. If calibration
is required before a survey, a suitable calibration area must be identified before reaching the survey area.
- A sound speed profile must always be taken within the survey area. In some areas the profile will vary, mostly due to fresh water inflows from rivers or currents from areas with different salinity. Surface sound speed
variation may be strongly affected by solar warming. If variations can be expected, where and when sound velocity profiles are to be
taken must be planned, and the survey line schedule adjusted to take this into account.
- If the measured sound speed value at the Sonar Head or the transducer array depth is continuously measured, it is recommended to
compared this to what is observed by the profiling instrument to evaluate the need for observing a new profile.
- Note that in some cases the coverage capability of the echo sounder cannot be fully utilized, because remaining errors in roll
and sound speed profile measurements, which are critical for maintaining the accuracy of the outer beams, become too large. The ray
bending effect (Snell’s law) may also reduce the online coverage since the energy can bend inwards.