Marine Survey Patterns

Underwater survey or search is called "mowing the lawn".  Another analogy is cleaning the ice at a skating rink; if both cases, the desire is to get complete coverage with as little overlap as possible.  If you miss a spot, it is called a "data holiday", and it might or might not be easy or possible to return and collect the data.

For some systems, like side scan sonar, the operator selected a swath width and then tries to fly the towfish at the appropriate depth.  For others, like multibeam bathymetry, where then sensor is hull mounted, the water depth determines the swath width because the coverage area is a pie shaped wedge that widens in deeper water.  The swath width determines the number of survey lines required to cover an area.  In reconnaissance work, or to determine the general characteristics of an area, gaps may be acceptable if it would take too much time or money for complete coverage.  In search operations where missing the target is a priority, complete coverage may be required.  For some operations where failure is not an option, such as charting obstructions to navigation where there are legal and financial costs to an incorrect conclusion, there may be requirement for 200% coverage, which is the requirement to remove a reported wreck from the NOAA data base. 

Multibeam survey along the Mid Atlantic Ridge.  The ridge axis lies to the east of the map area, and the coverage there approaches 100%.  In the less interesting region to the west, you can see that only half the tracks that would be required from complete coverage have been surveyed, and the turns are likely as tight as the survey ship could handle.  If a survey ship returned later, the coverage could be completed with minimal overlap.
Aeromag survey.  This was done in two days (red and blue) to achieve complete coverage of the area.  Survey lines runs NW-SE, following the tides in the area and the survey patterns of ships towing sidescan sonar.  Gentle turns skip several survey lines, but the survey plan eventually completely fills in the survey area.

The long length of the lines suggests aerial survey.

Three SW-NE tie lines allow "crossover analysis".  This was critical in pre-GPS navigation, when the best methods could be off by significant distances, and by the end of a cruise positions could have significant errors.  By looking at crossovers, with a survey covered the same date as a previous line, the positions could be adjusted to spread out and lessen the effects of the navigation errors.  While GPS navigation has decreased the need for tie lines in many cases, magnetic surveying requires a large number of corrections to the raw readings, and having the crossovers increases confidence in the final results.
Side scan sonar survey showing individual survey lines.  The search area had an "LL shape to abut an earlier survey to the southwest.

The lines run with or against the strong tide.  Note the much shorter survey lines compared to the aerial survey above.

Because data collected during turns with a side scan sonar has such low quality, it is usually discarded.  As a result we cannot see the actual turn patterns in the display of the records.  This map shows a bad GPS reading (there are other anomalies in the GPS when the map is zoomed in), as well as a missing survey line.  There are also two apparently random survey lines in the NW of the map.  The survey vessel was unable to complete the survey in the SE portion of the map because a large seismic survey ship asserted right of way, and the survey vessel moved a safe distance away to survey two lines that would be part of future planned operations area. 

Last revision 5/10/2016