CH Field Survey & Data Collection 2 — Questions and Answers
Question 1: What is the primary purpose of a vessel's motion reference unit (MRU) during hydrographic surveys?
- To measure vessel speed over ground
- To compensate for pitch, roll, and heave effects on sonar data (Correct answer)
- To track the vessel's heading relative to magnetic north
- To record water temperature at the transducer depth
Correct answer: To compensate for pitch, roll, and heave effects on sonar data
An MRU measures vessel dynamic motion (pitch, roll, heave) so these movements can be removed from depth measurements to obtain accurate seafloor depths.
Question 2: During a side-scan sonar survey, what does 'towfish altitude' refer to?
- The depth of the towfish below the sea surface
- The height of the towfish above the seafloor (Correct answer)
- The horizontal distance between the towfish and the vessel
- The vertical distance between the vessel and the towfish
Correct answer: The height of the towfish above the seafloor
Towfish altitude is the height of the side-scan sonar towfish above the seafloor, typically optimized at 10–15% of the total range to maximize coverage and resolution.
Question 3: What is 'layback' in the context of towed sonar operations?
- The time delay between ping transmission and reception
- The horizontal offset between the vessel's GPS antenna and the towfish position (Correct answer)
- The cable angle measured at the stern of the vessel
- The depth correction applied for cable catenary
Correct answer: The horizontal offset between the vessel's GPS antenna and the towfish position
Layback is the horizontal distance between the vessel's reference point (usually the GPS antenna) and the towfish, which must be accounted for in positioning.
Question 4: When establishing a horizontal control network for a hydrographic survey, which datum is most commonly used in the United States?
- NAD 27
- NAD 83 (Correct answer)
- WGS 72
- ITRF 2000
Correct answer: NAD 83
NAD 83 (North American Datum of 1983) is the standard horizontal reference datum used for hydrographic surveys conducted in the United States.
Question 5: What is the recommended minimum overlap between adjacent multibeam swaths in a hydrographic survey?
- 5%
- 10% (Correct answer)
- 20%
- 50%
Correct answer: 10%
A minimum of 10% swath overlap between adjacent lines is generally recommended to ensure full bottom coverage and detect any data gaps.
Question 6: In GNSS-based positioning for hydrographic surveys, what does PDOP stand for?
- Positional Dilution of Precision (Correct answer)
- Projected Datum of Position
- Phase Delay of Propagation
- Polar Dilution of Positioning
Correct answer: Positional Dilution of Precision
PDOP (Positional Dilution of Precision) is a measure of satellite geometry quality; lower PDOP values indicate better satellite geometry and more accurate positioning.
Question 7: Which water column phenomenon can cause a 'false bottom' return in single-beam echo sounding?
- Thermocline layer with strong acoustic impedance contrast (Correct answer)
- High salinity surface layer
- Turbulence near the vessel hull
- Sediment resuspension at depth
Correct answer: Thermocline layer with strong acoustic impedance contrast
A strong thermocline (sharp temperature gradient) creates an acoustic impedance contrast that can reflect sonar energy, producing a false bottom echo above the true seafloor.
CH Field Survey & Data Collection 2
What is the primary purpose of a vessel's motion reference unit (MRU) during hydrographic surveys?