CH Bathymetric Data Acquisition & Processing 2 — Questions and Answers
Question 1: Which sonar beam geometry best describes a multibeam echosounder (MBES) system?
- A single narrow beam sweeping mechanically across track
- A fan of beams projected perpendicular to the vessel track (Correct answer)
- Multiple beams projected along the vessel track
- A wide omnidirectional pulse with time-of-flight ranging
Correct answer: A fan of beams projected perpendicular to the vessel track
MBES systems transmit a fan of beams oriented across track, enabling wide swath coverage in a single pass.
Question 2: When processing MBES data, what is the primary purpose of applying a sound velocity profile (SVP)?
- To correct for vessel heave during data collection
- To compensate for beam refraction caused by water column velocity gradients (Correct answer)
- To calibrate the transducer sensitivity across the swath
- To remove GPS position offsets from the navigation data
Correct answer: To compensate for beam refraction caused by water column velocity gradients
SVP corrections account for ray bending (refraction) as acoustic beams travel through water layers with different sound velocities.
Question 3: A hydrographer notices outer-beam soundings consistently read shallower than inner-beam soundings over flat bottom. The most likely cause is:
- An incorrectly entered sound velocity at the sonar head (Correct answer)
- Excessive vessel speed causing Doppler shift
- Tide gauge malfunction affecting water level corrections
- GPS multipath errors during the survey
Correct answer: An incorrectly entered sound velocity at the sonar head
Using an incorrect surface sound velocity at the transducer face causes outer beams to refract incorrectly, producing characteristic 'smiling' or 'frowning' artifacts.
Question 4: In IHO S-44 standards, the 'Special Order' survey category is primarily intended for:
- Open ocean passage routes with depths over 200 m
- Areas where under-keel clearance is critical, such as harbor entrances (Correct answer)
- Coastal reconnaissance surveys with relaxed position tolerances
- Inland waterways with no commercial traffic
Correct answer: Areas where under-keel clearance is critical, such as harbor entrances
Special Order is the most stringent IHO category, reserved for critical harbor, berthing, and canal areas where every seafloor feature must be detected.
Question 5: What does the term 'patch test' refer to in multibeam survey operations?
- A software calibration routine that removes thermal noise from raw data
- A field calibration procedure to determine transducer mounting angle offsets (Correct answer)
- A post-processing filter applied to remove spike noise from swath data
- A tidal zoning correction applied across a large survey area
Correct answer: A field calibration procedure to determine transducer mounting angle offsets
A patch test (or MBES calibration run) measures residual pitch, roll, yaw, and latency offsets in the transducer installation.
Question 6: Which motion sensor parameter, if incorrectly measured, would cause a depth error that increases with heave amplitude but is independent of swath angle?
- Roll bias
- Pitch bias
- Heave (Correct answer)
- Yaw (heading) bias
Correct answer: Heave
Heave errors directly add or subtract from measured depth values uniformly across all beams, independent of beam angle.
Question 7: During data cleaning of MBES soundings, a 'hockey stick' artifact at the outer swath edges most commonly indicates:
- A calibration error in the vessel's GPS antenna offset
- Residual refraction error from an inaccurate or poorly timed SVP cast (Correct answer)
- Incorrect vessel draft entry in the acquisition software
- Magnetic compass interference affecting heading data
Correct answer: Residual refraction error from an inaccurate or poorly timed SVP cast
Residual refraction errors from an outdated or inaccurate SVP cause characteristic upward or downward curving artifacts at the outer beams.
CH Bathymetric Data Acquisition & Processing 2
Which sonar beam geometry best describes a multibeam echosounder (MBES) system?