CH Bathymetric Data Acquisition & Processing 3 — Questions and Answers
Question 1: What is the principal advantage of using a Kongsberg SIS or similar acquisition software's 'runtime parameters' feature during an MBES survey?
- It permanently recalibrates the transducer between lines
- It allows real-time adjustment of swath width, power, and gain without stopping acquisition (Correct answer)
- It automatically updates the sound velocity profile from ambient sensors
- It exports data directly to a chart production system
Correct answer: It allows real-time adjustment of swath width, power, and gain without stopping acquisition
Runtime parameters enable the operator to dynamically tune sonar performance settings while maintaining continuous data collection.
Question 2: When using a single-beam echosounder (SBES), the depth recorded represents:
- The average depth across the full beam footprint area
- The minimum depth detected within the conical beam footprint (Correct answer)
- The slant range to the first return within the pulse length
- The nadir depth directly below the transducer center
Correct answer: The minimum depth detected within the conical beam footprint
SBES records the shallowest (earliest) return within the beam cone, which may not be the nadir point if bottom topography is irregular.
Question 3: A Conductivity-Temperature-Depth (CTD) cast is taken at the start of an MBES survey line but conditions change significantly by midday. The best practice is to:
- Continue using the morning SVP for the entire day to maintain consistency
- Take additional CTD casts at regular intervals or when water mass properties change (Correct answer)
- Extrapolate the morning SVP linearly to the afternoon conditions
- Apply a single fixed sound speed of 1500 m/s for the entire survey
Correct answer: Take additional CTD casts at regular intervals or when water mass properties change
Sound velocity profiles should be updated whenever environmental conditions change, particularly in areas with tidal mixing, thermoclines, or freshwater input.
Question 4: In the context of bathymetric data, 'backscatter intensity' primarily provides information about:
- Exact water depth at each sounding location
- Vessel motion at the time of data collection
- The acoustic reflectivity and hardness characteristics of the seafloor (Correct answer)
- The refraction correction applied to each beam
Correct answer: The acoustic reflectivity and hardness characteristics of the seafloor
Backscatter intensity reflects the acoustic energy returned from the seafloor, which correlates with sediment type, roughness, and hardness.
Question 5: Which statement best describes the relationship between pulse length and depth resolution in an echosounder?
- Longer pulse lengths improve depth resolution by averaging more returns
- Shorter pulse lengths improve depth resolution by reducing the along-track footprint
- Shorter pulse lengths improve range resolution by reducing the minimum detectable range difference (Correct answer)
- Pulse length has no effect on depth resolution in modern digital systems
Correct answer: Shorter pulse lengths improve range resolution by reducing the minimum detectable range difference
Range (depth) resolution is approximately half the pulse length in water; shorter pulses allow the system to distinguish between targets that are closer together in depth.
Question 6: For a hydrographic survey using RTK GPS, what is the primary limitation compared to using a tide gauge for water level correction?
- RTK cannot provide positions accurate enough for hydrographic use
- RTK requires a geoid model to convert ellipsoidal heights to chart datum, introducing potential model errors (Correct answer)
- RTK systems are incompatible with multibeam echosounder data formats
- RTK corrections are only valid within 5 km of the reference station
Correct answer: RTK requires a geoid model to convert ellipsoidal heights to chart datum, introducing potential model errors
RTK GPS provides ellipsoidal heights, and conversion to the sounding datum (e.g., MLLW) requires an accurate geoid model, which may introduce uncertainty.
Question 7: During MBES data processing, the term 'swath editor' or 'sounding editor' is used for:
- Automatically computing chart datum reductions for all soundings
- Manually or automatically flagging and removing erroneous soundings from the dataset (Correct answer)
- Generating final contour products from the cleaned sounding grid
- Calibrating the motion reference unit offsets
Correct answer: Manually or automatically flagging and removing erroneous soundings from the dataset
A swath or sounding editor is a post-processing tool that allows hydrographers to identify and reject erroneous soundings through visual inspection and automated filters.
CH Bathymetric Data Acquisition & Processing 3
What is the principal advantage of using a Kongsberg SIS or similar acquisition software's 'runtime parameters' feature during an MBES survey?