CH Field Survey & Data Collection 3 — Questions and Answers
Question 1: What is the purpose of running 'check lines' (cross lines) during a hydrographic survey?
- To calibrate the vessel's GNSS receiver
- To verify internal consistency of depth data between survey lines (Correct answer)
- To establish tidal datum benchmarks
- To measure acoustic velocity at the transducer face
Correct answer: To verify internal consistency of depth data between survey lines
Check lines run perpendicular to main survey lines; depth values at intersections are compared to assess systematic errors and data consistency.
Question 2: A sound velocity profile (SVP) cast is considered 'stale' when it no longer represents current water column conditions. What factor most commonly necessitates a new SVP cast?
- Change in vessel speed
- Significant change in water temperature or salinity with time or location (Correct answer)
- Transition to a new survey line
- Change in multibeam ping rate
Correct answer: Significant change in water temperature or salinity with time or location
Sound velocity is primarily driven by temperature, salinity, and pressure; changing oceanographic conditions (tidal mixing, fronts, freshwater inflow) require updated SVP casts.
Question 3: What does the term 'nadir' refer to in multibeam sonar surveys?
- The outermost beam angle of the swath
- The point directly below the transducer, at the center of the swath (Correct answer)
- The highest quality region at the swath edges
- The angle at which acoustic energy is most attenuated
Correct answer: The point directly below the transducer, at the center of the swath
Nadir is the point directly below the transducer (0° beam angle), which typically produces the most accurate depths but covers the narrowest portion of the seafloor.
Question 4: When conducting a bar check to verify echo sounder accuracy, a target plate is lowered to a specific depth. What is the correct procedure if the echo sounder reads shallower than the measured bar check depth?
- Increase the sound velocity setting in the echo sounder
- Decrease the sound velocity setting in the echo sounder (Correct answer)
- Adjust the heave compensation offset
- Recalibrate the transducer draft setting
Correct answer: Decrease the sound velocity setting in the echo sounder
If the echo sounder reads shallower than the true depth, the instrument is using too high a sound velocity; decreasing the sound velocity setting will increase the displayed depth.
Question 5: In tidal datum establishment, what is the relationship between Mean Lower Low Water (MLLW) and Mean Low Water (MLW) in a mixed semidiurnal tidal environment?
- MLLW and MLW are identical in all tidal regimes
- MLLW is always lower than MLW because it averages only the lower of each day's two low waters (Correct answer)
- MLW is always lower than MLLW in the US
- MLLW is used only in diurnal tidal areas
Correct answer: MLLW is always lower than MLW because it averages only the lower of each day's two low waters
MLLW averages only the lower low water of each tidal day, making it lower than MLW (which averages all low waters), providing a more conservative nautical chart datum.
Question 6: What is 'patch testing' (or 'system calibration') performed on a multibeam echosounder system designed to determine?
- The optimal sonar frequency for a given water depth
- Systematic angular offsets between the sonar, MRU, and vessel reference frame (Correct answer)
- The minimum detectable object size at various ranges
- The maximum operational water depth for the system
Correct answer: Systematic angular offsets between the sonar, MRU, and vessel reference frame
Patch testing determines roll bias, pitch bias, heading (yaw) bias, and latency offsets between the sonar transducer, motion sensor, and positioning system.
Question 7: What is the primary advantage of using Real-Time Kinematic (RTK) GNSS over DGPS for hydrographic positioning?
- RTK has a longer range from the base station
- RTK provides centimeter-level positioning accuracy versus decimeter-level for DGPS (Correct answer)
- RTK does not require a shore-based reference station
- RTK works better in deep-water environments
Correct answer: RTK provides centimeter-level positioning accuracy versus decimeter-level for DGPS
RTK uses carrier-phase measurements to achieve centimeter-level horizontal accuracy, significantly better than the sub-meter decimeter accuracy of standard DGPS.
CH Field Survey & Data Collection 3
What is the purpose of running 'check lines' (cross lines) during a hydrographic survey?