Field Survey & Data Collection Flashcards

What is 'acoustic backscatter' data collected during multibeam surveys primarily used for?

Answer: Characterizing seafloor sediment type and habitat classification

Acoustic backscatter intensity reflects how much sonar energy returns from the seafloor, which varies with sediment type, roughness, and hardness, enabling bottom classification.

When GPS satellite signals are temporarily blocked by a bridge or cliff face during a survey, what technique is commonly used to maintain positioning?

Answer: Dead reckoning using vessel speed and heading from the IMU/gyro

Dead reckoning uses the vessel's last known position plus integrated velocity and heading data from the IMU to estimate position during brief GNSS outages.

What is the recommended practice for collecting a sound velocity profile (SVP) in a stratified estuary with significant freshwater inflow?

Answer: Cast at multiple locations representing different salinity zones and at regular time intervals

Stratified estuaries have strong horizontal and vertical sound velocity gradients that change with tidal phase, requiring spatially distributed and time-repeated casts throughout the survey.

In a hydrographic survey, what is 'tidal zoning' used for?

Answer: Dividing survey areas to apply different tidal datum corrections based on local tidal characteristics

Tidal zoning divides the survey area into zones where different tide gauge records or tidal models are applied for depth reduction, accounting for spatial variation in tidal phase and range.

What is the purpose of the 'roll bias' component determined during multibeam patch testing?

Answer: To correct the systematic angular offset between the transducer mounting and the vessel's true roll axis

Roll bias corrects for misalignment between the transducer's reference plane and the MRU, which would otherwise cause the swath to appear tilted relative to the true horizontal.

Which factor primarily determines the maximum range of a side-scan sonar system for a given operating frequency?

Answer: Acoustic absorption and spreading losses in the water column

Higher frequencies provide better resolution but suffer greater absorption over distance, limiting range; lower frequencies travel farther but produce coarser imagery.

During a hydrographic survey, a surveyor notices that check-line crossings show consistent depth differences with the main survey lines on one side of the survey area. What is the most likely cause?

Answer: A systematic error such as a tidal reduction error or unmodeled sound velocity gradient

Consistent, spatially correlated discrepancies at crossings typically indicate a systematic error — such as an incorrect tidal zone boundary, stale SVP, or unmodeled refraction — rather than random noise.