CH Field Survey & Data Collection 5

Question 1

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

Accepted Answer

Characterizing seafloor sediment type and habitat classification

Answer

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

Question 2

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

Accepted Answer

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

Answer

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.

Question 3

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

Accepted Answer

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

Answer

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.

Question 4

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

Accepted Answer

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

Answer

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.

Question 5

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

Accepted Answer

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

Answer

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.

(CH) Certified Hydrographer Practice Test

(CH) Certified Hydrographer Practice Test

CH Field Survey & Data Collection 5