Bathymetric Data Acquisition & Processing Flashcards

What is the primary function of a multibeam echo sounder in hydrographic surveying?

Answer: Mapping underwater features using multiple beams

A multibeam echo sounder (MBES) is a sophisticated sonar system that emits multiple sound beams in a fan-shaped pattern perpendicular to the vessel's track. By measuring the time it takes for each beam to return, it can simultaneously collect depth data from a wide swath of the seafloor. This allows for efficient and detailed 3D mapping of underwater topography and features, providing comprehensive bathymetric coverage.

Why is sound velocity profiling important in bathymetric surveys?

Answer: To correct echo sounder data for refraction

Sound velocity varies with water temperature, salinity, and pressure, causing sound waves to refract (bend) as they travel through the water column. Sound velocity profiling measures these changes, providing data to correct the path and travel time of the sound pulses emitted by echo sounders. Without accurate sound velocity corrections, measured depths and positions of seafloor features would be inaccurate due to the bending of the sound beams.

Which of the following instruments is used to measure the depth of the seafloor?

Answer: Single-beam echo sounder

A single-beam echo sounder (SBES) is a fundamental instrument in hydrographic surveying used to measure the depth of the seafloor directly beneath the vessel. It emits a single pulse of sound vertically downwards and calculates the depth based on the time it takes for the sound to travel to the seafloor and return. This provides a precise depth measurement at a specific point, forming a profile along the survey line.

What is the role of motion sensors in bathymetric surveys?

Answer: Correcting for vessel motion

Motion sensors, such as Inertial Measurement Units (IMUs), measure the vessel's pitch, roll, heave, and yaw. These movements can significantly distort bathymetric data collected by echo sounders, causing inaccurate depth and position measurements. By integrating motion sensor data, these effects are precisely corrected, ensuring that the recorded seafloor depths are accurately referenced to a stable platform, regardless of sea state.

What does side-scan sonar primarily provide in a hydrographic survey?

Answer: Seafloor imagery

Side-scan sonar (SSS) is used to create acoustic images of the seafloor, providing a detailed visual representation of its morphology and any objects lying on it. Unlike echo sounders that measure vertical depth, SSS transmits sound beams sideways, mapping the intensity of the backscattered sound. This allows for the detection of features like wrecks, pipelines, and geological formations, complementing bathymetric data with textural information.

Which correction is typically applied to echo sounder data to account for tidal variations?

Answer: Tide correction

Tides cause the water level to fluctuate significantly throughout the day, directly impacting measured depths. To ensure all soundings are referenced to a consistent vertical datum, a tide correction is applied. This involves subtracting or adding the measured or predicted tidal height at the time of the sounding, effectively normalizing all depths to a common reference plane, such as Mean Lower Low Water (MLLW).