Monitoring the Coastal Environment; Part III: Geophysical and Research Methods
Acoustic and electromagnetic geophysical methods are widely used in coastal studies for determining water depth, identifying bottom sediment type and surficial features, locating man-made objects and hazards, and studying sub-bottom geology and structure. Acoustic methods are broadly divided into categories: 1. high-frequency systems such as echo-sounders whose main purpose is to determine water depth (minimal seafloor penetration); 2. lower-frequency systems with the ability to penetrate bottom sediments. Coastal hydrographic surveys must be conducted by qualified personnel with meticulous quality-control procedures. The maximum practicable achievable accuracy for coastal surveys using echo sounders is about ± 0.5 ft (0.15 m). High-resolution seismic surveys are used for engineering and for beach fill studies. The thinnest bed or layer that can be detected is about λ/4, where λ is the wavelength of the acoustic source.