Explain how sound is used to map the seabed and sub-surface.
Acoustic remote sensing is used to collect information about the shape and material properties of the seabed and subsurface. Sensors, traditionally deployed from ships and boats, are increasingly being installed on remotely operated or autonomous vehicles. These active remote sensing techniques transmit sonar (sound/acoustic) pulses and measure the two-way travel time for the pulse to be reflected, scattered, or refracted from the seafloor or subsurface and returned to the sensor. Knowing the speed of sound in water, sediment and rock, the relative position of the target on or below the seafloor can be determined. Absolute positioning of the target is determined by the integration of the sensor platform with ancillary Global Navigation Satellite Systems (GNSS) and motion referencing units (MRU). Reflected energy is used to determine the shape (or morphology) of the target, and scattered energy is used as a proxy for material properties. Applications of these methods have expanded significantly in recent years as coastal and marine landscapes are being transformed due to climate change impacts and anthropogenic forcing, with cables, pipelines, offshore renewables, and a broad range of complex engineering structures now ubiquitous on the seafloor.