How does wire-drag surveys determine the presence of underwater obstructions?

The method of wire-drag surveys effectively determines the presence of underwater obstructions by employing a taut wire that forms a "V" shape. This technique involves fishing a wire down into the water while the boat moves forward. As the wire is dragged along the seabed, any obstructions will create disturbances in the wire tension. When the wire encounters an object, the tension changes, which can be sensed and recorded by the survey crew. This change in tension indicates the presence and sometimes the nature of the obstruction beneath the water.

This method is particularly advantageous because it allows surveyors to identify obstacles that might not be detectable through other means, such as sound waves or visual inspections. Wire-drag surveys are often used in locations where other survey techniques may not be feasible or effective due to environmental conditions.

In contrast, temperature changes do not provide direct information regarding underwater obstructions, sound wave echoing is primarily used in sonar surveys and is more suited for mapping or detecting objects rather than the physical presence of obstacles, and visual inspection from a boat may miss submerged structures, especially if they are deeper than the water's surface visibility.