Underwater observation unit and system

A method for monitoring a spatial profile of a submerged surface or structure in time, using the underwater observation system, the method comprising:

• deploying first and second observation units underwater at non-coinciding first and second positions on or along the submerged surface or structure, followed by:
• emitting a first light signal with the first light source of the first observation unit towards the second observation unit;
• acquiring first image data of the first light signal with a second underwater imaging device of the second observation unit;
• emitting a second light signal with the second light source of the second observation unit towards the first observation unit;
• acquiring second image data of the second light signal with a first underwater imaging device of the first observation unit;
• determining first positional data of the second light source relative to the first imaging device, and second positional data of the first light source relative to the second imaging device, and
• storing the first and second positional data with timestamps, to form a dataset of time-dependent positional data.

A method for using an observation unit including an underwater imaging device with a digital imaging sensor, the method comprising:

• providing the underwater imaging device with refractor optics with a focal length that slightly exceeds a distance to the imaging sensor;
• acquiring image data by imaging a point-like light from a light source that is located within the FOV of the underwater imaging device out of focus, to generate a blob in the image data that extends over multiple adjacent pixels of the 2D pixel array of the imaging sensor;
• identifying a center of the blob relative to the 2D pixel array, and
• determining sub-pixel coordinates for the center of the blob, representative of an angular position of the light source relative to the imaging sensor.