The Acoustic Advantage

Seawater is an excellent conductor of sound but a terrible medium for light. While high-frequency radio waves are quenched instantly, a $12\,\text{kHz}$ acoustic pulse can travel through the entire depth of the Marianas Trench and back.

Multibeam Echosounders (MBES)

Modern survey vessels use Multibeam Echosounders to generate a 'swath' of depth data. Unlike a single-beam sounder that only looks down, an MBES fires a fan of acoustic beams (up to 512 beams) across the ship's path.

By measuring the Time of Flight (ToF) and the angle of each returning echo, the system builds a 3D digital elevation model (DEM) of the sea floor.

Sub-Bottom Profiling (SBP)

For cable burial, knowing the sea floor's surface is not enough. We must know what lies beneath the sand. Sub-Bottom Profilers use low-frequency "chirps" that penetrate the top 50-100 meters of sediment.

Reflections from various geological layers allow engineers to distinguish between soft silt (easy to plow), hard clay, and solid bedrock (which could strip the armor off a cable).

Underwater Navigation: USBL & INS

GPS does not work underwater. To track an ROV (Remote Operated Vehicle) laying cable at 3,000 meters depth, we use Ultra-Short Baseline (USBL) positioning.

A transducer on the ship sends an acoustic signal to a transponder on the ROV. By measuring the phase difference of the reply across three localized receivers on the ship's hull, the system calculates the ROV's position relative to the ship.

Calculating the angle of arrival ($\theta$) using phase difference ($\Delta \phi$).

Conclusion

Marine survey physics is the invisible guide for our digital world. By mastering the behavior of sound in a high-pressure, liquid environment, we ensure that the physical infrastructure of the internet remains safe and stable on the planet's most inhospitable frontier.