Method and system of depth triggers for marine geophysical survey cable retriever systems

A ballast system comprising:

• a ballast attachment block comprising:
• a first end and a second end, each end defining an outside diameter configured to couple to an inside diameter of a geophysical survey cable;
• a first attachment location medially disposed on the ballast attachment block;
• a passage extending between the first end and the second end, the passage disposed within the ballast attachment block;
• a first ballast weight abutting the first attachment point, the first ballast weight defining a first aperture;
• a first depth release mechanism coupling the first ballast weight to the ballast attachment block, the first depth release mechanism comprising
• a piston disposed within a cylinder defined in the ballast attachment block, the piston defining an outer face exposed to ambient pressure;
• a link member coupled to the piston and to the first ballast weight, wherein at depths in water above a predetermined depth the link member holds the piston in a non-triggered state;
• and
• wherein at the predetermined depth and below ambient pressure acting on the outer face of the piston creates a tension force that breaks the link member and releases the first ballast weight from the first attachment point.