Constant tensioner for a seismic marine cable

A system for maintaining a substantially constant towing force and for minimizing the average displacement relative to a shipboard reference of a seismic detector cable towed by a ship, the detector cable having a tow line secured to its head end, comprising:

• a tensiometer operatively interfaced with said tow line for measuring and sampling, at desired intervals, the intantaneous towing force exerted by said towed cable;
• a servo-controlled capstan having a rotatable capstan drum mounted on said ship, the leading end of the tow line being secured to said capstan drum;
• means including a multiturn shaft encoder coupled to said capstan drum for measuring and sampling, at desired intervals, the instantaneous displacement of said tow line relative to a shipboard reference;
• a microprocessor interconnected with said tensiometer, said multiturn shaft encoder and said servo-controlled rotatable capstan drum for maintaining a running average of the instantaneous force measurements, for determining a desired statistic therefrom and for applying said statistic to said running average to define upper and lower force limits of a towing force window so that when the instantaneous towing force exceeds the upper force limit, the microprocessor causes said capstan to pay out a length of tow line and when the instantaneous towing force becomes less than the lower force limit, the microprocessor causes said capstan to retrieve a length of tow line;
• said microprocessor including further means for determining a running average of the sampled tow-line displacement measurements and for establishing fore and aft displacement limits relative to said running average displacement;
• and
• means associated with said microprocessor for causing said servo-controlled capstan drum to restrict the amount of tow-line payout when the running average displacement approaches said aft displacement limit and to restrict the amount of tow-line retrieval when said running average displacement approaches said fore displacement limit.