Method and system for joint monitoring of non-dispersive frequency band and mechanical tension in structures

A computing device for jointly estimating a tension T and a critical frequency fc in a tensionable element, the computing device comprising:

• an interface for receiving data measured with plural particle motion sensors along the tensionable element, wherein the data is associated with waves that propagate along the tensionable element;
• and
• a processor connected to the interface and configured to,
• apply a low-pass filtering to the data, in a non-dispersive frequency band [0, fc], to obtain a phase velocity of the wave;
• estimate the tension Tin the tensionable element based on the phase velocity of the wave;
• calculate the critical frequency fc corresponding to the tension T based on a model which links the critical frequency to the tension;
• and
• repeat the steps of applying, estimating, and calculating until a difference between a previous tension Tor a previous critical frequency fc and a current tension T or a current critical frequency fc, respectively, is smaller than a given threshold value,
• wherein the critical frequency fc defines a border between the non-dispersive frequency band and a dispersive frequency band of the tensionable element.

A streamer to be towed in water for collecting seismic data, the streamer comprising:

• a body;
• plural particle motion sensors located inside the body;
• and
• a computing device located inside the body, the computing device including,
• an interface for receiving data measured with plural particle motion sensors along the tensionable element, wherein the data is associated with waves that propagate along the tensionable element;
• and
• a processor connected to the interface and configured to,
• apply a low-pass filtering to the data, in a non-dispersive frequency band [0, fc], to calculate a phase velocity of the wave;
• estimate the tension T in the tensionable element based on the phase velocity of the wave;
• calculate the critical frequency fc corresponding to the tension T based on a model which links the critical frequency to the tension;
• and
• repeat the steps of applying, estimating, and calculating until a difference between a previous tension Tor a previous critical frequency fc and a current tension T or a current critical frequency fc, respectively, is smaller than a given threshold value,
• wherein the critical frequency fc defines a border between the non-dispersive frequency band and a dispersive frequency band of the tensionable element.