1. Field of the Invention
This invention relates generally to marine seismic exploration, and particularly to a method and apparatus for reducing turbulent noise associated with cable balancing weights.
2. Discussion of the Related Art
Often in marine seismic exploration, an acoustic source and a seismic streamer cable are towed behind a vessel steaming above a subsurface zone of interest. At periodic intervals the acoustic source is activated, generating an acoustic signal which propagates through the earth and is reflected towards the surface by acoustic impedance differences existing in the subsurface. The reflected acoustic signals are received by detectors positioned along the length of the streamer cable positioned behind the vessel. The detectors may generate an optical or electrical signal indicative of the detected acoustic signal which is sent up the streamer cable to a remote recording and storage device on the vessel.
Both the acoustic source and the streamer cable are towed at a predetermined depth in the water to maximize signal strength and to reduce the noise floor of the received signals. In order to maintain the depth of the streamer cable, cable depth controllers (birds) and lead weights are necessarily employed. The lead weights are generally attached to the cable by a heavy-duty strength linen tape. The lead weights, usually in one pound sheets, are molded to the outer circumference of the cable and held fast by the tape. Often the layers of weight and tape are substantial, radically increasing the diameter of the cable several millimeters and up to over a centimeter within a lateral distance of less than two inches. In other words, rise to run ratio is great and the profile of the cable forms a steep profile. This sharp increase in the cable diameter causes turbulent flow as the streamer cable glides through the water. This turbulent flow is one factor which contributes to the noise floor present in the streamer cable.
Another disadvantage in the present technique of taping weight to the cable is expediency in application. The streamer cable must be reeled in to the point where the weight is to be applied. The one pound sheets of weight are formed to the cable exterior by hand and retained there by wrapping the weight in the linen tape. The fixing of the weight may take on the order of five minutes to assure the weight will not be lost. The cable is laid out to test if the appropriate amount of weight was applied. This procedure may be repeated several times until the cable is properly balanced.
Yet another disadvantage to the procedure of taping weights to the cable exterior is that the lead weights are often lost. Because the cable is emersed in water and the tape is exposed to the turbulent flow, the tape often unwraps, dropping the weight. Additionally, as the tape unravels, the trailing strand of tape introduces noise which is detected by the hydrophones adjacent the trailing tape.
It is an object of this invention to provide an exterior weight which reduces the noise floor caused by turbulent flow.
It is another object of this invention to provide an apparatus for applying weight to the exterior of the streamer cable in a fast and efficient manner.
It is another object of this invention to provide a device to retain the weight on the cable without requiring tape.