Offshore oil exploration requires use of a complex signal detection system, of which the marine streamer is the transducer portion. The integrity of the streamer directly affects the quality of the recorded seismic data.
Most streamers in use today are transformer coupled. This gives satisfactory signal-to-noise ratios and frequency response characteristics over the long lead lines to the boat. The marine streamer test system MARSTEST was developed to thoroughly evaluate each transformer-coupled hydrophone group (section) before each survey. It also continuously monitors the streamer during the survey, for output anomalies that might develop. MARSTEST was designed to "see through" the transformer, thus enabling actual evaluation of the hydrophone sensors - an ability not available in other test systems. Figure 1 is a photograph of the prototype unit.
As illustrated in Figure 2, a marine seismic streamer is composed of different, coupled sections. These sections are:
Ree1-to-doghouse cable - signal connection between storage reel and instrument room (doghouse) patch panel.
Lead-in section - signal cable with a strain member; connects head of streamer to storage reel and reel-to-doghouse cable.
Pig section - heavy section used to depress head end of streamer.
Stretch section - unit to absorb shock and reduce mechanical noise.
Live section - primary section, containing hydrophone group.
Depth-transducer/water break section - contains depth-detector unit and water break hydrophone
A streamer section (except the reel-to-doghouse cable and the lead-in section) is made up of a tubular plastic jacket over a section carcass that includes: the strain wires; spacers separating the strain wires and protecting other components; operating devices (hydrophones or a depth transducer); and a signal wire bundle. Couplers on each end of the section provide mechanical and signal continuity between adjacent sections and act as fluid seals. Each section is filled with a low-density, incompressible fluid that provides the buoyant force necessary to float the streamer. Desired depth is adjusted by the combination of fluid fill pressure and adding lead plates, or by pressure-actuated wings on a depth controller.4
Detailed design and carefully controlled construction have allowed streamers to acquire seismic data commensurate with the digital standards of today. The live section-hydrophone transformer circuits are dynamical1y designed to provide flat frequency response in the 10- to 100 Hz seismic range. Although signal amplitudes are in the microvolt region, satisfactory signal-to noise ratios have been achieved through the use of acceleration-canceling hydrophones, stretch sections, and streamlined cable.
However, even the best design sometimes exhibits a flaw, especially in a hostile environment. The bitter foe of the streamer is salt water, in which it is forced to exist for days at a time. Minute penetrations can cause leakage paths across the many connections that the signal must pass through. Even worse, a slight residue across a hydrophone circuit can drastically disrupt its dynamic characteristics.
