An autonomous four-component seabed system offers a deepwater alternative to ocean-bottom cable.

Offshore seismic exploration is focusing more and more on water depths below 3,000 ft (915 m). Tenders for seismic surveys to 10,000-ft (3,050-m) water depths are on the increase. In addition, for enhanced resolution and an indication of the fluid content of a hydrocarbon reservoir, the acquisition of shear waves is becoming of interest. Therefore, data acquisition has to be carried out at the seabed.
These surveys are predominantly conducted with ocean-bottom cables (OBC), which have the following shortcomings:
• the cable substantially distorts the horizontal shear wave components;
• in rugged terrain, several three-component (3-C) geophones may not be coupled to the ground at all;
• in jagged areas, the cable may be damaged or cut;
• in rough seas, the vessel may run into trouble; and
• below 6,000 ft (1830 m), the weight/strain relation of the feeder cable becomes uneconomical.
Therefore, self-landing and -ascending ocean-bottom seismic (SLA-OBS) systems, which have been used by the scientific community for the past 20 years, are a promising alternative, especially for the ultradeep sea.
OBS in operation
In essence, an SLA-OBS system is an autonomous marine seismic station consisting of a buoyancy unit, an anchor weight, a recording package with a highly accurate clock, a hydrophone, a sensor assembly containing a 3-C geophone, and a battery pack. The geophone is used to detect the vertical and two horizontal displacements or accelerations of the ground; the hydrophone reacts to pressure variations in the water. These make up the four signal components of a 4-C system. Recent improvements in low-power electronics for data acquisition and storage (A/D converters, digital signal processors, storage cards) and crystal oscillator technology allow surveyors to produce data loggers for continuous four-channel data recording, capable of long-term deployment (up to 12 months) with only a few kilograms of batteries.
SLA-OBS systems operate autonomously. They do not need a cable link to a vessel. For deployment, they are heaved overboard by a crane and sink to the ocean bottom. They operate in water depths to 20,000 ft (6,100 m).
After landing and deploying the 3-C geophone, they begin recording the seismic signals. Time marks from an internal, temperature-compensated clock are inserted into the data stream to allow correlation to the shot events later. When shooting along profiles at the seabed is completed, the anchor is released using an acoustic trigger, and the SLA-OBS starts to ascend due to its buoyancy unit. Once it floats on the surface, a crane will pick it up, and the data can be retrieved for post-processing.
One benefit of its autonomous operation is the possibility of an arbitrary layout configuration with varying receiver point distances. Second, a survey can be conducted using only one ship. At the beginning, the systems will be deployed. The same ship serves as a shooting vessel, and in the end the systems are picked up again.
OBS systems also can be used to complement streamer surveying. The execution of refraction or wide-aperture measurement at the sea bottom allows surveyors to determine a detailed velocity model for depth conversion of the seismic.
Types of OBS
During the years, two types of SLA-OBS systems have emerged.
For scientific use, a glass sphere doubles as buoyancy unit and pressure case for the electronics, anchor release system, 3-C geophone and batteries. Although it has a low initial acquisition cost, the fragility of the glass sphere, long turnaround time from recovery to redeployment (1 to 2 hours) and the need for highly trained personnel are clear disadvantages.
For commercial exploration, the buoyancy unit is made from syntactic foam, and the electronics, anchor release system and batteries are contained in pressure cylinders. Although it costs about twice as much, its robustness and quick turnaround time (20 to 30 minutes for onboard service between pickup and redeployment) make it the more economical solution in a production environment. In addition, with the latter system, the 3-C geophone can be deployed a few feet away from the OBS structure, providing the necessary coupling for seismic frequencies above 40 Hz - a must for commercial exploration.
Design details
Figure 1 shows the production system. It originally was developed at the German Marine Research Institute (Geomar). Equipped with a recording unit that works at low power levels using an internal clock with excellent stability, it provides 30 days continuous recording at a 2 millisecond (ms) sampling rate. For seismological research, its recording period can be extended to 15 months using additional battery packs and a highly specialized long-term recorder.
The frame of this SLA-OBS consists of a rod carrying a floatation unit made from syntactic foam at the top and a tripod at the lower end. The anchor, attached to the tripod and kept in place by an acoustical release system, can be made from iron or concrete. One pressure cylinder contains the release system, another contains the seismic recorder and batteries. The hydrophone is attached next to the pressure cylinders. Once the instrument touches the seafloor, a crane arm system drops the 3-C geophone on the floor 3 ft (1 m) from the OBS. For recovery and positioning, a flag, radio beacon and flasher are mounted on top of the OBS.
Applications and data example
SLA-OBS systems have been used successfully in the scientific community for 20 years. For example, the Geomar system has been deployed more than 2,000 times with only six losses. Crustal studies using refraction measurements, 2-D reflection experiments and small 3-D surveys have been conducted. Gas hydrate research has been carried out using high-resolution reflection seismics at 0.1-ms sample rates, discerning layers only a few hundred feet thick.
The data example in Figure 2 shows a horizontal wave component of the 3-C geophone. The corresponding OBS was deployed at a depth of 10,000 ft (3,050 m) on the Pacific seabed. The seismic section shows 1,200 shot positions along a profile using two 32 l (2,000 in. each) air guns. Meaningful signals could be recorded with up to 50 miles (80 km) between shot position and OBS location.
Within the section, the earliest first breaks are generated by the "direct wave at the seabed" mark on the OBS position. Looking at a horizontal registration, it comes as no surprise that the "refracted P at basement" is rather weak. But less than a second later, the "refracted P converted to S" waves are clearly visible. An additional shear wave component is triggered by incident P waves, visible as "PS refraction."
The future
For 4-C measurements at the ocean bottom, SLA-OBS systems are a promising alternative to OBC systems. They can be used under rough sea conditions as well as in rugged terrain, and a survey can be conducted using just one ship for deployment, shooting and recovery.
For the ultradeep sea below 6,000 ft (1,800 m), SLA-OBS systems are the enabling technology. The scientific community is using them down to 20,000 ft (6,100 m) on a routine basis. The recovery rate is higher than 99.5% when carefully designed, rugged systems are used.
The next generation of SLA-OBS systems will have an optimized shape such that 40 systems will fit into one 20-ft (6-m) container. All metallic parts will be made from titanium, extending the system's life span. A consequent modular design will make it possible to adapt and improve components like the geophone, recorder, float and anchor according to various operational and geophysical requirements. The compactness simplifies the shipment to remote areas, thus avoiding the transfer cost of dedicated OBC vessels. Additional cost reductions will be incurred by reducing the maximum deployment depth to 12,000 ft (3,600 m) and simplifying the structure, building it from only a few modular parts that can be combined to realize systems for different payloads. This will open the possibility for 4-D surveys in the ultradeep sea.
Acknowledgement
Data examples have been extracted from "Geomar Report 96," courtesy of Geomar, Research Centre for Marine Geosciences.

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