Applied Seismology: Introduction and Principles
4.6 Detection and recording of seismic waves
4.6.2 Hydrophones and streamers
A hydrophone is in essence a waterborne equivalent of a geophone, although the principle of operation is quite different. A hydrophone responds to variations in pressure, whereas a geophone responds
μ < 1; Underdamped; most sensitive; oscillates μ = 1; Critically damped; returns to zero most quickly μ > 1; Overdamped; returns to zero slowly
TIME
DeflectionOutput/input Natural Frequency
16
8
4
2
1
30%μc 50%μc 70%μc
½ fo fo 2fo
FREQUENCY
4fo 8fo 16fo
(A)
(B)
(C)
0
Seismic detector response curve Output vs frequency Type SM4
Natural frequency : Coil resistance : Sensitivity 0.73 V-I-S Shunt Ω
10 Hz 375 ohms 0.29 V-CM-S Damping % Model B
8.0 6.0 4.0
2.0
Output V-I-S
1.0 0.8 0.6 0.4
0.2
1 2 3 4 5 6 8 10 20
Frequency - Hz
30 40 50 60 80100 200 400 600 25 39 48.5 68.5 93 OC
3.9 K 2.2 K 1.0 K 510
Figure 4.35 (A) Deflection as a function of time for underdamped, critically damped and overdamped modes of a geophone. (B) The amplitude response as a function of frequency in three states of underdamping to critical damping (µc). (C) Variation of response with frequency for a typical moving-coil geophone. Courtesy of Sensor Nederland BV.
Figure 4.36 The earliest known seismoscope was developed in 132 by Heng Zhang and was used successfully to indicate the occurrence of an earthquake in the year 138. Courtesy of Stop Disasters and the World Intellectual Property Organization.
to ground particle motion. Hydrophones can be used in open wa- ter, down a water-filled borehole, and in water-saturated marshy conditions.
A hydrophone consists of two piezoelectric ceramic discs (e.g.
barium titanate or lead zirconate) cemented to a hollow sealed cop- per or brass canister (Figure 4.37A). A pressure wave effectively squeezes the canister and bends the piezoelectric disc, thus generat- ing a voltage between the top and bottom of each disc. The two discs are polarised and connected in series so that the voltage generated by the passage of a pressure wave (i.e. seismic wave) are added, but those due to acceleration will cancel (Figure 4.37B). Piezoelectric hydrophones have a high electrical impedance and thus any signals must pass through impedance-matching transformers before being transmitted through to the recording instruments.
In marine work, hydrophones are deployed in a streamer, a ma- rine cable up to 9 km long, which is designed to be towed contin- uously through the water. A streamer is made up of a number of elements (Figure 4.38). The main section comprises active or live sections (groups) in each of which 30 hydrophones are connected in parallel with impedance-matching transformers. In multiplexing cables, the signals from each hydrophone group are amplified, mul- tiplexed, and converted into digital form within the cable, and then transmitted to the recording vessel along a single wire. The individ- ual hydrophones are acoustically coupled to the surrounding water by being immersed in oil, which also assists in keeping the streamer neutrally buoyant. Such streamers are typically towed in the depth
range of 5–9 m. High-resolution streamers, as used in engineer- ing type investigations, are normally towed at the water surface. In 2007, PGS introduced a new form of streamer (GeoStreamerR) that is filled with a Buoyancy Void Filler, which is introduced into the streamer as a fluid but transforms into a solid gel during a curing process. The GeoStreamerR also uses dual pressure and particle ve- locity sensors that allow the streamer to be towed at greater depths, typically 15–25 m, and is thus less affected by surface weather noise.
The dual sensor also permits the receiver ‘ghost’ wave to be removed, thereby improving the signal-to-noise ratio across all frequencies and giving better vertical resolution. The reflection wavelet’s ghost reflection (from the reflection of the upgoing wave on the under- side of the sea surface), as sensed by the particle velocity sensor, has the opposite polarity to that of the same ghost reflection wavelet as sensed by the pressure sensor. Combining the two cancels out the ghost reflection (Carlson et al., 2007; Tenghamn and Dhelie, 2009).
The streamer skin, which is kept from collapsing by plastic bulk- heads in the case of oil-filled streamers, is made either of PVC for use in warm water, or of polyurethane for use in cold water. If the wrong skin is used – for example, if a warm-water skin is used in cold water – the streamer stiffens, which generates acoustic noise as the cable is towed through the water.
From the front end of the streamer, the first section encountered is the towing bridle, which takes the strain of towing and connects the hydrophone communication cables to the ship’s recording systems.
At the rear end of the lead-in section is a depressor hydrovane (paravane), which is used to keep the nose of the streamer at a given level below the surface. Immediately aft of this is a compliant or stretch section which is designed to absorb and attenuate the jerks caused by uneven tow rates and/or sea conditions, and to isolate the streamer from shocks to the ship (such as ploughing through a swell). Another stretch section is located right at the end of the streamer where it is connected to the tail buoy, which is a polystyrene raft housing a radar reflector and radio antenna, and is used to isolate the streamer from jerks from uneven movement of the tail buoy.
Along the line of the streamer, depth controllers are located at spe- cific points. These devices have servo-controlled fins which change their aspect in order to keep the streamer at a predefined depth below the surface. Pressure sensors on each controller are used to measure float depth, and if the streamer deviates from the required level the fin angles are adjusted to compensate. Also, a series of compasses is located along the length of the streamer (Figure 4.39) and each transmits signals back to the ship so that the position of each active group of the streamer can be determined and plotted (see Section 6.2.2.3). Different commercial operators may deploy their own systems. One system (NautilusR, developed by Sercel) comprises a series of tri-finned acoustic transceivers (Figure 4.39) spread along the streamer on airgun sub-arrays and the tailbuoy or navbuoy. These provide accurate and quality-controlled acoustic range measurements to the navigation system. If the ship is towing a long streamer across a side current, which causes the streamer to drift off track (called feathering ), the feather angle and the streamer position can still be determined (Figure 4.40). Such information is
Figure 4.37 (A) Hydrophone construction, and (B) acceleration-cancelling hydrophone. Courtesy of SSL.
vital for subsequent data processing. Knowing the position of each towed element is vital not only for the success of the seismic survey, but also for marine safety. Seismic surveys are sometimes under- taken in busy shipping lanes, and other ships need to know where the streamers are so as to be able to avoid snagging them. When a streamer is not being used it is coiled up on a drum on the ship’s deck (Figure 4.41).
Knowing the precise location of the streamers deployed is all the more important when three-dimensional seismic data are being acquired. Usually, up to four streamers, each up to 6 km long, are deployed from two ships steaming parallel to each other. In addi- tion, there may be four seismic gun arrays deployed from the same
two ships. The most modern seismic vessels are capable of deploying 12–16 streamer arrays each 6 km long, and longer in special cases, 9 km not being unusual. In order to be able to deploy so many stream- ers, a specially designed fleet of ships has been built (the Ramform series) based on the Norwegian intelligence-gathering ‘stealth’ ship, the Marjatta. The 83 m long Ramform Explorer, which is oper- ated by the Norwegian Petroleum Geo-Services Group (PGS), was radically different from previous seismic vessels when she entered service in 1995 in that she is triangular in shape with a 40 m aft beam (Figure 4.42A) and automatic streamer spooler system. The latest Ramform S-class vessels are designed to tow up to 22 streamers.
The wide beam and massive deck space of this ship are sufficient
T
ail buoy with radar reflector Cable reel on stern of ship
Towing bridle Lead-in section
Depressor paravane
Compliant section to isolate streamer shocks from ship
Depth controller
Live section containing
~ 20-100 hydrophones in 12.5- 100 metres length.
Group 1 Dead section
Group 2
Depth controller on dead section
Group 48 (or 96)
Compliant tail section to isolate from tail
buoy jerking
Figure 4.38 Basic structure of a hydrophone streamer. From Sheriff (2002), by permission.
(A)
(B) (C)
Figure 4.39 (A) Three-dimensional multi-streamer deployment. Each streamer has a series of fins (B) by which to control its position – Nautilus system (courtesy of Sercel); (C) integrated tailbuoy in use (courtesy of Seamap). [C]
Direction of cross current
Direction of ship’s movement
Direction of cross current
Successive streamer positions
Figure 4.40 Schematic plot of successive streamer locations – significant feathering occurs caused by strong cross-track currents.
to accommodate a Chinook-size helicopter operation. This allows for complete crew changes at sea, thereby increasing the ship’s pro- ductive time. The simultaneous use of up to 17 streamers is for the acquisition of 3D seismic data. The Ramform Sovereign, which be- came operational in 2008, set the record (to date in February 2009) for towing 17 streamers, the widest tow (14×100 m), and the largest total streamer length deployed (14×8100 m). When fully deployed these ships are each towing in excess of$60 million worth of sensi- tive seismic equipment (Greenway, 2009). The distinctive and novel Ulstein X-BOW (Figure 4.42B) is being used for a new generation of seismic survey vessels that came into operation in 2009, the first of which was WG Columbus, operated by WesternGeco. These ships are capable of deploying up to 12 streamers each and are designed for both 2D and 3D data acquisition, depending upon the individual vessel specification.
Towed streamers have also been developed for use on land (‘land streamers’) and have been in use since the mid-1970s (e.g. Kruppen- bach and Bedenbender, 1975, 1976). However, a significant amount
Figure 4.41 Coiled streamers on the aft deck of the Ramform Sovereign survey vessel. From Greenway (2009), by permission. [C]
(A)
(B)
Figure 4.42 (A) Ramform Explorer in operation showing the distinctive broad stern. Photograph courtesy of PGS Exploration, Norway. (B) The distinctive Ulstein XBOWR design of a new series of seismic survey vessels operated by WesternGeco. c Polarcus Ltd. Both types of vessels can deploy up to 12 streamers simultaneously. [C]
Figure 4.43 Detail of a geophone mount and stabilising wing on a land streamer. Courtesy of Geostuff. [C]
of development has taken place in the last decade and commercial systems are now available. Geometrics has provided an overview of land streamer systems in their March 2003 issue of Geoprofiles.
In essence, the traditional seismic spread layout is exactly the same as for a fixed land survey, except the geophones have their ground spikes removed and are instead fixed to some form of baseplate to aid ground coupling and in turn broad webbing or other non- stretch material that is used to tow the system (Figure 4.43) but which itself does not provide any inter-geophone coupling. The geophones are connected to the usual multi-core take-out cables that are secured to the top of the webbing. Wings are variably available to provide stability to the spread as it is being towed so that it does not flip over or become entangled as it is moved along.
Van der Veen and Green (1998) and Van der Veen et al. (2001) provided more details of a system developed at ETH in Switzerland.
Some workers have used wooden blocks to mount and protect each geophone (COWI, Denmark); Kansas Geological Survey mounts the geophones inside heavy duty fire hose. Each geophone is screwed into an external three-pointed cutter that carves a groove into the ground surface over which it is being dragged. Loose material is pushed aside and light vegetation is sliced, leaving firmer ground that improves coupling. Additional weights are added to improve the ground coupling as necessary. The fire hose protects the cable, reduces tow noise, isolates each receiver and is strong enough to be pulled by a vehicle. Where standard geophones are used in a towed array, they must be used correctly and be protected from damage through being dragged over the ground surface. It is important therefore that the geophone array is the correct way up when being dragged across the ground and when in use. In Montana Tech’s system, the geophones are gimballed and mounted onto a broad weighted rubber mat with the cables secured on the upper side to avoid becoming entangled. This system has been developed further so that four separate land streamers in a 96-channel 3D array can be towed behind a vehicle simultaneously with a separate vehicle deploying the seismic source.
Unlike marine deployment of streamers where the data acqui- sition is continuous, the land streamer method is ‘stop-start’. The streamer is dragged to the required position and left to stand. The seismic source is moved to the back of the array and fired to pro- vide the initial shot record. The source is then moved up the array in sequence. Once the source has arrived at the front of the array the shot-front receiver offset is maintained. After each shot the source and land streamer are moved up one shot increment and, once the streamer has settled for a few seconds, another shot is fired.
The process is then repeated as many times as required to cover the designated profile length.
For use in snow-covered areas, a gimballed geophone ‘snow streamer’ has been developed (Eiken et al., 1989) after field trials for an exploration survey in 1988 in Svalbard. The system comprised a main cable with a central stress member surrounded by insulated conductors. Half-gimballed geophones were connected to the main cable by short geophone cables up to a few metres long. The sur- vey was carried out using a tracked over-snow vehicle towing the snow streamer. Each geophone sensor was self-orientating along one horizontal axis and was enclosed in an oil-filled cylindrical metal case (20 cm long, diameter 4.5 cm with a mass of 1 kg). The weight of the sensor in its case coupled well with soft snow. For each shot, the streamer was stationary. No appreciable difference in data quality was noted between data acquired using the streamer and using standard spiked geophones implanted within the snow cover. However, the increased speed of surveying achieved using the streamer resulted in significant savings in survey time and hence costs. The snow streamer has also been used in the Antarctic. Given the increased amount of exploration work, particularly in the Arc- tic, both onshore and over sea-ice, the use of the snow streamer is likely to increase.