Applied Seismology: Introduction and Principles
4.5 Seismic energy sources
4.5.6 Vibrators .1 Vibroseis
On land, an alternative to explosive or impulsive sources is the use of a vibrating plate to generate a controlled wave train. A system developed by Conoco, known as Vibroseis and described in detail by Baeten and Ziolkowski (1990), consists of a vibrator plate mounted on the underside of a special truck. When on location the plate is set on the ground surface and the truck is jacked up so that its weight is transferred to the plate (Figure 4.28). A low-amplitude sinusoidal vibration of continuously varying frequency (between 60 and 235 Hz) is applied over a sweep period which lasts between 7 and 60 seconds.
Progressively increasing frequencies are used in upsweeps and progressively decreasing frequencies with time are used in down- sweeps. Usually the change of frequency is linear, although non- linear sweeps may be used where higher frequencies are used for longer to compensate for loss of high-frequency information through the propagation of the signal. The resulting field record is the superposition of the reflected wavetrains (Figure 4.29) and is correlated with the known source sequence (pilot sweep). At each sample point along each trace the pilot is cross-multiplied with the superimposed signals to produce a correlogram trace. When the pilot sweep matches its reflected wavetrain (i.e. autocorrelates) it produces a significant zero-phase wavelet (known as a Klauder
Vaporchoc (A)
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Heat insulation
TUNNING THE GUNS WITHIN ONE SUBARRAY
REDUCING THE EFFECTS OF THE FORERUNNERS WITHIN ONE SUBARRAY
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Figure 4.25 (A) The Vaporchoc system. (B) and (C) The use of Starjet to modify the source characteristics.
wavelet) on the trace (Figure 4.29). The display of adjacent cor- relogram traces produces a seismic section or correlogram which resembles its conventional seismic counterpart obtained using a high-energy impulsive source.
A major advantage of the Vibroseis method is that it is a rapid and easily repeatable source that is convenient to use, especially in urban areas. The cross-correlation data-reduction method enables the extraction of sensible signals even in areas with high cultural noise. Furthermore, the Vibroseis method does not damage the ground surface and thus can be used with care on macadamised roads and over public utilities.
Table 4.5 Principal applications of commercial waterborne site investigation using high-resolution seismic methods.
Near/inshore marine/estuarine environments:
Bridges, tunnels, viaducts
Harbours, jetties, quay walls, marinas, canals Pipelines and sewage outfalls
Dredging for access channels to ports/harbours Marine:
Hydrocarbon pipelines
Hydrocarbon production platforms/wellheads Siting of drilling rigs
Shallow gas surveys Sand and gravel resources
Dredge and contaminated spoil-dump surveys Wind-farm foundation surveys
In order to increase the energy input into the ground for greater depth penetration, a number of vibrators can be used simultane- ously in a group provided they are phase-locked, i.e. they all use the same sweep at exactly the same time. Multiple sweeps can be used and summed (stacked) to improve the signal-to-noise ratio. It is possible to improve the results from vibroseis methods for shal- low investigations using the groundforce signal of the vibrator as a correlation operator and applying sweep deconvolution (Buness, 2007).
Very large vibrators have also been developed for crustal and earthquake studies where the actuator mass is of the order of 100 tonnes (Kovalevsky et al., 2009). Vibroseis interferometry was used with 10-minute periods of vibration at three separate frequencies (7, 8 and 9 Hz).
4.5.6.2 Small-scale land vibrators
On a much reduced scale, the Vibroseis principle can be used in the form of a number of small vibrator sources. One of these is the Mini-Sosie source which consists of a pneumatic hammer im- pacting on a baseplate. The thumper delivers a random series of blows to the baseplate on which a detector is located which records the source sweep. The variable-frequency source wavetrain with low amplitude is transmitted into the ground. The detected sig- nals are cross-correlated with the recorded pilot sweep to produce correlograms. The source can produce 10 pops/second and sev- eral hundred pops are summed at each shot point. The frequency range of this method is higher than for dynamite, thereby provid- ing higher resolution. Another type of P- and S-wave vibrator is Elvis (Electrodynamic Vibrator System) (Figure 4.30A), which uses sweep frequency control software to control the operation of both the seismograph (e.g. Geode) and the vibrator from the same com- puter. It can generate a peak force of∼450 N, has a frequency range of 5–360 Hz and can generate signals to about 100 m, and to 200 m for zero-offset vertical seismic profiles (VSPs).
For a shallow hydrocarbon exploration, mining or geotechnical surveys, a number of small land vibrators were developed during the early 1990s. One of these was described by Christensen (1992).
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Figure 4.26 (A) Boomer plate with a surface-tow catamaran; (B) a Squid sparker system, and (C) a Delta sparker system where the electrode tips are the spikes within the A-frame. (B) and (C) courtesy of Applied Acoustics.
His device consists of a 0.91 m diameter baseplate on which a 136 kg mass is located (Figure 4.30B). A small engine drives a hydraulic pump which energises the vibrator over a frequency bandwidth of 10–550 Hz at 4.4 kN output. The peak force is 30.5 kN. By slanting the mass-actuator through 45◦(Figure 4.31), both P- and S-waves can be generated. A PC-based controller drives the vibrator using open-loop amplitude and phase control. It also measures the
Table 4.6 Theoretical resolution and depth penetration of three common high-frequency source types.
Frequency Resolution Depth of Source bandwidth (kHz) (m) penetration (m)
Pingers 3.5–7 0.1–1 ≤tens
Boomers 0.4–5 ≈1 tens to 100+
Sparkers 0.2–1.5 2–3 ≥1000
output of the vibrator, utilising appropriate sensors, and records this information for signal correlation purposes.
It is becoming increasingly important to measure in situ stiffness of the ground rather than relying solely on measurements made on samples in a laboratory. One way to achieve this is to use a Rayleigh wave generator. A mass is suspended in a small gantry (Figure 4.32) that can be excited by a signal oscillator. The inertia of the mass is such that the vibrations that are in a horizontal plane are used to strain the ground by generating Rayleigh waves (Abbiss, 1981;
Powell and Butcher, 1991). See Section 5.6.2.
4.5.6.3 Marine vibrators
In 1988, Western Geophysical introduced the MultipulseTM(Hy- droacoustics Inc.) marine vibrator, which operates over the fre- quency range 5–100 Hz and has a sweep period of 6 seconds. The
Squid 2000 at 1500 J
Delta Sparker at 6000 J
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Figure 4.27 Typical pulses from (A) a squid sparker at 1.5 kJ and (B) a delta sparker at 6 kJ. Courtesy of Applied Acoustics.
advantages of this system over traditional airgun arrays are the minimal disturbance to marine life due to reduced cavitation; a smaller radius of interference away from the source so that it can be operated near production platforms without affecting diving activ- ities; and particularly, that the source characteristics are extremely well known and controlled. The vibrator plates are powered by a pneumatic-nitrogen supply and compressed air, and are deployed in a flotation gantry. Marine vibrators have been discussed in more detail by Baeten et al. (1988).
For smaller scale surveys, a system has been developed by GeoA- coustics Ltd, UK, (now part of Kongsberg Maritime, Norway), called the GeoChirp. The system can be deployed in either an over-the-side mounted assembly or as a towed fish (Figure 4.33). The instrument consists of up to four source transducers with frequency bandwidths of either 2–8 kHz or 1.5–11.5 kHz. An upsweep of frequencies (the
‘chirp’) is transmitted in a pulse lasting either 16 or 32 ms, and repeated 4 or 8 times a second.
Immediately behind the source mounting, which looks like the head of a tadpole, is located an 8-element streamer that is about 1 m long. As the source transducer characteristics have been measured by the manufacturer, the system is programmed with these charac- teristics in order to provide an autocorrelated ‘de-chirped’ analogue output. This can be printed directly on to a hard-copy device, such as a thermal linescan printer. Alternatively, the analogue signals can
be put through a sonar enhancement processing unit that provides facilities for real-time filtering, gain control, and so on, and then output as required.
Another Chirp-type system has been introduced by IXSEA (Echoes 1500), which uses a single transducer repeatable sound source. The single transducer is suspended from a buoy and towed as necessary. The manufacturer claims the system has a 27 cm res- olution.
A shear-wave source that is being developed uses the principle of a vibrating mass embedded within the ground (VibroPile). The source vibrates horizontally, thereby generating shear waves. For reservoir monitoring, a configuration would have 25 installed piles into the seabed with one moveable S-wave source. Each pile is
∼0.6 m in diameter and has a length of about 2 m. The vibrator itself weighs around 1.5 tonnes. The total weight with the submerged power unit is 6 tonnes. A linear sweep through 10 Hz to 80 Hz is used with a duration of 10 s. It is thought that by using shear waves it is possible to study reservoirs beneath gas clouds, to monitor fracture development and to help optimise production in oil fields.