3.8 Seismic data acquisition systems
3.8.1 Seismic sources and the seismic/acoustic spectrum
A seismic source is a localized region within which the sudden release of energy leads to a rapid stressing of the surrounding medium. The archetypal seismic source is an explosion. While explosives are still used, there is an increasing number of more sophisticated and efficient (and safe!) ways to collect seismic data.
The main requirements of the seismic source are:
• Sufficient energy across the broadest possible fre- quency range, extending up to the highest recordable frequencies.
• Energy should be concentrated in the type of wave energy which is required for a specific survey, either P-
wave or S-wave, and generate minimum energy of other wave types. Such other unwanted energy would degrade the recorded data and be classed as coherent noise.
• The source waveform must be repeatable. Seismic sur- veys almost always involve comparing the seismograms generated by a series of sources at different locations.
Variations on the seismograms should be diagnostic of the ground structure, not due to random variations of the source.
• The source must be safe, efficient, and environmen- tally acceptable. Most seismic surveys are commercial operations which are governed by safety and environ- mental legislation.They must be as cost-effective as pos- sible. Sometimes the requirements for efficiency lead to higher safety and environmental standards than legally enforced.Whether involving personal injury or not, ac- cidents are referred to as ‘lost-time incidents’. Safety aids efficiency as well as being desirable from many other viewpoints.
The complete seismic/acoustic spectrum is shown in Fig. 3.14.There is a very wide variety of seismic sources, characterized by differing energy levels and frequency characteristics. In general, a seismic source contains a wide range of frequency components within the range from 1 Hz to a few hundred hertz, though the energy is often concentrated in a narrower frequency band.
Source characteristics can be modified by the use of several similar sources in an array designed, for example, to improve the frequency spectrum of the transmitted pulse. This matter is taken up in Chapter 4 when dis- cussing the design parameters of seismic reflection surveys.
Explosive sources
On land, explosives are normally detonated in shallow shot holes to improve the coupling of the energy source with the ground and to minimize surface damage. Ex- plosives offer a reasonably cheap and highly efficient seismic source with a wide frequency spectrum, but their use normally requires special permission and pre- sents logistical difficulties of storage and transportation.
They are slow to use on land because of the need to drill shot holes. Their main shortcoming, however, is that they do not provide the type of precisely repeatable source signature required by modern processing tech- niques, nor can the detonation of explosives be repeated at fixed and precise time intervals as required for efficient reflection profiling at sea carried out by survey vessels underway. Since explosive sources thus fail at least two,
and usually three, of the basic requirements for modern surveys, their use is steadily declining and limited to lo- cations where alternative sources cannot be used.
Non-explosive land sources
Vibroseis®is the most common non-explosive source used for reflection surveying. It uses truck-mounted vibrators to pass into the ground an extended vibration of low amplitude and continuously varying frequency, known as a sweep signal. A typical sweep signal lasts from several seconds up to a few tens of seconds and varies progressively in frequency between limits of about 10 and 80 Hz. The field recordings consist of overlapping reflected wave trains of very low amplitude concealed in the ambient seismic noise. In order both to increase the signal-to-noise ratio (SNR) and to shorten the pulse length, each recorded seismogram is cross-correlated (see Section 2.4.3) with the known sweep signal to produce a correlated seismogram or correlogram. The correlogram has a similar appearance to the type of seismogram that would be obtained with a high-energy impulsive source such as an explosion, but the seismic arrivals appear as symmetrical (zero phase) wavelets known as Klauder wavelets(Fig. 3.15).
The Vibroseis®source is quick and convenient to use and produces a precisely known and repeatable signal.
The vibrator unit needs a firm base on which to operate, such as a tarmac road, and it will not work well on soft ground.The peak force of a vibrator is only about 105N
and, to increase the transmitted energy for deep penetra- tion surveys, vibrators are typically employed in groups with a phase-locked response. Multiple sweeps are com- monly employed, the recordings from individual sweeps being added together (stacked) to increase the SNR.
A particular advantage of vibrators is that they can be used in towns since they cause no damage or sig- nificant disturbance to the environment. The cross- correlation method of extracting the signal is also capa- ble of coping with the inherently high noise levels of urban areas. Some Vibroseis®trucks are adapted so that the vibration direction can be horizontal rather than ver- tical. In this case the truck can also be used as an S-wave source. A principal disadvantage of the Vibroseis® method is that each fully configured truck costs of the order of half a million dollars. While the method is effective for major hydrocarbon surveys, the costs are prohibitive for small surveys. Small electro-mechanical vibrators have been developed for shallow geophysical surveys, and these are gaining increasing acceptance as seismographs capable of receiving and correlating the signals are developed.
Mini-Sosie adapts the principle of using a precisely known source signature of long duration to cheaper, lower energy applications. A pneumatic hammer deliv- ers a random sequence of impacts to a base plate, thus transmitting a pulse-encoded signal of low amplitude into the ground. The source signal is recorded by a detector on the base plate and used to cross-correlate with the field recordings of reflected arrivals of the
Echo sounders Pingers
Boomers Sparkers
Air guns Vibroseis Quarry blasts Earthquake body waves Earthquake surface waves
10–2 10–1 1 101 102 103 104 105
Frequency (Hz) (log scale) Fig. 3.14 The seismic/acoustic spectrum.
pulse-encoded signal from buried interfaces. Peaks in the cross-correlation function reveal the positions of reflected signals in the recordings.
Weight drops and hammers. Perhaps the simplest land seismic source is a large mass dropped on to the ground surface. Weight drops have been manufactured in a wide variety of forms from eight-wheel trucks dropping a weight of several tonnes, to a single person with a sledgehammer. If the source energy required is relatively low, these types of sources can be fast and efficient. The horizontal impact of a weight or hammer on to one side of a vertical plate partially embedded in the ground can be used as a source for shear wave seismology.
Shotguns, buffalo guns and rifles. One solution to gaining additional energy for small-scale surveys is to use the compact chemical energy in small-arms ammunition.
Rifles have been used as seismic sources by firing the bullet into the ground. While effective as a very high- frequency source, this is banned by legislation in many countries. An alternative is to fire a blank shotgun car- tridge in a hole using a suitable device, commonly termed a buffalo gun (Fig 3.16).The blank shotgun car- tridge offers an impulsive source giving considerably more energy than a sledgehammer, with few of the safety problems of explosives.
Vibroseis sweep signal
Reflection from base of layer 1
Reflection from base of layer 2
Reflection from base of layer 3 (phase inverted)
Field recording (superposition of above reflections)
Output trace resulting from correlation of field recording with sweep signal
Time t = 0
Fig. 3.15 Cross-correlation of a Vibroseis®seismogram with the input sweep signal to locate the positions of occurrence of reflected arrivals.
Firing pin
Ground surface
Auger flight
Firing chamber
Fig. 3.16 Schematic cross-section of a typical buffalo gun.The cartridge is fired by dropping a simple firing pin on to the cartridge.
Marine sources
Air guns(Fig. 3.17(a)) are pneumatic sources in which a chamber is charged with very high-pressure (typically 10 –15 MPa) compressed air fed through a hose from a shipboard compressor. The air is released, by electrical triggering, through vents into the water in the form of a high-pressure bubble.A wide range of chamber volumes are available, leading to different energy outputs and frequency characteristics. The primary pulse generated by an air gun is followed by a train of bubble pulsesthat increase the overall length of the pulse. Bubble pulses are caused by the oscillatory expansion and collapse of secondary gas bubbles following collapse of the initial bubble. They have the effect of unduly lengthen- ing the seismic pulse. Steps can, however, be taken to suppress the effect of the bubble pulse by detonating near to the water surface so that the gas bubble escapes into the air. While this does remove the bubble pulse
effect, much energy is wasted and the downgoing seismic pulse is weakened. More sophisticated methods can be used to overcome the bubble pulse problem while preserving seismic efficiency. Arrays of guns of differing dimensions and, therefore, different bubble pulse periods can be combined to produce a high- energy source in which primary pulses interfere constructively whilst bubble pulses interfere destruc- tively (Fig. 3.18). For deep penetration surveys the total energy transmitted may be increased by the use of multiple arrays of air guns mounted on a frame that is towed behind the survey vessel. Air guns are mechani- cally simple and can operate with great reliability and repeatability. They have become the standard marine seismic source.
Water guns(Fig. 3.17(b)) are an adaptation of air guns to avoid the bubble pulse problem. The compressed air, rather than being released into the water layer, is used to drive a piston that ejects a water jet into the surrounding
Air
Variable chamber size
(a) (b)
Water
Fig. 3.17 Schematic cross-sections through (a) a Bolt air gun and (b) a Sodera water gun to illustrate the principles of operation. (Redrawn with permission of Bolt Associates and Sodera Ltd.)
water.When the piston stops, a vacuum cavity is created behind the advancing water jet and this implodes under the influence of the ambient hydrostatic pressure, gener- ating a strong acoustic pulse free of bubble oscillations.
Since the implosion represents collapse into a vacuum, no gaseous material is compressed to ‘ bounce back’ as a bubble pulse. The resulting short pulse length offers a potentially higher resolution than is achieved with air guns but at the expense of a more complex initial source pulse due to the piston motion.
Several marine sources utilize explosive mixtures of gases, but these have not achieved the same safety and reliability, and hence industry acceptance, as air guns. In sleeve exploders, propane and oxygen are piped into a sub- merged flexible rubber sleeve where the gaseous mix-
ture is fired by means of a spark plug. The products of the resultant explosion cause the sleeve to expand rapid- ly, generating a shock wave in the surrounding water.
The exhaust gases are vented to surface through a valve that opens after the explosion, thus attenuating the growth of bubble pulses.
Marine Vibroseis®.Whilst vibrators were developed for land surveys, it is of interest to note that experiments have been carried out using marine vibrator units, with special baseplates, deployed in fixtures attached to a survey vessel (Baeten et al. 1988).
Sparkersare devices for converting electrical energy into acoustic energy. The sparker pulse is generated by the discharge of a large capacitor bank directly into the sea water through an array of electrodes towed in a frame
(a)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 s
(b)
Single 270 in3 air gun
Seven – gun array (1222 in3 total volume)
Fig. 3.18 Comparison of the source signatures of (a) a single air gun (peak pressure: 4.6 bar metres) and (b) a seven- gun array (peak pressure: 19.9 bar metres).
Note the effective suppression of bubble pulses in the latter case. (Redrawn with permission of Bolt Associates.)
behind the survey vessel. Operating voltages are typi- cally 3.5–4.0 kV and peak currents may exceed 200 A.
This electrical discharge leads to the formation and rapid growth of a plasma bubble and the consequent genera- tion of an acoustic pulse. For safety reasons, sparkers are increasingly being replaced by other sources.
Boomerscomprise a rigid aluminium plate attached below a heavy-duty electrical coil by a spring-loaded mounting. A capacitor bank is discharged through the coil and the electromagnetic induction thus generated forces the aluminium plate rapidly downwards, setting up a compressional wave in the water. The device is typically towed behind the survey vessel in a catamaran mounting.
Sparkers and boomers generate broad-band acoustic pulses and can be operated over a wide range of energy levels so that the source characteristics can to some ex- tent be tailored to the needs of a particular survey. In gen- eral, boomers offer better resolution (down to 0.5 m) but more restricted depth penetration (a few hundred metres maximum).
Pingers consist of small ceramic piezoelectric transducers, mounted in a towing fish, which, when ac- tivated by an electrical impulse, emit a very short, high- frequency acoustic pulse of low energy.They offer a very high resolving power (down to 0.1 m) but limited pene- tration (a few tens of metres in mud, much less in sand or rock). They are useful in offshore engineering applica- tions such as surveys of proposed routes for submarine pipelines.
Chirpsystems are electro-mechanical transducers that produce an extended, repeatable, source waveform which allows greater energy output. This longer signal can be compressed in processing to give greater resolu- tion and/or better signal-to-noise ratio.
Further discussion of the use of air guns, sparkers, boomers and pingers in single-channel seismic reflec- tion profiling systems is given in Section 4.15.