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This series on marine seismic sources will summarize salient points for geoscientists who need to sharpen their rusty skills in seismic source technology. It will also discuss the effect seismic sources have on marine life.
Martin Landrø and Lasse Amundsen
A seismic source is defined as any device which releases energy into the earth in the form of seismic waves. The major source type in marine exploration is the air-gun array, which since the 1970’s has been by far the most popular. The air-gun can be described as a chamber of compressed air that is released rapidly into the surrounding water to create an acoustic pulse. The air gun is the most commonly used source because the pulses are predictable, repeatable and controllable, it uses compressed air which is cheap and readily available, and it has only a minor impact on marine life.
An air-gun volume is measured in litres (l) or more commonly, by the conservative petroleum geophysicist, in cubic inches (in3). Typical volumes of individual air-guns used by the exploration industry vary from 20 in3 (0.3 l) to 800 in3 (13.1 l), while academic seismic refraction studies can use volumes up to 1600 in3 (26.2 l). An air-gun array consists of 3 - 6 sub-arrays called strings, each string containing 6 - 8 individual guns, so that the array usually involves between 18 and 48 guns, although in special cases as many as 100 guns an array can be used. The air-gun array volume is the sum of the volumes of each gun, and is typically in the range 3,000-8,000 in3 (49.2-131.6 l).
The air-guns hang in the sea beneath floats between 3m and 10m below the sea surface, generally at about 6m, except for refraction studies when a deeper deployment is needed. The gun pressure mostly used by the seismic industry is 2,000 psi (138 bar) and during a survey, the guns fire every 10-15 seconds.
It is common to arrange several (2-4) air-guns in a cluster, with the guns so close together that they behave as a larger single gun. The main purpose of clustering is to improve signal characteristics, since the bubble motion (see below) is reduced by this configuration.
The energy sent out by air-gun arrays is dominantly directed vertically downwards. The broad band of frequencies from the array form a pulse with peak-to-peak amplitude in the range 14-28 bar-m, corresponding to 243-249 dB re 1 mPa-m vertically downward. The amplitude levels emitted horizontally tend to be 15-24 dB lower. These numbers are frequency dependent. By filtering out high frequencies there is less deviation between amplitude levels vertically and horizontally.
Here, ‘dB re 1 mPa-m’ means decibel value peak-to-peak relative to the reference pressure one micropascal at a reference distance of one metre. Confusing units? Read Box 1 and we will guide you through the physical principles of air-guns and the basic sound measurement units. Our focus in the present article is on the vertically downward travelling ‘far-field’ signature of an air-gun array as this signature provides a quantitative measure of the array’s performance.
When compressed air is suddenly released into the water an oscillating bubble forms. This process is described in Parkes and Hatton (1986):
Initially, the pressure inside the bubble greatly exceeds the hydrostatic (external) pressure. The air bubble then expands well beyond the point at which the internal and hydrostatic pressures are equal. When the expansion ceases, the internal bubble pressure is below the hydrostatic pressure, so that the bubble starts to collapse. The collapse overshoots the equilibrium position and the cycle starts once again. The bubble continues to oscillate, with a period typically in the range of tens to hundreds of milliseconds.
The oscillation is stopped due to frictional forces, and the buoyancy of the bubble causes it to break the sea surface. If we could stop this cyclic motion immediately after the first expansion of the bubble, the air gun would create an ideal signal close to a single spike and similar to a dynamite explosion. However, due to the cyclic bubble motion, the signal from a single air gun is far from ideal.
As explained in Box 2 on bubble motion, the dominant frequency of the bubble oscillations decreases with increasing gun volume, or with increasing gun pressure, or with decreasing source depth. Therefore, small guns emit higher frequencies and big guns emit lower frequencies – like a bell choir. The geophysicist wants a broad band of frequencies.
However, as we will explain, the source ghost (see Box 3) has a detrimental effect on broad-band spectrum.
The time domain pressure pulse that is emitted by the single air-gun in the vertical direction is called the pressure signature. However, another pulse simultaneously travels upward from the source, is reflected down at the sea surface and joins the original downward-travelling pressure pulse. This delayed pulse, reflected at the sea surface, is called the source ghost. From a data processing point of view, the source ghost is considered to be an intrinsic feature of the source wavefield, and therefore often included in the definition of the source pressure signature.
As illustrated in Box 3, the effect of the source ghost on the frequency content of the source signature is substantial. Therefore, like gun size, the gun depth is an important parameter in source and survey design.
The useful frequency band of seismic data is between the first (always at 0 Hz) and the second zeroes in the amplitude spectrum of the source signature. The zeroes in the spectrum, caused by the source ghost and therefore called ghost notches, implies that there is no signal at these particular frequencies.
If high-resolution seismic of the shallow subsurface is the objective, it is important to extend the high-frequencies as much as possible. For a source at a depth of 3.75 m, the second ghost notch is at frequency f1=200 Hz, but the low-frequency amplitudes are much lower than for a source depth of, for example, 15 m. As low frequencies improve penetration into the subsurface, a shallow source is not recommended for a deep-looking survey. By comparison, a 15 m source depth has nice low-frequency characteristics for good penetration, but the ghost notch at f1=50 Hz has a detrimental effect on resolution. A survey with both low-frequency and high-frequency objectives is difficult to realize. It can be seen from this that the survey objective dictates the source depth.
DefinitionsHydrophones are sensitive to sound pressure, which is measured in micropascals (mPa). By tradition, the geophysicist uses a different pressure unit, microbar (mbar). One bar is equivalent to 1011 mPa. The far-field signature of an air-gun array, measured vertically beneath, is used to define the nominal source level. This is the acoustic pressure 1 m away from its hypothetical point source equivalent that would radiate the same amount of sound in the far-field as the actual source. Units are in bars at 1 m, abbreviated as bar-m. For example, 100 bar-m means that if the array were a point source and a hydrophone were 50 m away, then the hydrophone would detect a pressure of 2 bar. In presenting sound measurements, acousticians use ratios of pressures; in underwater sound the adopted reference pressure is one mPa. Further, acousticians adopt the decibel (dB) scale so that sound pressure level (SPL) of a sound of pressure S is SPL (dB) = 20 log10(S/S0) where S0 is the reference pressure. The standard for specifying air-gun signal levels is the peak-to-peak (P-P) level, which is the maximum negative-to-positive measurement of the air-gun signature. The seismic survey literature refers to P-P pressure amplitudes in bar-m. P-P can be converted to source level Ls in dB re 1 mPa-m as follows: Ls (dB re 1 mPa-m) = 20 log(P-P)+220. Acoustic pressures of 10-20 bar-m correspond to 240-246 dB re 1 mPa-mP-P. The source amplitude spectrum level gives source amplitude strength versus frequency. It is common to normalize the amplitude spectrum in dB relative to 1 mPa/Hz at 1 m, abbreviated as dB re 1 mPa/Hz-m. Because the reference pressure 1 mPa is a small pressure, a moderately sized air-gun array will have a spectrum that peaks above 200 dB above this reference level. |
The air-gun ‘signature’, recorded by a hydrophone below the gun, has three characteristic features: the direct arrival produced when the gun fires; the source ghost; and the bubble pulse caused by the expansion-collapse cycle of the air bubble. The signature is characterized by two parameters: the primary pulse peak-to-peak (P-P) amplitude, or “strength”, the useful part of the signal, and its bubble period. These two characteristic parameters depend on the air-gun’s size, initial firing pressure, and depth.
Two important parameters of an air-gun array signature are its peak-to-peak (P-P) strength and primaryto-bubble ratio (PBR). The PBR should be as high as possible so that the air-gun array signature is close to an ideal pulse. To find the P-P strength of an array, its signature is measured at a distance from the source known as the far-field point, a point where the output signals of the individual guns interfere constructively, typically 250-300m beneath them. This far-field signature is then used to define a nominal point-source level, at 1 m from the centre of the array, by multiplying the signature by the radial distance from this point to the hydrophone. This nominal point-source level is a theoretical sound pressure level. Because of partial destructive interference between the signals of the individual guns, the actual level at this point in reality tends to be 10 times (20 dB) lower than the nominal level. The P-P strength (related to this nominal source level) is defined as the difference in absolute amplitude between the peaks of the primary and ghost arrivals.
The P-P amplitude of, for example, the 3,397-in3 array is 102 bar-m, corresponding to 260 dB re 1 μPa-m. Typical arrays tend to produce levels of 243-249 dB re 1 μPa-m, corresponding to 14-28 bar-m. The seismic industry sometimes gives output levels in root-meansquare (rms) peak-to-peak amplitudes (rms P-P). The dB reduction of rms P-P compared to P-P is around 3 dB since the maximum peak and trough amplitudes are approximately equal.
It is important to remember that the time-domain specifications of P-P strength and PBR depend strongly on the frequency bandwidth of the array’s signature. When high frequencies are cut the P-P strength and PBR both decrease.
There are several reasons for deploying air-guns in arrays. The first is to increase the power of the source. The basic idea is that a source array of n single sources produces n times the power of the single source. The second is to minimize the PBR by tuning the array: Guns with different volumes will have different bubble periods, leading to a constructive summation of the first (primary) peak and destructive summation of the bubble amplitudes.
Bubble motionThe physics of air bubbles from marine air-guns is complex. The temperature inside the bubble can drop to around minus one hundred degrees Celsius, causing ice crystals to form. There is also evidence that the bubble comprises a number of small bubbles rather than one large one. Nevertheless, to a first approximation, the parameters that govern the time behavior of the bubble oscillation are surprisingly “simple”: T = C1 P1/3 V1/3 / P05/6. Here, P and V are the initial internal pressure (firing pressure) and volume of the air before it is released and P0 is the hydrostatic pressure. C1 is a constant that depends upon the air gun details. The dominant frequency of the bubble motion, f=1/T, then decreases with increasing gun volume, or with increasing gun pressure, or with decreasing hydrostatic pressure (and thus, source depth). By tradition, the seismic industry quotes firing pressure in pound force per square inch, psi (psi=6894.76 Pascals; a Pascal is a Newton per square meter). Air-gun array strengthThe strength of an array is: (1) Linearly proportional to the number of guns in the array. All else being equal, a 40-gun array generates twice the amplitude of a 20-gun array. (2) Close to linearly proportional to the firing pressure of the array. A 3000-psi array has 1.5 times the amplitude of a 2000-psi array. (3) Roughly proportional to the cube root of its volume. The number of guns provides the greatest influence on the array strength, not the firing pressure nor the total volume. |
The physics of the source ghost related to marine sources is simple. The upward travelling part of the source signal cannot escape into the air, as the sea surface acts as a mirror, and reflects the signal downwards with opposite polarity. This source ghost pulse is delayed in time with respect to the initial downwards primary pulse from the source. Geometrically, the source ghost appears to originate from the mirror image of the seismic source. The time delay therefore is given by t =2d cosq/c, where d is the source depth, c is the sound of speed in water, and q is the offset angle of the initial downwards primary pulse measured from the vertical axis.
The frequency spectrum of the composite signal (primary and ghost) is
|G(f)| = 2 sin(2 p f d cosq/c).
This spectrum has zeroes called ‘ghost notches’ at frequencies fn = n c/(2 d cosq), where n is zero or positive integer. The first notch (n=0) is always at f0=0 Hz. The useful frequency band in seismic is inside the first and second notch frequencies (inside f0 and f1) of the spectrum of the composite signal.
Acknowledgment
We thank WesternGeco for providing useful illustrations to this article.
References
A Ziolkowski 1970; A method for calculating the output pressure waveform from an air gun: Geophys. J. R. astr. Soc. 21, 137-161.
G Parkes and L Hatton 1986; The marine seismic source: Kluwer.
J Langhammer 1994; Experimental studies of energy loss mechanisms in air-gun bubble dynamics: PhD thesis, NTNU.
B Dragoset 2000; Introduction to air guns and air-gun arrays: Leading Edge, 8, 892-897.
J Caldwell and B Dragoset 2000; A brief overview of seismic air-gun arrays: Leading Edge, 8, 898-902.
| Article from GEO ExPro Magazine NO1 - 2010 |
Updated: 02.06.2010 14:46 by Alf Kvassheim
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