GB2481840A

GB2481840A - Positioning seismic sources beneath an acoustic ceiling to reduce source ghosts and bubble pulses

Info

Publication number: GB2481840A
Application number: GB1011526.9A
Authority: GB
Inventors: Lasse Amundsen; Harald Westerdahl; Mark Thompson
Original assignee: Statoil ASA
Current assignee: Equinor ASA
Priority date: 2010-07-08
Filing date: 2010-07-08
Publication date: 2012-01-11
Anticipated expiration: 2030-07-08
Also published as: GB201011526D0; WO2012004587A2; GB2481840B; WO2012004587A3; NO20130216A1; NO345333B1

Abstract

A marine seismic source comprises at least one acoustic source 22 and an acoustic ceiling 20 positioned above the source so that a reflection of said acoustic wave from said ceiling is reduced in amplitude compared to reflection of said acoustic wave from a boundary between said water and air. The ceiling may be the underside of a vessel, and the sources may be rigidly attached underneath the ceiling, or suspended by flexible cables. An array of sources may be used, at least one of which is not located under the ceiling so that it does produce a source ghost. The sources in the array may produce acoustic waves having different frequencies, such as 1 Hz to Hz and 10 Hz to 120 Hz. The source may be an air gun or a water gun. The acoustic ceiling may have a high impedance, a high attenuation, or both.

Description

IMPROVED SEISMIC SOURCES AND METHODS OF CONDUCTING A

SEISMIC SURVEY

Field of the invention

The invention relates to the field of seismic surveys, and particularly marine seismic surveys.

Background of the invention

A marine seismic exploration survey with the objective to map hydrocarbon deposits within geological formations typically involves deploying one or several seismic sources beneath the sea surface and seismic sensors at predetermined locations as illustrated in Figure 1. Figure 1 shows a marine seismic survey 2 in which a vessel 4 tows a source 6 and a streamer 8. The source 6 may comprise one or more air guns 10. The streamer comprises one or more seismic sensors, which may be hydrophones. Both the source 6 and the streamer 8 are towed beneath the surface of the water 14.

The sources generate seismic waves which propagate into the geological formations 12 below the water 14. Changes in elastic properties of the geological formations 12 reflect, refract and scatter the seismic waves, changing their direction of propagation.

Part of the reflected energy reaches the seismic sensors which can be hydrophones sensitive to pressure changes and/or geophones sensitive to particle motion. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon accumulations.

A seismic source is defined as any device which releases energy into the earth in the form of seismic waves. The major type in marine exploration is air-gun array sources, which since the 1970's have been by far the most popular ones. The air-gun can be described as a chamber with compressed air that is released to the surrounding water through port(s) to create an acoustic pulse. The main reasons for their popularity are that their pulses are predictable, repeatable and controllable, and that they use compressed air which is cheap and readily available.

Another type of source is the water gun. In the following, the discussion will refer to air guns, although the principles we discuss are not limited to air guns.

The time domain pressure pulse that is emitted by an air gun in the downwards direction is called the primary pulse. However, air guns produce unwanted pulses commonly called "source ghost" pulses and "bubble" pulses. These pulses produce unwanted components in the seismic signal that is transmitted into the subsurface of the geological formation (12).

The source ghost pulse is the signal which travels upward from the source, is reflected down at the sea surface and joins the original, primary downward-traveling pressure pulse from the source. From a seismic data processing point of view, the source ghost is often considered to be an intrinsic feature of the source wavefield, and therefore often included in the definition of the source signature.

The physics of the source ghost in relation to marine sources is illustrated in Figure 2.

The upward traveling part of the source signal cannot escape into the air. The sea surface acts as a mirror, and it reflects the signal downwards with opposite polarity.

This source ghost pulse is delayed in time with respect to the initially downgoing primary pulse from the source. Geometrically, the source ghost appears to originate from the mirror image of the seismic source. The time delay of the ghost pulse is given by r=2dcosO/c (1) where d is the source depth, c is the sound of speed in water, and 0 is the offset angle of the initial downgoing primary pulse measured from the vertical axis.

We assume that the initially downgoing pulse has amplitude A=1/2. In the frequency domain, the effect of the ghost then is to modify the initial pulse by the so-called ghost factor G(f) = (1/2)(1 + i exp(2irfdcos9 Ic)) (2) where f is frequency and the reflection coefficient r0=-1 for a free surface with vanishing pressure (i.e. the air above the water). Because of the effectively zero pressure at the sea surface the ghost pulse gets the opposite polarity in relation to the initial pulse.

The frequency spectrum of the ghost factor then is G(f) sin(2irfdcos9/c) (3) The spectrum has zeroes called ghost notches' at frequencies f, nc/(2dcos9) (4) where n is zero or positive integer. The first notch (n=O) is always at f0=O Hz. This is a strong contributing reason to the fact that in seismic data acquisition we can lose the low frequency components in relation to noise. The useful frequency band of seismic data is between the first and the second zeroes in the amplitude spectrum of the source signature (inside f0 and f1). The existence of zeroes in the spectrum, caused by the source ghost and therefore called ghost notches, implies that there is no signal, but only noise, at these particular frequencies. As the offset angle increases, the ghost notch frequency increases. The inability to use low frequencies as a result of the first notch is particularly problematic when exploring regions containing basalt volcanic rock and salt layers. In order to effectively "see through" basalt and salt layers it is necessary to use low frequencies, for example in the range 1 to 10 Hz.

The ghost notches are seen to result from the interference between the primary downgoing pulse and the secondary ghost pulse. The phase differences between the two pulses causes attenuation and enhancement of spectral component within the bandwidth of the source signature. Attenuation is most severe at the frequencies where the pulses are 180 degrees out of phase. Enhancement is most significant at the frequencies where the pulses are in phase.

Since the source ghost effect depends on source depth the gun depth is an important parameter in source and survey design. In Figure 3 we display the effect the source ghost has on the amplitude spectrum of the source signature in the vertical direction (0=0) for source depths of 3.75 m, 7.5 m and 15 m.

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 in conventional exploration the survey objective dictates the source depth.

An introduction to this area of seismology is provided in Ikelle, L. T. and Amundsen, L., 2005, Introduction to petroleum seismology: Society of Exploration Geophysics.

Air guns, which generate an acoustic wave by suddenly releasing compressed gas into the water, also generate bubble pulses. When the compressed gas is released, the pressure inside the bubble greatly exceeds the hydrostatic (external) pressure. The air bubble 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.

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.

Use of a single air gun will result in a seismic signal whose frequency spectrum will exhibit a series of peaks and notches related to the bubble pulse oscillation period. To produce a seismic signal with a flatter frequency spectrum, it is common practice to deploy a number of different air-guns in arrays. Guns with different volumes have different bubble periods, leading to a constructive summation of the first (primary) peak and destructive summation of the bubble amplitudes.

Summary of the invention

The invention provides a marine seismic source and a method of conducting a seismic survey as set out in the accompanying claims.

Brief description of the figures

Figure 1 shows a marine seismic survey; Figure 2 shows the geometry of a primary pulse from a seismic source and a ghost pulse reflected from the sea surface; Figure 3 shows the effect which the ghost pulse has on the amplitude spectrum of the source in the direction vertically downwards, for 3 different source depths of 3.75m, 7.5m and 15m; Figure 4 is a schematic diagram showing a ceiling at the surface of the water and above a seismic source, in the form of an array of air guns; Figure 5 is a schematic diagram showing a ceiling below the surface of the water and above a seismic source, in the form of an array of air guns; Figure 6 is a schematic diagram in which an array of air guns is mounted to a ceiling at the surface of the water, with the air guns on the underside of the ceiling; Figure 7 shows frequency spectra of a ghost factor for effective reflection coefficients from 0 to-i; Figure 8 shows frequency spectra of a ghost factor for effective reflection coefficients from0to+i; Figure 9 shows an acoustic ceiling in the water which has an upper part which is air-filled and a lower part which is high-impedance; and Figure 10 shows the results of modelling the frequency responses of signals 200 m below the acoustic ceiling of Figure 9.

Description of preferred embodiments

Figure 4 shows an arrangement in which a ceiling structure 20 is positioned above an array or air guns 22. In this embodiment the ceiling floats on the surface of the water and is towed by a vessel 24. The air guns 22 are tethered to the ceiling 20 by means of connectors 26, which can be flexible cables.

Figure 5 shows a similar arrangement in which corresponding parts are given the same reference numerals. The difference in this embodiment is that the ceiling is positioned below the surface of the water, but still above the air guns 22.

Figure 6 shows a further embodiment, in which corresponding parts are again given the same reference numerals. However, in this embodiment the air guns 22 are firmly mounted to the underside of the ceiling 20.

In each of these embodiments the ceiling has an impedance that differs from the impedance of the sea water. The impedance can vary vertically and laterally. In contrast to the sea surface which has a reflection coefficient that is close to r0 = -1, the ceiling has an effective reflection coefficient r which may vary vertically and laterally and may depend on frequency.

The ceiling has finite extent. The seismic source therefore should be deployed close to the device.

The ceiling, which is preferably a floating object in the sea, can take any suitable form.

In one realization of the ceiling, the object can be a vessel that is autonomous or towed above a seismic source. In another realization the ceiling is a man-made construction that can be towed above the seismic source.

In one embodiment the ceiling is a barge containing sand and/or gravel or a similar material. The barge or other ceiling may have dimensions of about 20 metres. For example, the length may be in the range 5 to 30 metres, and the width may be in the range 5 to 30 metres. Other dimensions are also possible. The impedance and the attenuation of the ceiling should be as high as possible in order to attenuate the ghost pulse.

The effective reflection coefficient provided by the ceiling should be greater (i.e. more positive) than -1, which is the reflection coefficient of the water/air boundary. In different embodiments the effective reflection coefficient provided by the ceiling may be greater (more positive) than -0.9, greater than -0.7 or greater than -0.5. Alternatively the effective reflection coefficient may be in the range -0.9 to +0.9, or in the range -0.7 to +0.7. Ideally the effective reflection coefficient is in the range -0.5 to +0.5, the range -0.3 to +0.3, or the range -0.1 to +0.1.

We refer to the effective reflection coefficient because in practice there may be a number of reflections involved. Some of the energy of the ghost pulse may be reflected from the sea/air interface around the ceiling. Some of the energy of the ghost pulse may pass through the ceiling and be reflected at the ceiling/air boundary above the ceiling. The effective reflection coefficient of the ceiling is the reflection coefficient of the ceiling taking into account these factors.

The effect of the ceiling is to change the effective reflection properties of the sea surface. The method involves deploying one or several objects, here for brevity called the ceiling, on or below the sea surface, above one, several or all of the seismic sources.

The difficulties generated by the presence of a source ghost and bubbles can be remedied if the ceiling is configured to suppress the generation of the ghost pulse and bubbles.

To illustrate the impact and benefit of the ceiling in changing the reflection property of the sea surface, we display in Figures 7 and 8 frequency spectra of the ghost factor (see equation (2) above) for the special cases of constant reflection coefficients ranging from values r=-1 to r=0 and from r=0 to r=1, respectively. The case r0 means that there is no reflection. We observe that as the reflection coefficient of the ceiling increases (i.e. becomes more positive) from r=-1, the amplitude of the ghost factor at low frequencies gets significantly larger. Furthermore, no notches are present in the frequency spectra.

Fig 9 shows an example of a ceiling which can have attenuation to decrease the ghost contribution (free surface contribution), in particular for low frequencies. In this case, the effective reflection coefficient of the ceiling may be frequency dependent.

Figure 9 shows an acoustic ceiling 30 in the water 32 which has an upper part 34 which is air-filled and a lower part 36 which is high-impedance with or without attenuation.

Impedance = density x seismic velocity. High impedance may refer to values that are higher than water impedance (ie about l000kg/m3 x lSOOmIs = 1 500 000 kg/m2s). The impedance of the lower part 36 is also typically less than (10 000 kg/m3 x 6000 mls)= 000 000 kg/m2s.

The acoustic ceiling 30 is arranged above an acoustic source 38. The effective seismic property of the material between the upper and lower surface of the acoustic ceiling 30 represents either high impedance or high attenuation or both.

High attenuation is typically more than 0.5dB/m for a plane wave, where dB = 2OloglO(NAO). A= amplitude at im along the propagation direction, A0= amplitude at Om. 0.5dB/m attenuation means the amplitude is 0.5dB less at im compared to 0 m, i.e. -0.5dB reduction (assuming the plane wave propagates in the positive direction).

That is, the amplitude at im compared to atOm: A1/A0=10 05I20= 0.9441, i.e. about 5.6% reduction. The amplitude is preferably reduced by at least 5%.

As mentioned above, the acoustic ceiling 30 may be high impedance, high attenuation or both. These combinations will now be discussed, with examples of the construction of the acoustic ceiling 30.

Four examples of an acoustic ceiling with high impedance and low attenuation are as follows: 1) Dense water saturated mud I drilling fluid (eg. including a large portion of the heavy rock forming mineral Barite, BaSO4) with densities over 3000 kglm3 (and seismic velocity between 1300 and 2000 m/s) 2) Homogenous solid, such as concrete (water saturated or dry) (density=2000 -3000 kg 1m3, velocity 4000-6000m/s).

3) Homogenous heavy and/or stiff metal, such as massive steel (density about 7000 kg/m3, velocity about 6000m/s).

4) Water saturated porous solid from 2) and 3) above, where the permeability is low (eg less than 0.1 Darcy).

Three examples of an acoustic ceiling with high impedance and high attenuation are as follows: 1) The same as above, but saturated with a mix of water and air, i.e. partly water saturated.

2) Partly saturated of fully water saturated assembly of blocks of concrete, rock or heavy metal (steel), where the average diameter of each block element is more than 5-10 cm, -ensuring high permeability(> 0.1 Darcy) for fluid flow.

3) A dense steel structure, assembled by steel bar elements, where the connection between each steel element is made up by damping springs.

In the case of an acoustic ceiling having high attenuation and low impedance, the impedance is preferably not lower than 500 000 kg/m2s.

The acoustic ceiling 30 can be, for example, a vessel hull containing, or filled with, heavy granular or blocky material, or a heavy structural skeleton, surrounded by a fluid (liquid and/or gas). In this embodiment the upper part of the ceiling 30 may be air-filled to provide the vessel with sufficient buoyancy, thus allowing it to float with the high impedance lower part 36. In this context heavy means a density of more than 2,000 kg/rn3.

To enhance friction and attenuation inside the acoustic ceiling 30, in addition to the possible viscous attenuation created by a fluid, special materials like polymers can be added to the solid contact points of the blocky material, as described below.

The material structure or skeleton referred to above is built up of elements which have connection points I joints between each element, to bind the structure together. In our case we wish to damp the motion between the structural elements (which may be bars) at each joint. That is possible using damping materials in the joints acting as springs 3fl84272-1 -isaijith with dash-pots, for example normal vibration damping springs, or special damping materials such as polymer, like sorbothane The acoustic ceiling 30 can be constructed in a layered manner, with different acoustic/elastic and attenuating properties in different layers.

Figure 10 shows the results of modelling the frequency responses (in the range 2-50 Hz) of signals 200 m below the acoustic ceiling 30 of Figure 9 in the two cases where the upper part 34 of the acoustic ceiling 30 is air-filled and the lower part 36 is high-impedance with or without attenuation. The source 38 is 2 m below the ceiling 30. For the sake of comparison, the response with no ceiling is included. The example illustrates the benefit of including a ceiling above the seismic source during surveying.

The model used was a 2D domain model, with the following parameters: 20m wide scatter object, 2m thick, density= 3000kg/m3, velocity1500m/s, attenuation = 10dB/rn (red curve), attenuation = OdB(blue curve), source 2rn below the lower boundary of the ceiling The embodiments described change the effective reflectivity of the sea surface above a seismic source with the object of attenuating the source ghost reflection pulse and bubble oscillations.

The ceiling also has an improved effect on the bubble pulses. In addition to suppressing reflection of bubble pulses from the surface of the water, the ceiling has an effect on the bubbles themselves. The bubbles from an air gun rise upwards at a speed of about 1 meter per second. The bubbles continue to oscillate as they rise. For a gun depth of 7 metres the bubbles therefore oscillate for up to about 7 seconds. However the presence of the ceiling prevents the bubbles from rising, and causes the bubbles to break up. A particular advantage is obtained if the ceiling is relatively close to the air gun, such as in the case where the air gun is mounted to the ceiling, as in the embodiment of Figure 6 for example. The ceiling may for example be positioned no more than 1 meter above the air gun or guns.

Benefits of the proposed method include: * More low frequencies will be available from the source, which are important for improved seismic imaging below complex overburden and seismic inversion.

* The effect of ghost notches will be significantly reduced, which will increase signal to noise ratio and enhance seismic resolution.

* The bubble effect may be reduced, giving source signatures with higher primary to bubble ratios.

The presence of a ceiling above an air gun is particularly beneficial in allowing low frequencies to be used, for example between 1 Hz and 10 Hz, as we have discussed above. This allows the system to see through basalt and salt layers. The seismic survey may also wish to use normal frequencies, for example in the range 10 -120 Hz.

It is therefore possible, in certain embodiments, to position a ceiling above only some of the air guns. These air guns can be used to collect data using low frequencies. At the same time other air guns can be used to collect data using normal frequencies.

The air guns with and without ceilings can be timed appropriately so that they go off at different times. The air guns with and without ceilings may form part of the same air gun array, and be towed by the same vessel, and used to collect data in the same survey. In this way a more accurate model of the geological formations below the water can be produced.

Claims

CLAIMS: 1. A marine seismic source for use in conducting a seismic survey of a geological formation below water, said source comprising: at least one acoustic source for emitting an acoustic wave in said water, and an acoustic ceiling for positioning, in use, above said acoustic source so that a reflection of said acoustic wave from said ceiling is reduced in amplitude compared to reflection of said acoustic wave from a boundary between said water and air.
2. A marine seismic source as claimed in claim 1, wherein said acoustic source is physically connected to said acoustic ceiling.
3. A marine seismic source as claimed in claim 2, wherein said acoustic source is connected to said acoustic ceiling by a flexible cable.
4. A marine seismic source as claimed in claim 2, wherein said acoustic source is connected to said acoustic ceiling in a rigid manner, so that said acoustic source and acoustic ceiling are substantially fixed in position relative to each other.
5. A marine seismic source as claimed in any preceding claim, wherein said acoustic source is positioned beneath the surface of said water.
6. A marine seismic source as claimed in any preceding claim, wherein said acoustic ceiling has a lower surface which is positioned beneath the surface of said water.
7. A marine seismic source as claimed in claim 6, wherein said acoustic ceiling has an upper surface which is also positioned beneath the surface of said water.
8. A marine seismic source as claimed in claim 6, wherein said acoustic ceiling is provided by the underside of a vessel.
9. A marine seismic source as claimed in claim 8, wherein said vessel is a powered vessel which comprises power means for powering itself through said water.
10. A marine seismic source as claimed in any preceding claim, wherein when placed in said water said acoustic ceiling has an effective reflection coefficient of greater than -0.9, that is more positive than -0.9.
11. A marine seismic source as claimed in claim 10, wherein when placed in said water said acoustic ceiling has an effective reflection coefficient of greater than -0.7.
12. A marine seismic source as claimed in claim 11, wherein when placed in said water said acoustic ceiling has an effective reflection coefficient of greater than -0.5.
13. A marine seismic source as claimed in any of claims 10 to 12, wherein when placed in said water said acoustic ceiling has an effective reflection coefficient between -0.9 and +0.9.
14. A marine seismic source as claimed in any preceding claim, wherein said acoustic ceiling is positioned in the path of a ghost pulse, which is a pulse generated by said acoustic source which, in the absence of said acoustic ceiling, travels substantially parallel with a primary pulse from said acoustic source after reflection from the surface of said water.
15. A marine seismic source as claimed in any preceding claim, wherein said at least one acoustic source is an array of acoustic sources.
16. A marine seismic source as claimed in claim 15, wherein said array of acoustic sources comprises at least one acoustic source which produces a ghost pulse which has no acoustic ceiling positioned in the path of the ghost pulse.
17 A marine seismic source as claimed in any preceding claim, wherein at least one of said acoustic sources produces an acoustic wave in the frequency range 1 Hz to Hz.
18. A marine seismic source as claimed in any preceding claims, wherein at least one of said acoustic sources produces an acoustic wave in the frequency range 10 Hz to 120 Hz.
19. A marine seismic source as claimed in any preceding claim, wherein at least one of said acoustic sources is an air gun.
20. A marine seismic source as claimed in any of claims 1 to 18, wherein at least one of said acoustic sources is a water gun.
21. A marine seismic source as claimed in any preceding claim, wherein said acoustic ceiling has an acoustic impedance of greater than 1,500,000 kg/m2s.
22. A marine seismic source as claimed in any preceding claim, wherein said acoustic ceiling has an attenuation of more than 0.5dB/rn.
23. A method of conducting a seismic survey of a geological formation below water, the method comprising: placing at least one acoustic source in the water; placing an acoustic ceiling above said acoustic source; and emitting an acoustic wave from said acoustic source so that at least part of said acoustic wave reflects from said acoustic ceiling, and so that the reflection of said acoustic wave from said ceiling is reduced in amplitude compared to reflection of said acoustic wave from a boundary between said water and air.
24. A method as claimed in claim 23, which comprises analysing acoustic waves in the frequency range 1 Hz to 10 Hz reflected from said geological formation.
25. A method as claimed in claim 23 or 24, which comprises analysing acoustic waves in the frequency range 10 Hz to 120 Hz reflected from said geological formation.
26. A method as claimed in claims 24 and 25, which comprises using different acoustic sources for the analysis in the 1 Hz to 10 Hz range and the 10 Hz to 120 Hz ranges, and introducing a timing difference between said different acoustic sources.
27. A method as claimed in any one of claims 23 to 26, which further comprises: placing at least one secondary acoustic source in the water; and emitting an acoustic wave from said secondary acoustic source so as to produce a primary pulse travelling downwardly from said secondary acoustic source, and a ghost pulse which travels substantially parallel with said primary pulse after reflection from the surface of said water; wherein no acoustic ceiling is positioned in the path of said ghost pulse.
28. A method as claimed in claim 27, wherein a timing delay is introduced between the acoustic waves produced by said at least one acoustic source and said at least one secondary acoustic source.
29. A method as claimed in any one of claims 23 to 28, wherein said at least one acoustic source and said acoustic ceiling form parts of a marine seismic source which has the features of any of claims 1 to 22.