Method of vertical seismic profiling

Abstract: In a method and system for creating a vertical seismic profile (VSP), the spacing interval (6) between units (20) either seismic sources or receivers, in a borehole (4) increases exponentially as their distance (8) from the surface (32) increases, and the spacing interval between units (30), either seismic sources or receivers, along the surface of the earth (32) decreases exponentially as their distance (20) from the surface intercept (12) of the wellbore increases. In a preferred embodiment of this invention, the common depth point (CDP) fold of the data gathered is substantially flattened across the reflectors (14) surveyed.

Description: This invention relates to a method for vertical seismic profile surveys (VSP's). A vertical seismic profile (VSP) is a seismic data gathering activity in which either the seismic receivers or the seismic sources are positioned at various depths in a drilled well. VSP surveys are closely related to horizontal seismic surveys. Both involve the propagation of seismic energy, from seismic sources, through the earth, to seismic receivers. The arrival times and character of the seismic waves are recorded. This data is typically further processed to eliminate or minimize spurious signals, and the resulting record provides significant information about the section of the earth through which the seismic energy passed. The geometry of the VSP presents particular problems, however, which are not encountered in horizontal seismic surveys. In a horizontal survey, the positions available for placement of seismic sources and receivers are virtually unlimited. In a VSP, one leg of the survey is fixed, and is limited to the location of the wellbore. Depth positions are limited to depths approximately between the surface intercept of the wellbore and the well's total depth. This limitation presents a problem of diminishing fold across the reflectors surveyed. In both horizontal and VSP surveys, the record of signals received by a receiver or single group of receivers is called a trace. During the processing stage, traces are stacked to eliminate spurious signals and the stacked traces represent a reflection from a common depth point (CDP) or point of reflection on the reflector surveyed. The number of traces stacked, which depends upon the number of source-receiver pairs which have received data reflected from a common depth point, is the fold. Because of the limited geometry of the VSP, not all common depth points on a reflector will have the same fold, or number of traces stacked to represent them. Conventional practice positions sources and receivers at equally spaced locations along the surface and in the wellbore. Typical spacing between sources or receivers in the borehole is 10 to 25 meters, and typical spacing along the surface of the earth is hundreds of meters. With this equally spaced positioning, CDP fold for a reflector surveyed falls rapidly as the CDP point moves radially away from the borehole. This results in VSP data which has poorer resolution and a higher noise to signal ratio away from the wellbore. VSP data with fold fall-off is also poorly comparable with horizontal sections with which one may wish to "tie" the VSP survey. Efforts to eliminate the reflection signal's amplitude degradation with increasing distance from the borehole due to CDP fold fall off problem in VSP's have centered on the processing phase of seismic exploration. Data is collected in a traditional manner, and then processed to compensate, somewhat, for a lack of fold. Most commonly, a technique known as amplitude gaining is employed to enhance data received from areas of the reflectors with fewer fold. This technique is not totally satisfactory because it destroys the true amplitude of the traces. True amplitude is useful both in making an accurate tie with horizontal surveys and in newer exploration techniques, such as direct hydrocarbon imaging (DHI). DHI requires retention of true amplitudes in a final record, as these amplitudes may be indicative of the fluid type (i.e. hydrocarbon) in the rock. GB-A-1204364 discloses a method for attenuating horizontally travelling noise by combining an array of transducers, either sources or detectors, into a single unit having an improved wavenumber response. The individual elements of the transducer array are spaced apart along a substantially straight line at intervals that are in relative proportion to each other as the intervals between the centroids of the same number of equal areas under a truncated sinc function based on the ideal wavenumber response for the array. The array functions as a unit to provide either a single source or a single detector. US-A-3,893,539 discloses an underwater seismic source for generating an improved seismic signal. The source uses a spaced array of air guns of different sizes to generate a seismic signal having an initial bubble impulse and to suppress the cumulative effects of secondary bubble impulses. The sizes, minimum spacings, and secondary bubble suppression techniques are combined to provide an underwater seismic source having an acoustic pressure signature wherein the amplitude of the initial impulse is from eight to twelve times the amplitude of any subsequent bubble impulses. To accomplish this, the individual sources are spaced apart from one another at increasingly greater distances sufficient to prevent coalescence of adjacent bubbles. All of the air guns are activated simultaneously to produce a single seismic signal. According to the present invention, there is provided a method of vertical seismic profiling of an earth formation including a plurality of seismic reflectors, wherein a plurality of seismic signals are sent into the earth formation towards the reflectors (14:114) from a plurality of seismic sources (30:130), and wherein said plurality of seismic signals are reflected by said reflectors (14:114) to form a plurality of reflected seismic signals which are received at a plurality of seismic receivers (2:102) and recorded, said receivers being positioned along or near the surface of the earth (32:132) while said seismic sources are positioned in a wellbore (4:104), or vice versa , and wherein each plurality of sources, signals and receivers can be the same or different; the method being characterized in that said sources and/or receivers are positioned so that a first distance (134:34) between a pair of adjacent seismic receivers (136:138) or sources (36:38) positioned along or near the surface of the earth (132:32) decreases exponentially as a second distance (120:20) from the closer (136:36) of the pair of adjacent seismic receivers (136:138) or sources (36:38) to the surface intercept (112:12) of the wellbore (104:4) increases, and/or a first distance (106:6) between a pair of adjacent sources (110:116) or receivers (10:16) positioned in the wellbore (104:4) increases exponentially as a second distance (108:8) from the closer (110:10) of the pair of adjacent seismic sources (110) or receivers (10) to the surface intercept (112:12) of the wellbore increases. The invention will be better understood by referring, by way of example, to the accompanying drawings, in which :- FIGURE 1 illustrates pictorially the basic components of a vertical seismic profile, including sources and receivers. FIGURE 2 illustrates pictorially the basic components of a reverse vertical seismic profile. FIGURE 3 illustrates graphically a division of the available points in the surface offset (r) and depth (z) plane into CDP bins. The borders of this figure represent constraints imposed by reflection imaging. FIGURE 4 (Prior Art) This figure graphically illustrates the distribution of CDP stacking fold or a range of CDP offset radii, r CDP , for an example in which the target reflector is located at a depth of 3,050 meters. Line 401 illustrates the distribution of fold for conventional VSP shooting, with equally-spaced source and receiver positions. FIGURE 5 graphically illustrates the distribution of CDP stacking fold for a range of CDP offset radii, r CDP , for an example in which the target reflector is located at the same depth as in Figure 4. Line 501 illustrates the improvement achieved spacing the sources and receivers as disclosed hereinbelow. Referring to FIGURE 1, a system for sending and receiving seismic signals comprises a plurality of seismic sources positioned at or near the surface of the earth, and a plurality of seismic receivers positioned, as indicated at 2 in FIGURE 1, in a wellbore, as indicated at 4. The distance between a pair of adjacent seismic receivers, Δz, as indicated at 6 in FIGURE 1, increases exponentially as a second distance, (z), as indicated at 8 in FIGURE 1, measured from the closer of the pair of adjacent seismic receivers, indicated at 10 in FIGURE 1, to the surface intercept of the wellbore, indicated at 12 in FIGURE 1, increases. Thus, in contrast to conventional VSP shooting, where seismic receiver positions are located at equally-spaced intervals, the seismic receivers in this system are positioned increasingly further apart as one goes further from the surface intercept of the wellbore. The sources in this system may be positioned at any locations at or near the surface of the earth. In a preferred embodiment of this invention, a distribution of CDP fold recorded by this system is substantially equalized, or flattened, across a reflector surveyed, indicated at 14 in FIGURE 1. In a more preferred embodiment of this invention, a plurality of seismic receivers, indicated at 2 in FIGURE 1, are positioned such that Equation I is satisfied: (I) Δz = (e βz ) (z') A first distance, Δz, indicated at 6 on FIGURE 1, is measured between a pair of adjacent seismic receivers, indicated by 10 and (16) in FIGURE 1. A second distance, z, as indicated at 8 in FIGURE 1, is less than or equal to the total depth of the wellbore and is measured from the closer of the pair of adjacent seismic receivers, indicated at 10 in FIGURE 1, to the surface intercept of the wellbore, indicated at 12 in FIGURE 1. β is a constant, and e is the exponential constant, being approximately equal to 2.718. The variable is an initial depth spacing interval based on aliasing considerations. To prevent data from having uncertain phase associated with sampling less densely than the Fourier criterion of at least 2 sample locations per wavelength, the standard procedure is to: (1) Determine the mimimum speed of sound among the geologic layers for P-waves, S-waves, and Stonely or "tube" waves in a borehole (or Rayleigh waves, also known as "ground roll", on the surface); (2) Determine the maximum frequency of seismic signals useful for reflection imaging, apply analog filters during field operations to remove the higher frequencies, and then (3) Set the spatial sampling density, in this case z', equal to the minimum velocity divided by twice the maximum frequency. In a more preferred embodiment of this invention, the constant β in Equation I is calculated by using linear regression to determine the slope of a plot of z versus log e A(z), where A(z) is determined by solving Equation II: (II) A(z) = 2DΔc -½(Δc) 2 -rΔc D-z 2D-z + DΔc log e [(2D-z)D] [(D-z)r + (2D-z)Δc] + rD D-z 2D-z log e [r(D-z)r] [r(D-z)r + (2D-z)Δc] for a series of values of z=(z')(n). The variable z' is the initial depth interval used in Equation I and n is an integer. Of necessity, z is less than or equal to the depth of the wellbore. The variable r is set equal to r', where r is an initial surface offset interval also based on aliasing considerations. D, indicated at 18 FIGURE 1, is the depth to a reflector being imaged, and Wc is a CDP increment, which may be chosen to be r' or r'/2. In a most preferred embodiment of this invention, the seismic sources are positioned at a plurality of distances, r, indicated by 20 in FIGURE 1, and the seismic receivers are positioned at a plurality of depths, z, indicated at 8 in FIGURE 1, such that the plurality of distances, r, and the plurality of depths, z, which are available for mapping a reflector at depth, D, are grouped into CDP radial offset bins. CDP radial offset bins are sets of data that represent a common depth point on a reflector being mapped. In the most preferred embodiment of this invention, the plurality of distances, r, and the plurality of depths, z, are grouped into CDP radial offset bins according to the Equation III: (III) r CDP = (D-z)r 2D-z In this equation, r CDP , indicated at 22 in FIGURE 1, is a distance measured radially from an axis running vertically through the wellbore, indicated by 24 in FIGURE 1, to a common depth point, indicated at 26 in FIGURE 1, on the reflector being mapped, indicated at 14 in FIGURE 1. The variable r is measured from the surface intercept of the wellbore, indicated at 12 , to the position of a seismic source at or near the surface of the earth. The depth z is measured from the surface intercept of the wellbore to the position in the wellbore of a seismic receiver. Of necessity, in shooting reflection seismic, the variety of depths, z, and the variety of surface offsets, r, which may be employed are limited by the constraint that an angle ϑ, indicated at 28 on FIGURE 1, must be ≤45°. The depth, z, must be less than or equal to the reflector depth D, and the surface offset, r, will be less than or equal to twice the reflector depth, D. D is measured from the surface of the earth, along a perpendicular to said surface, to the reflector, indicated at 14 in Figure 1. FIGURE 3 illustrates the division of available surface offset and depth positions into CDP bins. The example in FIGURE 3 illustrates the situation where a reflector being surveyed lies at a depth of 3048 meters in a constant velocity material, with a velocity of 2438 meters per second. The borders of the diagram represent the constraints just discussed. Embodiments noted as most preferred provided the best flattening of CDP fold in experimental tests. Another embodiment of the invention provides a system for sending and receiving seismic signals comprising a plurality of seismic receivers positioned in a wellbore, and a plurality of seismic sources, indicated at 30 in FIGURE 1, placed along or near the surface of the earth, indicated at 32 in FIGURE 1. In this system a first distance, Δr, indicated by 34 in FIGURE 1, between a pair of adjacent seismic source positions, indicated at 36 and 38 in FIGURE 1, decreases exponentially as a second distance, r, indicated at 20 in FIGURE 1, measured from the closer of the pair of adjacent seismic sources, indicated at 36 in FIGURE 1, to the surface intercept of the wellbore increases. Thus, in contrast to conventional VSP shooting, where source positions are located at equally-spaced intervals, the sources in this system are positioned increasingly closer together as one goes further from the surface intercept of the wellbore. The receivers in this system may be positioned at any locations in the wellbore. In a preferred embodiment of this invention, seismic sources in this system are positioned such that a distribution of CDP fold recorded by the system is substantially equalized across a reflector surveyed. A more preferred embodiment of the invention provides that the plurality of seismic sources, indicated at 3 0 in FIGURE 1, are positioned such that Equation IV is satisfied: (IV) Δr = (r') (e -αr ) In this equation, Δr is a first distance, indicated at 34 in FIGURE 1, measured between a pair of adjacent seismic source positions, r is a second distance, indicated 20 in FIGURE 1, measured from the closer of the pair of adjacent seismic source positions to the surface intercept of wellbore, indicated at 12 in FIGURE 1, and r' is an initial surface spacing interval based on aliasing considerations known in the art. The variable r' must be less than or equal to 2D, (α) is a constant, and e is a constant approximately equal to 2.718. In a more preferred embodiment of the invention, α is calculated by using linear regression to determine the slope of a plot of r versus log e A(r), where A(r) is determined by solving Equation V for values of r: (V) A(r) = 2DΔc -½(Δc) 2 D-z 2D-z + DΔc log e [D(2D-z)] [r(D-z)r + Δc(2D-z)] + rD D-z 2D-z log e [r(D-z)] [r(D-z) + Δc(2D-z)] The depth of the seismic receivers, z, as indicated at 8 in FIGURE 1, is held equal to zero, and the equation is solved for a series of values for r equal to n(r'), where n is an integer, and r' is the initial surface spacing interval used in Equation IV. D is a depth to a reflector, indicated by 18 in FIGURE 1, and Δc is a CDP increment, chosen to be either r' or r'/2. Another embodiment of the invention provides a system for sending and receiving seismic signals comprising seismic sources positioned in a wellbore and seismic receivers, indicated at 102 in FIGURE 2, positioned along or near the surface of the earth, indicated at 132 in FIGURE 2. A first distance, Δr, indicated at 134 in FIGURE 2, measured between a pair of adjacent seismic receivers, indicated at 136 and 138 in FIGURE 2, decreases exponentially as a second distance, r, indicated at 120 in FIGURE 2, from the closer of the pair of adjacent seismic receivers, indicated at 136 in FIGURE 2, to the surface intercept of the wellbore, indicated at 112 in FIGURE 2, increases. In a more preferred embodiment of the invention, the seismic receivers are positioned such that a distribution of CDP fold recorded by the system is substantially equalized across a reflector surveyed. An example of a reflector surveyed is indicated by 114 in FIGURE 2. Preferably the seismic receivers are positioned such that Equation IV is satisfied. Even more preferably α is calculated by using linear regression to determine the slope of a plot of r versus log e A(r), where A(r) is determined by solving Equation V for values of r equal to n(r'). A most preferred embodiment of the invention provides that the seismic sources are positioned at a plurality of depths, z, indicated 108 in FIGURE 2, and the seismic receivers are positioned at a plurality of distances, r, indicated at 120 in FIGURE 2, such that the plurality of depths, z, and the plurality of distances, r, which are available for mapping a reflector at a depth, D, are grouped into CDP radial offset bins according to Equation III. CDP radial offset bins are sets of data that are treated as representing a common depth point on the reflector being mapped. In this instance r CDP is a distance, indicated at 122 in FIGURE 2, measured radially from an axis running vertically through the wellbore, indicated at 124 in FIGURE 2, to a common depth point, indicated at 126 in FIGURE 2, on a reflector being mapped, indicated at 114 on FIGURE 2. The distance r is measured from the surface intercept of the wellbore, indicated at 112 , to the position of a seismic receiver at or near the surface of the earth. The depth z is measured from the surface intercept of the wellbore to the position in the wellbore of a seismic source. Another preferred embodiment of the invention provides a system for sending and receiving seismic signals comprising a plurality of seismic receivers positioned at or near the surface of the earth and a plurality of seismic sources, indicated at 130 in FIGURE 2, are positioned in a wellbore, indicated at 104 in FIGURE 2. A first distance, Δz, indicated at 106 in FIGURE 2, between a pair of adjacent seismic source positions increases exponentially as a second distance, z, indicated at 108 in FIGURE 2, measured from the closer of the pair of adjacent seismic source positions, indicated at 110 in FIGURE 2, to the surface intercept of the wellbore, indicated at 112 in FIGURE 2, increases. In a preferred embodiment of the invention, sources are positioned such that a distribution of CDP fold recorded by the system is substantially equalized across a reflector surveyed. A more preferred embodiment of the invention provides that the seismic sources indicated at 130 in FIGURE 2, are positioned such that Equation I is satisfied. In this instance, Δz, indicated at 106 in FIGURE 2, is a first distance, measured between a pair of adjacent seismic sources, indicated at 110 and 116 in FIGURE 2. The depth z, indicated at 108 in FIGURE 2, is a second distance, less than or equal to the total depth of the wellbore and measured from the closer of the pair of adjacent seismic receivers, indicated at 110 in FIGURE 2, to the surface intercept of the wellbore, β is a constant, z' is an initial depth data spacing interval based on aliasing considerations known in the art, and e is a constant approximately equal to 2.718. In a more preferred embodiment of this invention, the constant β in the above equation is calculated by using linear regression to determine the slope of a plot of z versus log e A(z), where A(z) is determined by solving Equation II for a series of values of z less than or equal to the depth of the wellbore, where z=(z')(n). The variable z' is the initial depth interval used in Equation I, and n is an integer. In solving this equation, r is held equal to r', an initial surface offset interval based on aliasing consideration, D, indicated at 118 FIGURE 2, is a depth to a reflector, and Δc is a CDP increment, which may be chosen to be r' or r'/2. FIGURES 1 and 2 illustrate positions of both sources and receivers that are positioned according to this invention. Positioning of sources and positioning of receivers may be practiced independently, however, and should result in data of more even fold than conventional spacing. FIGURE 5 illustrates results for a 3048 meter reflector, imaged through rock of a constant velocity, positioning both seismic sources and receivers as taught by the invention. Figure 4 illustrates results obtained for the same conditions using conventional positioning. Curve 401 illustrates the extreme drop off of CDP stacking fold under conventional VSP spacing, and curve 501 in Figure 5 illustrates the results achieved through use of this system. The computer program contained in Table I, written in FORTRAN, illustrates one preferred embodiment of the invention.

Claims: A method of vertical seismic profiling of an earth formation including a plurality of seismic reflectors, wherein a plurality of seismic signals are sent into the earth formation towards the reflectors (14:114) from a plurality of seismic sources (30:130), and wherein said plurality of seismic signals are reflected by said reflectors (14:114) to form a plurality of reflected seismic signals which are received at a plurality of seismic receivers (2:102) and recorded, said receivers being positioned along or near the surface of the earth (32:132) while said seismic sources are positioned in a wellbore (4:104), or vice versa, and wherein each plurality of sources, signals and receivers can be the same or different; the method being characterized in that said sources and/or receivers are positioned so that a first distance (134:34) between a pair of adjacent seismic receivers (136:138) or sources (36:38) positioned along or near the surface of the earth (132:32) decreases exponentially as a second distance (120:20) from the closer (136:36) of the pair of adjacent seismic receivers (136:138) or sources (36:38) to the surface intercept (112:12) of the wellbore (104:4), increases and/or a first distance (106:6) between a pair of adjacent sources (110:116) or receivers (10:16) positioned in the wellbore (104:4) increases exponentially as a second distance (108:8) from the closer (110:10) of the pair of adjacent seismic sources (110) or receivers (10) to the surface intercept (112:12) of the wellbore increases. A method according to claim 1, wherein the seismic sources (30) are positioned along or near the surface of the earth (32) and each pair of adjacent seismic receivers (36,38) have said exponentially increasing first distance between them, and wherein the plurality of reflected seismic signals are received at a plurality of seismic receiver positions such that a distribution of CDP fold recorded by the system is substantially equalized across a reflector (14) surveyed. A method according to claim 2, wherein: the plurality of seismic receivers (2) are positioned such that equation: Δz = (e βz ) (z') is satisfied, wherein Δz is a first distance, measured between a pair of adjacent seismic receivers (10,16), z is a second distance, measured from the closer (10) of the pair of adjacent seismic receivers (10,16) to the surface intercept (12) of the wellbore (4), z' is a predetermined value representing an initial depth interval based on aliasing considerations, β is a constant, and e is the exponential constant, being approximately equal to 2.718. A method according to claim 3 for sending and receiving seismic signals, wherein: β is is calculated by using linear regression to determine the slope of a plot of z, log e A(z), where A(z) is determined by solving the equation: A(z) = 2DΔc -½(Δc) 2 -rΔc D-z 2D-z + DΔc log e [(2D-z)D] [(D-z)r + (2D-z)Δc] + rD D-z 2D-z log e [(D-z)r] [(D-z)r + (2D-z)Δc] for values of z=n(z'), where z≤D, n is an integer, z' is an initial depth interval based on aliasing considerations, D is a depth to a reflector (14) being surveyed; Δc represents a CDP increment, chosen to be either z' or z'/2, r is set equal to r', and r' is an initial surface offset interval based on aliasing considerations. A method according to claim 4, wherein: the plurality of seismic sources (30) are positioned at a plurality of distances, r, measured from the surface intercept (12) of the wellbore (4) to a position in the wellbore of a seismic receiver (36) at or near the surface of the earth (32), and the plurality of seismic receivers (2) are positioned at a plurality of depths, z, in the wellbore, measured from the surface intercept (12) of the wellbore (4) to a position in the wellbore of a seismic receiver (10), such that the plurality of distances, r, and the plurality of depths, z, which are available for mapping a reflector (14) at depth, D, measured from the surface of the earth (32), along a perpendicular to said surface, to the reflector (14), are grouped into CDP radial offset bins according to the equation: r CDP = (D-z)r 2D-z in which r CDP (22) is a distance measured radially from an axis (24) running vertically through the wellbore (4) to a common depth point (26) on the reflector (14) being mapped, and CDP radial offset bins are sets of data that are treated as representing a common depth point (26) on the reflector (14) being mapped. A method according to claim 1, wherein the seismic sources (30) are placed along or near the surface of the earth (32) with said exponentially decreasing first distance (34) between said pair of adjacent seismic source positions (36,38), and wherein the plurality of seismic signals are generated from a plurality of seismic source positions such that a distribution of CDP fold recorded by the system is substantially equalized across the reflector (14) surveyed. A method according to claim 6, wherein: the plurality of seismic sources (30) are positioned such that equation: Δr = (e -αr ) (r') is satisfied, wherein Δr is the first distance, measured between a pair of adjacent seismic source positions (36,38), r is the second distance, measured from the closer (36) of the pair of adjacent seismic source positions (36,38) to the surface intercept (24) of the wellbore (4), r' is a predetermined value representing an initial spacing interval based on aliasing considerations, α is a constant, and e is the exponential constant approximately equal to 2.718. A method according to claim 7, wherein: α is calculated by using linear regression to determine the slope of a plot of r, log e A(r), where A(r) is determined by solving the equation: A(r) = 2DΔc -½(Δc) 2 -rΔc D-z 2D-z + DΔc log e [D(2D-z)] [r(D-z) + Δc(2D-z)] + rD D-z 2D-z log e [(D-z)r] [(D-z)r + Δc(2D-z)] for values of r=n(r'), where r<2D, n is an integer, r' is the initial surface offset interval based on aliasing considerations, Z is set equal to zero, D is a depth to a reflector (14) being surveyed; and Δc represents a CDP increment, chosen to be either r' or r'/2. A method according to claim 8, wherein: the plurality of seismic receivers (2) are positioned at a plurality of depths, z, in the wellbore (4), measured from the surface intercept (12) of the wellbore (4) to a position in the wellbore of a seismic receiver (10), and the plurality of seismic sources (30) are positioned at a plurality of distances, r, measured from the surface intercept (12) of the wellbore (4) to a position, at or near the surface of the earth (32), of a seismic source (36), such that the plurality of depths, z, and the plurality of distances, r, which are available for mapping a reflector (14) at depth, D, measured from the surface of the earth (32), along a perpendicular to said surface, to the reflector (14), are grouped into CDP radial offset bins according to the equation: r CDP = (D-z)r 2D-z in which r CDP (22) is a distance measured radially from an axis (24) running vertically through the wellbore (4) to a common depth point (26) on the reflector (14) being mapped, and CDP radial offset bins are sets of data that are treated as representing a common depth point (26) on the reflector (14) being mapped.

Inventors: RICE JAMES ALLEN, RICE, JAMES ALLEN
Applicants: EXXON PRODUCTION RESEARCH CO, EXXON PRODUCTION RESEARCH COMPANY
Application year: 1992
Main classification: G01V1
IPC codes: G01V1