GB2090405A

GB2090405A - Determination of static correction from refraction travel time

Info

Publication number: GB2090405A
Application number: GB8127181A
Authority: GB
Original assignee: Mobil Oil Corp
Current assignee: ExxonMobil Oil Corp
Priority date: 1980-12-31
Filing date: 1981-09-09
Publication date: 1982-07-07
Also published as: DE3149959A1

Abstract

A method for making static corrections by measuring the difference in refracted wave travel times between a fixed pair of sources A, B and a detector e.g. C spaced some distance from the sources and in line with them. The difference in travel times is then determined with the detector on the opposite side of the sources. From the measured differences the difference in path lengths a, b from the sources to the interface at the bottom of the weathered layer 10 can be determined. The sources and detectors may be interchanged in their respective positions. <IMAGE>

Description

SPECIFICATION Determination of static correction from refraction travel time This invention relates to seismic prospecting for minerals such as oil and gas and more particularly to a method of determining the time taken for a sismic wave to travel through the "weathered layer" at the earth's surface.

It has been common for many years to utilize seismic techniques for the exploration of the earth for oil, gas and other minerals. These techniques involve generating acoustic waves in the earth's crust by explosion, by vibration of a heavy massive object, or by other means, and detecting the returned acoustic wave which has been reflected from an interface between subterranean layers of rock. The time taken by the various waves to travel from the source to the interface and back to the detector is measured and compared and this comparison can then be used to generate a representation of the subterranean interface.

Among the problems encountered with these techniques is the fact that the only data available are the travel times taken of the acoustic waves. If the travel times of these waves are to be used properly in determining the shape of the subterranean interfaces, corrections must be made for variations in the velocity of the wave's passage through various layers. For example, that a large portion of the earth's land surface is covered with a layer of "weathering"; that is, an area in which a large volume of comparatively loose soil has been deposited but which has not been compressed into rock, so that the speed of the acoustic wave through this "weathered layer" is very much slower than in the solid rock disposed thereunder.Typically, the velocity in such a layer is between 1500 and 2500 feet per second (457 to 762 m.sec-1), the velocity in the lower layer may be 5000 feet per second (1524 m.sec.-') or more. Accordingly, it is important that a "weathering correction" be made.

In addition to being partially reflected at interfaces between successive subsurface layers, a portion of the acoustic wave passing from a low velocity layer into a high velocity layer is partially refracted along the interface. This fact has been used to determine the difference in time taken by successive acoustic waves to pass through the weathered layer, thereby providing a relative measure of the weathering correction to be made to reflection data. U.S. Patent No.4,101,867 describes a technique in which the time taken for an acoustic wave to pass from a source downwardly through the weathered layer, by refraction along the interface between the weathered layer and the first sub-weathered layer, and upwardly through the weathered layer to a geophone located a given distance away on the earth's surface is measured.This time is compared with the time taken for a similar acoustic wave to pass along a similar path between a second source located some distance from the first source to a second geophone located the same distance from the first geophone. If a large quantity of these parallel measurements of wave paths between adjacent sources and adjacent geophones are then made, they can be summed according to a simple mathematical formula to average out and thus effectively eliminate variations in the distance between the sub-weathered layer and the geophones, leaving the distance between the sources and the sub-weathered layer as the only source of variation in travel time.

The method described in U.S. 4,101,867 is not without utility but no indication of the absolute velocity of the wave in the sub-weathered layer is provided. Moreover, the method requires a great number of measurements to be averaged so as to eliminate statistically variations in the interface-to-detector distances.

We have now devised an improved method for making weathering corrections. According to the present invention the method is based upon a determination of the difference in travel time required for acoustic waves to pass between points on the surface of the earth and along generally parallel paths at the interface between a weathered layer of the earth and a sub-weathered layer. In the first step of the method the difference in the travel times along a pair of such paths of different length is determined. These paths extend between a pair of fixed points at the surface of the earth and a third point which is co-linear with the fixed points.In the second step of the method, the difference in the travel times along a pair of such paths of different length is determined, but in this case the paths extend between the fixed points and a fourth location which is co-linear with the fixed points but on the opposite side of them to the third point. The measured differences are then summed to yield a result proportional to the difference in path length between the fixed points and the interface.

Because the ray paths are reversible either the seismic sources or the detectors may be located at the fixed points. The invention is described below with reference to an embodiment with the sources at the fixed points, but the alternative arrangement with the detectors at the fixed points is also feasible.

The results may be improved by employing a number of sources or detectors at the third and fourth points and averaging the values of the travel time differences over the paths to the detectors or sources, respectively. Thus, if the sources are located at the fixed points the travel-time measurements are taken for the paths between the pair of sources and a single detector; and the difference is recorded. A second set of measurement are taken over a path leading from the same sources to a second point of detection, a third, and so on.Averaging the values of the differences attenuates any noise, leaving a value equal to the difference in travel time from the sub-weathered layer to each of the sources plus that time taken for the wave to pass along the interface between the weathered and the sub-weathered layers from the intersection of the first wave path with the sub-weathered layer to that of the other. The same procedure is then repeated with respect to the same sources but with the detectors (geophones) located on the opposite side of the sources from the initial measurements, and the same computations are performed.When the sums and differences of the two resultant equations (yielding the values for the average time) are taken, the sum is an indication of the absolute time difference taken by an acoustic wave to pass from the two sources to the sub-weathered layer, while the difference is proportional to the time taken by the acoustic wave to pass along the interface in the sub-weathered layer from the intersection of the ray path connecting one source with the sub-weathered layer to the intersection of the other. If the assumption can be made that the interface between the weathered layer and the sub-weathered layer is essentially flat, this can be used to calculate the velocity of sound in that medium.

The invention is described below with reference to the accompanying drawings in which: Figure 1 represents the arrangement used in the method of U.S. 4,101,867; Figure 2 represents the ray path of acoustic waves during the first sequence of operations according to the present method; and Figure 3 represents the ray paths taken by acoustic waves during the performance of the second set of measurements according to the present method.

Reference will now be made to Figure 1 for a description of the method described in U.S. 4,101,867. Figure 1 represents a cross-section of the earth. A pair of sources of acoustic energy are located at points A and B on the surface of the earth. Acoustic waves emitted by these sources A and B are transmitted through a first weathered layer 10, thence through a sub-weathered layer 12, and finally to a second sub-weathered layer 14, from which they are reflected back upwardly through sub-weathered layer 12 and weathered layer 10, to eventually arrive at the surface at points C and D respectively. If geophones are located at these points they will detect reflections from the second sub-weathered layer 14. The time taken for the reflected acoustic wave to reach the detectors is measured.If enough such measurements are taken, the times can be compared, processed, and used to yield a picture generally corresponding to the configuration of the second sub-weathered layer 14.

However, in order that the second sub-weathered layer 14 can be accurately depicted, it is essential that a correction be made for the time taken by the acoustic waves to travel through the weathered layer 10. As noted above, the velocity of an acoustic wave in a weathered layer such as 10 is typically much less than that in a comparatively solid sub-weathered layer such as at 12 or 14, and therefore the measurement of the time taken for the acoustic wave to travel from a source as at A to a detector C can only yield a reasonably accurate picture of the sub-surface formation if a correction is made for this low velocity or weathered layer 10. As the weathered layer 10 may be quite deep, it can cause significant inaccuracy.

Also as noted above, in addition to travelling through the earth until reflected from a hard layer such as at 14, acoustic waves are also refracted when they reach an interface between a first lower velocity and a second hig her velocity layer (such as at the interface between the weathered layer 10 and the sub-weathered layer 12), and travel along the interface. If detectors are placed on the surface of the lower velocity layer such as on the surface of the earth at C and D in Figure 1, they will detect this refracted wave.Accordingly, ray paths as indicated in Figure 1, which go down through the sub-weathered layer from the sources A and B travel along the interface between the weathered layer 10 and the sub-weathered layer 12 and are detected upon passage upwardly through the weathered layer to detectors located at C, D, E, F,...N-1, N, also occur as indicated. These refracted waves are used according to the method of U.S. 4,101,867 to determine the relative time taken for the wave to pass downwardly from sources such as A and B to the interface between the weathered layer and the sub-weathered layer 12.Once the relative difference in path length between the path leading from source A to the sub-weathered layer 12 and that leading from source B to the sub-weathered layer 12 is known, this correction can be applied to the time taken for the wave originating at A and B and reflected from the second sub-weathered layer 14 and used to correct this experimental evidence to obtain a more accurate picture of the second sub-weathered layer 14.

As indicated on Figure 1, the ray path f-rom source A to detector C comprises three portions, a path a downwardly through the weathered layer 10, passage along a distance E between the weathered layer 10 and the sub-weathered layer 12, and passage upwardly through the weathered layer 10 along the path c to the detector C. A comparable path is established between a source B along a ray path b downwardly through the weathered layer 10, along an equal distance Eat the interface between the weathered layer 10 and sub-weathered layer 12 and upwardly along a ray path d in the weathered layer 10 to a detector D. The difference in time taken along the paths a and b, a - b, is what the method of U.S. 4,101,867 seeks to determine.Clearly the times taken from the rays to pass from A to C and from B to D are equal to (a + E + c) and (b + E + d) respectively. Simply substracting the two measurements from one another eliminates the E's; however, to eliminate c and d requires that technique which is described in U.S. 4,101,867. This technique eliminates c and d from equation (1) above by taking a large number of comparable measurements. In Figure 1 these would correspond to measurements of the time taken from A to C, A to D, A to E,... through A to N - 1. A comparable series of measurements from B to D, B to E, B to F, through B to N are taken. The components E + n AE and the paths d, e, f... n - 1 are each common to one of the measurements from source A or from source B. Therefore, if the time measurements taken with respect to source B are made negative and are summed together with the corresponding times taken from source A, the E's, dE's as well as all of the path lengths d, e,... n - 1 will cancel, leaving the sum equal to the quantity N (a - b) + (e - n). (1) If the average of this sum is now taken, one is left with Correction = (a - b) + (e - n)/N. (2) Since c is roughly equal to n, and since N is large, (c - n)/N may be assumed to approximate 0 and the total weathered correction a - b is given by this process. The method of U.S. 4,101,867 therefore comprises measuring the time taken for an acoustic wave to pass from two adjacent energy sources to adjacent pairs of detectors.By summing over a large number of pairs of detectors, differences in the actual distance between the interface between the weathered layer 10 and the sub-weathered layer 12 and the surface of the earth on which the detectors rest can be assumed to cancel out, leaving the difference between the path lengths between the two sources to the interface as the source of difference in the pairs of time measurements.

In contrast, the present method compares the time taken for the acoustic wave to pass from two adjacent sources to the same detector and uses a plurality of such time difference measurements merely to reduce experimental error. A second set of measurements taken with the detector(s) disposed on the opposite side of the sources of acoustic energy is used algebraically to eliminate the ray path along the interface from the time measurement, leaving the difference in ray paths in the weathered layer the only source of difference in the time measurements.

Referring now to Figure 2 a first arrangement of sources A and B and geophones C, D,...N is shown. The time of travel of an acoustic wave from a source, for example source A, downwardly along a path a through a weathered layer 10, along a distance Eat the interface between the weathered layer 10 and the subweathered layer 12 and upwardly along a path c to a detector C is measured. Unlike the method of U.S.

4,101,867, the path between each detector C, D,...N is relevant with respect to waves originating at both sources A and B. Thus, for example, the time taken for the wave to pass from source A to detector D (hereinafter styled AD) AD = a + E + AE + d Likewise for BD: BD = b + E + d, Subtracting these two values, we have: AD - BD = a + E + d + AE - b - E - d = a - b + AE. (3) A number of such measurements N can then be made, that is, the time taken for a wave to pass from a source A two a series of detectors, D, E,...N can be measured and compared with the time taken for a similar wave to pass from a source B to the detectors D, E,... N.The results can be summed and divided by N in order to yield an average value, thus eliminating any noise or other random inaccuracies from the measurement, leaving a resonably accurate value equal to a - b + AE.

The next step in the method involves taking similar measurements with a second set of geophones C', D' ... N' disposed on the opposite side of the pair of sources A and B from those used in Figure 2. The same measurements are similarly processed to yield the difference value: AD' - BD' = a-b- AE (4) Adding equations 3 and 4, we have: Tsum = (a - b + hE) + (a - b - hE) = 2 (a - b) (5) thus yielding the desired static correction; that is, a measure of the difference in travel time of acoustic waves between successive source locations and the interface. If a long series of sources are used, as is usual in geophysical exploration, the method can be performed with respect to each successive pair of sources, yielding a substantially continuous picture.

Furthermore, it will be appreciated that by subtracting equation 4 from equation 3 we have Tiff= (a - b + AE) - (a - b - AE) = 2 (hE) (6) AE is the travel time along the interface between the weathered layer 10 and the sub-weathered layer 12 between the ray paths a and b.If the distance X between the sources A and B is known and it is assumed that a and b are parallel, the refractor velocity V between A and B at the interface can be calculated: V = X/E (7) The method according to the present invention differs from that of U.S. 4,101,867 in that the measurement made in the prior method is an indirect one relying on the averaging of the difference in path from the interface between the weathered layer and the sub-weathered layer to the detectors for accuracy, whereas the present measurement is direct and uses a large number of measurements only in order to average out noise or other experimental inaccuracies. The data required to calculate the static correction for weathering by the present method is already a portion of the seismographic record used to determined the shape of the second sub-weathered layer 14 and only relatively straightforward additional computations need be performed in order to make the correction. The correlation (that is, the comparison of successive data records to determine the actual time of receipt of the acoustic waves by the detector) described in U.S.

4,101,867 is applicable to the present method. While the invention has been described in terms of a pair of sources operating with a large number of detectors, the inverse arrangement would be equally functional.

Other possibilities include displacing the arrays of detectors from the line connecting the sources, which might be of use in areas where straight line exploration is not possible.

Claims

1. A method of determing the difference in travel time required for acoustic waves to pass between points on the surface of the earth and along generally parallel paths at the interface between a weathered layer of the earth and a sub-weathered layer in which, in a first step, the difference between the travel times along a pair of such paths of different length extending between a pair of fixed points at the surface of the earth and a generally co-linear third point is determined and, in a second step, the difference between the travel times along a pair of such paths of different length is determined, the paths extending between the fixed points and a fourth point generally co-linear with the fixed points but on the opposite side of the fixed points to the third point and summing the differences to yield a result proportional to the difference in path length between the fixed points and the interface.

2. A method according to claim 1 in which sources of the acoustic waves are located at the fixed points and a detector at the third and fourth points.

3. A method according to claim 2 in which the differences are subtracted from one another to yield a result proportional to twice the distance between the intersections of the ray paths of the waves along the interface and to the velocity of the acoustic waves along the interface.

4. A method according to claim 2 or 3 in which a plurality of detectors are disposed co-linearly with the fixed points in each of the first and second steps with the resulting plurality of measurements of travel times in each step being averaged.

5. A method according to claim 1 in which detectors for the acoustic waves are located at the fixed points and a source at the third and fourth points.

6. A method according to claim 5 in which the differences are subtracted from one another to yield a result proportional to twice the distance between the intersections of the ray paths of the waves along the interface and to the velocity of the acoustic waves along the interface.

7. A method according to claim 5 or 6 in which a pluraity of sources are disposed co-linearly with the fixed points in each of the first and second steps with the resulting plurality of measurements of travel times in each step being averaged.