US2569411A - Seismographic prospecting - Google Patents

Description

Sept. 25, 1951 SEISMOGRAPHIC PROSPECTING Filed March 25, 1949 s mL a ma m 0 C A L a lid/4v TORNEYS FIG. 2.

Patented Sept. 25, 1951 UNITED STATES OFFICE SEISMOGRAPHIC PROSPECTING Lacoste G. Ellis, Beaumont, .Tex., assignor :to Sun Oil Company; Philadelphia, Pa",

of New Jersey;

a corporation Application -March*25, 1949;1SerialNo'; 83,363 1'Claim.-.v (Cl. 181".53-)

earth or within a relatively shallow shothole which, considering the depth of the reflecting horizons of interest, effectively locates the'shotatthe surface. Detectors are also provided at the surface or adjacent to the surface in shallow holes; the detectors being arranged in various arrays which are calculated to give the-most informative results. Such practices; however, have the disadvantage that in the event of deep reflections the point of reflection is so far removed from both the shot point and the detector=that there is, in cases of complex structure, considerable doubt of the exact point of'refiection. Furthermore, when the structureiscomplicated it is 'difficult to understand-and picture geometrically the travel paths of the waves and, accordingly, there are introduced corresponding uncertainties in the interpretation of the structural conditions.

In accordance with the present invention the difficulties indicated are,- to a majorextent, avoided by accomplishing shooting in wells which areto be abandoned either because they have notproduced or because their production has'ceased: tobe of sufficient quantity to warrant their main-:- tenance. By the use of the-present methodsuch wells may be effectively utilized tosecure further information concerning the conditions existinginthe adjacentfield. In brief, in' accordance withthe invention an array of detectors is pro-. vided at or adjacent to thesurface of-the ground and a series of shots are fired within thewell'ordinarily startingat the greatest depth and-progressing up the -'h01e S that even if destruction-occurs in the vicinity of and below the "shot points: a complete series of shots may be fired. The general objects of the invention will, be apparent from the foregoing and details thereof will become clearer from consideration of procedures as outlined below in conjunction with the accompanying drawing in. which Figuresland 2 illustrate diagrammatically generally vertical sections through the earth showing the fashions in 'which shooting is effected and the nature of the results attained thereby.

Referring to-Figure 1, there -isillustrated-therein a well 2 which may be considered a well about to be abandoned. In general, 'such a well'will have been surveyed so that its position at any given depth will be known to a considerable degree of accuracy and, furthermore, there will be known, as the result of coring or logging, the locations of the stratapenetrated by the well. With this. knowledge itwill'be evident that explosive charges:

maybe located within-the well at various known positions not only asto coordinates with respect tothe mouth'of the well but also with respect to the various strata which have been penetrated. In particular, shot points may be located in the well" between particular known interfaces from.

whichreflections may be caused to occur.

Thereis indicated at 4 the weathered layer ad jacent to the surface of the earthabove which or in which detectors may be located. At 6, 8, l0, l2, l4 and IG there are indicated interfaces between the strata which, in the course of ordinary reflection shooting, would constitute reflecting. horizons. For simplicity, of course, the number ofthese-is shown considerably less than would be usual in thecase ofan actual situation. The bottom of the hole-is illustrated at I 8 within the stratum between'theinterfaces l4 and I6.

Detectors may be arranged at the surface in any-desired arrays, for example radiating from the mouth of the holeas indicated at 29 or surrounding the mouth of the holein arcs or other fashions as indicated at 2B. These detectors will beof the type generally used for reflection shooting'and, as in-ordinary reflection shooting, will beconnected-to'recorders of conventional types. Inasmuch as the particular detecting apparatus usedmay be conventional it need not be detailed herein; It" may comprise, for example, filtering arrangements for the'purpose of accentuating reflections, automatic volume controls, means for determining the instant of firing, etc.

In accordance with the invention, with a given array of detectors, shots are fired, for example, as-indic'ated at S1, S2, S3, and S4 in succession, the shots being desirably located between different pairs of reflecting horizons. Considering specifically' Figure '1, it will be noted that the first shot point =S1 is located at or near the bottom of the well -betweenthe horizons l4 and 16. In order to' avoid undue complication in the figure there are illustrated only the rays reaching the second and 'sixtndetectors 20 located .at one side of the mouth of the hole. The direct rays from the shot point si'are indicated atm and D'1 and reflected rays from the horizon l6 are indicated at R10 and R:m; A consideration of Figure 1 will reveal thatiashottat. the point Si, which shot will be firedthrough' connections tothe surface which are not shown, may produce the indicated reflections from the horizon I6 and possibly from deeper horizons but that reflections will not occur from horizons such as [4, l2, etc. above the shot point S1, though some refraction of the rays inay occur at these upper horizons. At any rate, it will be evident that the travel times of the reflections, for example, from the horizon IE, will be only slightly greater, in general, than the travel times of the direct rays from this same shot point and, consequently, the reflections from the horizon 16- may be rather easily detected on the seismograph record in contrast with the reflections from deeper horizons. Furthermore, since the shot point S1 is accurately known preted to provide an accurate picture of the contour of the horizon 16. This determination of the contour of horizon 16 may be made much more accurately than if the shot point was located at the surface and the same is true of the contours of lower horizons in view of the fact that the shot point will be much closer thereto than a shot point at the surface. The travel times of the rays reflected by such lower horizons will, of

course, be considerably less than if the shot point was at the surface. Furthermore, there are eliminated reflections from higher horizons so that interferences by reflections from them will be eliminated from the record.

The firing of the shot at S1 will, of course, generally destroy the portion of the hole in its vicinity but since succeeding shots are fired thereabove there is no interference with the possibility of providing such shots.

The next shot may be fired at the point S2 above the next higher horizon 14 or above some still higher horizon since these may be rather closely spaced. It will be noted that the shot fired at S2 will produce not only reflections R20 and R'2o from the horizon 16 but also reflections R21 and R'Zl from the horizon l4. This means that the passage from below to above the horizon M for the new shot point will produce distinctly new reflections in the record which were not present in the record formed by the shot at S1. These new reflections may be easily identified. Of course, if several reflecting horizons existed between the two successive shot points a corresponding number of new reflections would appear in the record made by each of the detectors. Following this, new shots may be fired at the shot points S3, S4, etc.

It will be evident that the records thus produced not only show different numbers of reflections but will be more indicative of the nature of the horizons in the general vicinity of the well than records produced by shots at the surface. Local slopes, for example, can be much more readily determined. As is usual, of course, interpretations are based on the delays of the reflections from the instant of firing of the shot.

Another situation which may arise is that illustrated in Figure 2 in which the well 22 is illustrated as passing near a fault F. Shot points are indicated at S5, S6 and S7. While considerations as above described will also exist, 1. e., the appearance of new reflections as the shot point rises above reflecting horizons, there will also exist situations such as that exemplified by reflections from the horizon 2d, interrupted by the fault, due to a shot at the shot point S1. Various reflections are indicated at R3, R4 and R5. It will be noted that the reflections R3 and R4 are from portions of the horizon to the left of the fault. The reflection it will be evident that the seismographic record may be easily inter-v R5, on the other hand, is from a portion of the horizon to the right of the fault and the relative transit times of the reflections R4 and R5 will be related to the fact that the horizon level is displaced by reason of the fault. Intermediate re flections between R4 and R5 will also be informative and, in fact, as will be apparent, one or more of these might not appear at all. Such an absence of reflections may be interpreted in terms of an accurate location of the surface of the fault.

The discussions of the types of reflections produced have, of course, been somewhat idealized since, in general, considerably more complication is involved in interpretation of results than those indicated; but those skilled in this art are accustomed to take into account the geometrical complications which may appear and are able to provide complete analyses of the records which are produced. As indicated, the advantage of the present invention lies in the fact that the reflections are from points on the horizons relatively close to the shot points and analysis is considerably simplifled with more accurate evaluation of the significance of the various records. It may be noted that attempts to reverse the directions of the rays will not be nearly as satisfactory in view of the fact that many more shots would be required to secure the same results and those detectors located in the hole would receive their signals from various directions so that there could not be gained the usual advantages of using the detectors commonly used in reflection shooting which have sensitivity characteristics which are directional.

As a result of the invention deep reflecting horizons may be mapped to a high degree of accuracy when their depths are comparable with the depths of successive shot points.

What I claim and desire to protect by Letters Patent is:

A method of reflection shooting comprising providing an array of detectors, adapted to receive reflections from deep strata, adjacent to the surface of the earth at different distances from the top of a bore hole penetrating deep reflecting horizons the orientation of which is sought, and firing successively within said bore hole a series of shots, one of said shots being fired below one of said deep reflecting horizons, and a subsequent shot being fired above said horizon and below the next higher reflecting horizon, so that in the case of said second shot reflections to said detectors will occur from said first mentioned horizon, from which no direct reflection to said detectors will occur due to said first mentioned shot, the reflections from said horizons being received by said detectors substantially later than direct waves from said shots.

LACOSTE G. ELLIS.

REFERENCES CITED The following references are of record in the file of this patent:

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