US2345288A - Method of seismic prospecting - Google Patents

Description

March 28, 1944. w. E PUGH METHOD OF SEISMIC PROSPECTING Filed Sept. 22, 1942 3 Sheets-Sheet, 1

WNW memEwL 0 my M MP .m m M WMWZZMW Patented Mar. 28, 1944 METHOD or SEISMIC rnosrsc'rmc William E. Pugh, Tulsa, Okla.,

Seismograph Service Corporation, Tulsa, Okla., a corporation of Oklahoma aasignor to Application September 22, 1942, Serial No. 459.325

4 Claims- (Cl- Isl-0.5)

This invention relate generally to the art of geophysical prospecting and more particularly to a method for conducting a continuous profile survey of the substrata of the earth's surface.

In order to obtain the data necessary from which a continuous profile of the substrata of the earth can be produced, it has been customary heretofore to record seismic waves which have been reflected from the interfaces of the substrata at detecting stations spaced at equal intervals throughout the length of the entire traverse. The detecting stations are usually divided into equal groups to form spreads and the waves detected at each group of stations recorded on a single seismogram in coordination with time. At each spread two records are made which represent waves originating at shot points that are equally spaced from the opposite ends of the spread. This procedure was thought necessary to obtain 100% continual subsurface control.

The instant invention is directed to a method of conducting a continuous profile of the substrata. of the earths surface for a 100% continuous subsurface control from only 50% of surface spread coverage and which will utilize onehalf the normal number of geophone set-ups for any given traverse distance.

Other outstanding advantages of the present invention over those practiced heretofore are: Acceleration of field operation, deep penetrating weathering control, elimination of spread and variable velocity effects and minimized calculation errors.

Other advantages of the present invention will become apparent from the following detailed description, when considered with the accompanying drawings in which:

Figure 1 is a diagrammatic view illustrating the positioning of detectors and sources of elastic wave along a traverse in accordance with the teaching of the instant invention;

' Figures 2a and 2b are illustrations of weathering data sheets;

Figure 3 is an illustration of a dip calculation sheet; and

Figure 4 is an illustration of a recapitulation sheet.

Referring to the drawings, particularly Figure 1, this method of obtaining a 100% continuous profile of the substrata by the use of onehalf the usual number of geophone set-ups for any given traverse is described in detail for a twelve-trace unit using a shot point interval of. for example, 1320 feet. The geophones in each spread are located at detecting stations that are spaced at -foot intervals with the first geophone 110 feet from the shot point in the direction of traverse and the last, or No. 12 geophone at 1320 feet from the shot point and offset 25 to 50 feet at-right angles to the line passing through the shot point position and the stations in the spread.

Assuming the geophones to be in the spread 2 position, records will be obtained from shot points S. P. 2 and S. P. 3. Then the No. 12 geophone at S. P. 4 is moved to S. P. 3 and records obtained from shot points S. P. 4 and S. P. 5. The geophones are then moved to spread 3 and the same procedure follows. In some areas it may be more desirable to use a geophone at each end of thespread. This may be accomplished by placing the geophones at -foot intervals, in which case it will not be necessary to move a geophone after the first two shot point records are obtained.

A constant shot point interval should be used. When it becomes necessary to alter the distance at any point, it is desirable that the odd interval distance be placed in the gap between geophone spreads. Thus, a constant spread distance will be maintained for the interlocking long spreads. This, however, is not necessary for if a change in the geophone spread cannot be avoided, the calculation of the results can readily be changed to compensate for this change in spread. In some areas of erratic weathering, it may be advisable to place the odd interval in the geophone spread position. This will allow equivalent penetration for the long weathering shots.

Where the survey is conducted in an area of measure this lag, but if the data is to be used for positive continuous control, some means of measuring the lag is required.

A 24-trace recording unit may be used in this method to further expedite and simplify the field operations. Using 12 traces for each adjacent pair of geophone spreads, the long and short spreads may be recorded simultaneously from each shot point. This procedure will also elimihate the need for extra shot holes to provide a measure of lag.

The calculation of the weathering correction for this method involves only the determination of relative weathering differences between respective geophones. When used for continuous profiling, it will be necessary to extend this weathering calculation to obtain absolute values, or an uphole correction system may be used for the geophones adjacent to the shot points.

Sample weathering computation sheets are shown in Figures 2a and 2?). Figure 2a assumes a horizontal surface with no elevation changes. I1 and I2 are the recorded intervals between successive first breaks, while 131 and is are the recorded times. The smallest summation Et is used as the base and 2W1- represents two times the relative weathering variation. With the assumed conditions of horizontal surface, Vo=2000- lit/sec. and Ve=6il00 ft./sec., the combined weathering and elevation correction (f corr.) is arrived at immediately by the relationship 2Wr t cornobtain the relative weathering and elevation correction, t corr. The actual numerical values will of course depend upon the V0 and Ve for the particular area.

From the calculated t corr. values, the weather ing correction to be applied to the traces used in the dip calculations is obtained. Thus, on Figure 2a the 111 correction is +.007, the 2-10 correction is +.007, the 3-9 correction is +.005, etc.

At the top of the weathering data sheets are placed the shot point numbers from which the refraction breaks were obtained while on the sides, in the distance rows, are placed the shot point numbers immediately adjacent to the geophone spread. The first shot point number of each pair represents the direction of traverse. Shot point numbers on records and data sheets are designated by the prefix c for short shots and L for long shots.

Where the weathering conditions are not particularly erratic, the short spread first breaks may be used for weathering calculations. This is recommended only when the long spread first breaks are not satisfactory.

Refraction breaks can often be counted more accurately by utilizing the trough after the first break and by counting intervals between successive troughs. Care should be exercised in noting frequency changes along the spread. That is, there is sometimes a change in time interval from first break to first trough between the first and last geophones. This time change must be corrected unless a. similar change occurs on the interlocking profile.

The fundamental principle involved in this method of dip calculations is that, under similar conditions of velocity, the variation in reflection times (after correcting for weathering and elevation) between any two geophones is a function of the dip of the reflecting horizon. Thus, for any given spread distance, the difference in reflection times (stepout) between any two geophones will remain constant for a horizontal bed. This constant will be designated as the normal stepout or mm. It follows that for a profile in direction of down dip the stepout will be greater than Atn and for a profile in direction of up dip, the stepout will be less than Atn.

For a special case in which the reflecting horison is considered as a plane surface let tr1=the reflection time to the first geophone trm==the reflection time to the mth geophone.

Then trm--tr1=the stepout A1 in the direction 1 to m. Similarly, reversing the direction of shooting, and maintaining the same shot distance as in normal continuous profiling tr1trm=stepout A2 in the direction 111. to 1 Aw=difference in weathering between 1 and m Al=Atn+At (due to slope of reflecting horizon) +Aw If the weathering influences are corrected then Al-A2=2Ata in which ZAtc: is a direct measure of the dip or difference in time datum in the subsurface between the No. 1 and No. m geophones It will be obvious from a study of Figure 1, that by a proper arrangement of geophone spreads and shot points, one obtains a similar subsurface control as at E and A.

Since Atn is also the spread correction, the latteris eliminated in Al-A2. Also it is seen that since the velocity may be considered as a constant locally, areal velocity variation effects have been reduced to a minimum by the simple device of subtracting from each other the stepouts observed continuously in two directions.

In other methods of dip shooting in which discontinuous one-directional observations of stepouts are made, it is necessary to know the absolute'velocity values before dips can be calculated. For absolute depth maps, it is, of course, necessary to have velocity information, but by this method, preliminary time maps may be constructed as the work progresses. If desired, these may later be changed to absolute depth maps when velocity information becomes available.

To apply the above principles to the field procedure, refer to Figure 1. Recording at spread 2 from shot point 3, the stepout A1 for the subsurface at B can be measured, and recording at spread 2 from shot point 4, the stepout A: for the subsurface at C can be measured. Then, using the proper algebraic signs Al-A2 represents the dip in the direction 3-4 for the subsurface B+C'. This dip is converted by the necessary factor K to represent the dip for the distance between shot points. For the conditions assumed in the diagram using the No. 1 and No. 11 geophones in which No. 1 is at feet and No. 11 is at 1210 feet from the shot point, the subsurface coverage for Ai-A: is 1100 feet and the factor K to convert to 1320 feet coverage is 1.2.

By the expedience of the long spread shots, the dip for the gap between the successive geophone spreads may be determined. Recording again at spread 2 from shot point 5 a measure of the stepout A: for the subsurface at D can be made. Then moving geophones to spread 3, one

- can record from shot point 4 and measure the stepout A1 at F. Here again Al-AZ represents 'the dip in the direction 4-5 for the subsurface per cent continuous control has been maintained.

The algebraic sign of Ai-A: willshow the direction of dip along the. line of traverse. If Ai-Az is plus the dip is down, if minus, the dip is up in the direction of traverse.

In practice, the 1 to 11 and the 2 to trace stepouts are usually averaged together for each directional stepout determination.

Referring to Figure 3, which is the preliminary calculation sheet and contains a full tabulation of reflection information obtained from each record, t; is the time of the first trace; A1 the reflection stepout from No. l to No. 11: Aw the combined elevation and weathering correction (t corr.) for 111 obtained from the weathering calculation; Am is the summation of columns 2 and 3; K the factor to convert 111 coverage to 1320 feet or to the shot point interval; and mm, final corrected stepout or column 4 times K, As is the stepout from No. 2 to No. 10; Aw the 2-10 relative weathering (t corr.); and the remainder of the items are self explanatory. AvKA1C is the average of the 111 and 210 KA1C's.

The elimination of one source of error can be obtained by the use of tabulation charts for the conversion of AlC to KAIC. If the record information is good, the KAIC (111) and the KAiC (2-10) should check reasonably well. If not, an error may be expected in the weathering, in the instruments or in the readings (very rarely due to subsurface). If the 111 and 2-10 As do not check, a new calculation using another pair of traces such as 3-9 may help to decide which of the original two is correct. If the latter check does not help, and the profile is in an important part of the area from a structural point of view, the same depth points can be repeated by moving the geophone spread from the original position to the gap position. In this manner, dip information from the same subsurface can be obtained at an entirely new geophone position.

The only computing digits necessary on the records are the first break counts and, for reflections, the first trace count and the stepout counts for the traces used. In making up the dip calculation sheets or, in other words, in making reflection picks and stepout counts, the weathering information should be considered. When stepouts of traces used for A1 and A: do not fit the weathering, records should be questioned and studied carefully. The observer should watch for bad traces particularly Nos. 1, 2, 10 and 11. Quite often, a lag ,or lead will be caused by a defective geophone.

Reflection picks should be made on each profile separately, disregarding the interlock record (unless continuous horizons are being followed). This is important since identical time interlock picks will often result in poor to bad picks bein made on one or both records. It is not necessary for dip calculation by A1--A2 that the trs be identical. It is merely necessary that they be reasonably close. The latter qualification may be evaluated by a consideration or study of the normal Art curve.

Second record checks should always be made. If there is any variation in count for the same reflection pick, either the best character pick should be used or if both picks are equally good, an average taken. For best results, it is recom mended that the same interpreter make picks on all records for one profile, these later to be checked arithmetically by another computer and reviewed by the interpreter.

From the preliminary calculation sheet, the computations are transferred to the final recapitulation sheet, see Figure 4. Tr and tr: are taken from the calculation sheet and averaged to obtain column 3 (Av Tr). Arm and AM: are combined to give which represents the dip component for 660 feet of subsurface. Changes in spread distances should be noted and the calculations revised to fit same. The Ais and Az'S must represent stepouts between the same absolute geophone distances or else the operation of AIA2 will not eliminate the spread correction.

Column 9 contains the quantity which represents the normal At or Atn. These latter values are tabulated as the work progresses. A reasonably large number of these Atn values when averaged and plotted against time will furnish an excellent normal At curve for the particular area. This curve may then be used to calculate velocities, dips from single direction stepouts and often to furnish a good check against erratic dips. For example, the reflection tabulated on Figure 4, Profile 4LE-5LW, Av Tr 1.585, shows a dip which is not consistent with the remainder of the dips for that part cular deter mination. The

or Atn value for this reflection is .043, whereas the Ain from the At curve is .038. If a reliable At curve has been obtained for the area, then these dips are either unreliable or do not repre-- sent dips from a continuous subsurface. Such dips should be plotted separately as single dips and subjected to additional study. Note that several dips have been calculated from the single direction stepout using the Atn values taken from the At curve. These dip values are placed in column 14.

In the tabulation of the lated values, the better the individual points will approach a smooth curve.

Average velocities are calculated from the values are plotted for 660 feet of subsurface coverage with the midpoint at the center of the shot point interval.

It is recommended that when the single dip determinations are plotted on the cross section, some method of plotting be used which will distingu-ished the single dip values from the 2 dip values since the latter should ordinarily be given the greater weight.

Phantom dips are then obtained by averaging the several picks at various levels depending upon the results and the structural peculiarities of the area under survey.

The recommended grading of reflection picks utilizes four grades; good, fair poor and questionable. Then in obtaining average dips, the use of respective weights of 4, 3, 2 and 1 should approach the best available average. In this connection, however, much depends upon the results at hand, and it is entirely possible that good looking reflection picks are fallacious. For instance, good reflections may be had from lithologic r stratigraphic lenses whose subsurface attitude is diiferent from the overall geologic structure. Normally, when the large majority of fair to good picks suggest a certain dip, other picks at variance to the majority may usually be disregarded without further consideration.

When individual dips approach the magnitude of 100 feet or more to the mile, the horizontal displacement of the depth points should be taken into account when the dips are plotted in cross section. Each pair of adjacent phantom dips, one obtained from long spread records, the other from short spread records, are averaged to obtain the dip component which is plotted for 660 feet of subsurface with the midpoint at the shot point, see Figure 1. It should now become apparent that by using A -Ag 2 and by averaging adjacent dip determinations for shot point dip components, the effect of error will be reduced. The weathering influence in the aaeaaee short spread dip determination is cumulative, hence by using the determination for 660 feet rather than 1320 feet. the error is reduced by 50 per cent. The ms and Az's obtained from the long spread records do not necessarily havev cumulative weathering effects because they are obtained from two different geophone spreads. Such a system of averaged computations should normally tend to reduce inherent errors to a minimum.

I claim:

1. A method 01 seismic surveyingthat come prises the steps of dividing a traverse into equal intervals, locating in alternate intervals aplurality of equally spaced detecting stations to form alternate spreads, creating seismic waves at both ends of each spread and also at the far extremity of the adjacent interval at opposite ends of the spread, separately detecting at each spread the waves which have been so created at each point. amplifying and recording the detected waves in coordination with time.

2. A method of obtaining data for a continuous profile of a subsurface formation that comprises the steps of dividing a traverse into equal intervals, locating a plurality of equally spaced detecting stations along alternate intervals to form alternate spreads, creating seismic waves at both ends of each spread and also at the far extremity of the adjacent interval at opposite ends of the spread, separately detecting the waves after they have been so created and which have been reflected from the interfaces of the substrata, amplifying and recording the detected waves in coordination with time.

3. The method of profiling a subsurface formation with 100 per cent subsurface control that comprises the steps of dividing the traverse along which the profile is to be made into equal intervals, locating along alternate intervals equally spaced detecting stations, the detecting stations in each interval forming a spread, creating seismic waves at both ends of each spread and also at the far extremity of the adjacent interval at opposite ends of the spread, detecting and recording on separate records at the first spread the waves which have been so created, advancing along the traverse to the next spread and repeating the operations until the entire traverse has been covered.

4. The method of profiling a subsurface formotion with 100 per cent subsurface control that comprises the steps of dividing the traverse along which the profile is to be made into equal intervals, locating along alternate intervals equally spaced detecting stations, the detecting stations in each interval forming a spread, creating seismic waves at both ends of each spread and also at the far extremity of the adjacent interval at opposite ends of the spread, detecting and recording on separate records at the first spread the waves which have been so created, and which

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