GB2179739A - Two dimensional seismic modeling - Google Patents

Abstract

A method for producing a cross sectional plot e.g. on paper 26 having a synthetic time response compatible with real seismic data is accomplished by establishing a field for distance as one axis (x) and time/depth on the remaining axis (z) a computer 10 then displays a seismic section on a monitor (12) having corresponding dimensions from stored seismic data. Velocity information is then entered for the velocity between the surface of the earth and the first event desired to be plotted. A digitizing cursor (21) is placed on the field and a first point selected. The computer instantly calculates the location of the point and through ray tracing and Snell's law plots a visible line or point on the monitor. If the line or point does not fall on the event selected, then a new point is selected and viewed on the screen. The process continues until the line or point falls exactly on the event selected and then the process is repeated continuously along the event until the length of the monitor screen has been completed or until the interpreter is satisfied with the distance that he has covered. Additional events can be plotted using the same process. <IMAGE>

Description

SPECIFICATION Two dimensional seismic modeling This invention relates to a two dimensional seismic modeling method that is fast, accurate and easy to use.

The method produces a cross sectional model that has a synthetic time response compatible with the real seismic data, or representation thereof. The preferred embodiment of the invention is accomplished by using a digitizing tablet with a paper overlay, a computer and a monitor for displaying a seis mic section in time as one axis and distance as the other axis.

The digitizing tablet is first registered so that the horizontal and vertical axis of the tablet are set to the desired dimensions on the paper which may be premarked as to distance and depth. The registration is then used by the computer to display the corresponding seismic data on the monitor. The layers of the earth are then drawn by the interpreter by plotting a point on the paper overlay of the digitizing tablet. This information is then digitized and transmitted to a computer which calculates the ray path from the plotted point to a location on the surface of the earth and is then communicated as distance and time to the monitor containing the seismic display.

The location and travel time are then plotted as a point on the seismic display. A visible line may be drawn from this location to the next immediate location previously plotted. As an alternative, only a point or mark may be plotted on the seismic display.

If the visual line, point or mark does not fall on the selected event, then the interpreter will preferably delete the line, point or mark through a deletion process and make another selection for the point. The process will provide continuous plotting of points on the tablet with instantaneous results appearing on the seismic monitor. The interpreter will then continue plotting points on the paper overlay of the digitizing tablet, maintaining the selected visual line or mark on the selected event until the entire event is plotted on the digitizing tablet.

The procedure may be then repeated for the next subsequent deeper event. The computer, using Snell's law, will calculate the path of the ray from the plotted point through the various layers until it reaches the surface of the earth and then communicates this information to the monitor containing a seismic section where it will display a location as a visual line or dot on the selected event.

The interpreter, as he progresses downward through each of the layers, will enter into the computer through a keyboard the selected velocities for these regions between each of the events, that is, the event currently being selected and the event just previously processed.

When the interpreter completes all of the events desired, the paper overlay will correspond to an actual cross sectional plot of the seismic section which is displayed on the visual monitor.

The interpreter has the ability to name the various layers and also has the ability to select various colors for each of the events being plotted on the seismic monitor.

Certain embodiments of the invention will now be described by way of example and with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a block diagram illustrating the preferred embodiment of this invention; Figure 2 shows a modified version of the apparatus; Figure 3a is a flow diagram of a procedure for the registration of the digitizing tablet illustrated in Fig. 3b; Figure 4 is a flow diagram illustrating the processing of the event from the selection of the point on the digitizing tablet to the plotting of the location on the seismic monitor; Figure 5a is a flow diagram of the computer process for tracing the ray from the plotted point to the surface of the earth so that the intersection with the earth and the travel time can be calculated; Figure 5b is a cross sectional plot illustrating the ray tracing process; Figure 6 illustrates the method for plotting and viewing the results of this modeling process;; Figure 7 illustrates the method for providing an enlargement of a region so that the actual match of the location and the event can be further enhanced; and Figure 8 illustrates one possible source of error in the method illustrated in Figs. 1 through 7.

DETAILED DESCRIPTION OF THE INVENTION Referring to all of the figures, but in particular to Fig. 1, a computer 10 is coupled through a cable means 11 to a graphic monitor 12. Computer 10 obtains its alpha-numeric input through a keyboard 13 coupled through a cable 14 to a terminal display 15 and through cabling means 16 to computer 10. A digitizing tablet 20 is of the kind manufactured by CalComp Corporation, Series 9000 electromagnetic digitizer. Such a digitizing tablet converts the physical position of a stylet or cursor 21 on an activated digitizer surface into a digital output that can be used for communication, processing and display purposes.

While the above referenced digitizing tablet 20 performs the function extremely well, other forms of digitizing can be used including manually inserting the location of the data point from a grid drawn on a sheet of paper which can be over laid on the paper. Digitizing tablet 20 is coupled through a cable 22 to a microprocessor unit 23 which is utilized to convert the information to that usable for computer 10. Micro-processor unit 23 communicates the information to computer 10 through cable 24.

Cursor 21 is coupled through a cable 25 to an input 27 on tablet 20. Input 27 is coupled internally to micro-processor unit 23. In use a paper sheet 26 is attached in any usual manner such as by pressure sensitive tape to digitizing tablet 20. The paper overlay 26 will provide the cross sectional plot of the seismic section to be processed. Such a plot and the method of making the plot will be discussed in a later portion of the specification.

Referring to Fig. 2, an alternative embodiment of that as shown in Fig. 1 is provided.

Thus, instead of a digitizing tablet 20, a graphic type monitor 30 is utilized having a screen 31 where the plot can be drawn.

Rather than using cursor 21, as shown in Fig.

1, a light pen 32 is used which in turn is coupled through a cable 33 to a micro-processor 34 which is coupled as previously described through wire 24 to computer 10.

Since graphics monitor 30 will not render a permanent image, the image must be transferred from monitor 30 to a permament recording. This can be accomplished in one of two ways. Either a paper overlay can be placed over the seismic monitor and the cross sectional plot drawn or traced onto the overlay, or computer 10 can be coupled through cable 35 to a graphic recorder 36 which can record on a paper 38 the lines displayed on screen 31 of monitor 30.

Referring to Figs. 1, 3a and 3b the registration of tablet 20 with seismic display monitor 12 is described. For purposes of illustrating the registration of tablet 20, in Fig. 3b tablet 20 is shown as previously described connected through cable 24 to computer 10 and through cable means 11 to seismic monitor 12. A seismic display 37 is illustrated on the face of monitor 12 with monitor cursor locations 40, 42 and 43. Cursor 21 is also shown provided with an entering button 41 mounted on the surface of cursor 21.

Normally, prior to beginning work on tablet 20, paper overlay 26 is laid out with the dimensions for the x and z axes marked on the paper. There is no particular limit on the actual dimensions selected so long as the dimensions are within the digitizing portion of tablet 20. Generally, the distance selected for x or distance should be the same or less than the data stored in the computer. The z axis is then marked for time which is also normally equal tq or less than the stored seismic data.

The paper 26 is then mounted on tablet 20 by any method such as taping. Once paper 26 is mounted on tablet 20 the location 0,0 which is the intersection of the x and z axes is determined by placing cursor 21 over the location determined for the 0,0 point. Button 41 is then pressed and the digitized information is communicated to computer 10. Cursor 21 is then moved to point 0,x which represents the maximum distance for the data to be cross sectioned where button 41 is again depressed, sending the digitized location of cursor 21 to computer 10 through cable 24.

The vertical distance is then entered by placing cursor 21 over location z,0 which represents the maximum time of the seismic data and depth for the cross section and button 41 depressed to send the digitized information to computer 10. It is, of course, obvious that other coordinates can be entered and still register the document, such as 0,0 and z,x.

Once computer 10 receives the location of the points 0,0, 0,x and z,0 or z,x computer 10 will then select the data from memory and corresponding to the internal coordinates and transmit the seismic section having the same dimension as those set out in tablet 20, to monitor 12 as display 37.

Since tablet 20 was registered in distance as the x axis and time as the z axis, the z axis must be equated to depth in feet, if the resulting plot is to result in a depth model. To equate the z axis in feet, the depth value equal to coordinate z,0 is entered (such as 12,000 feet) using keyboard 13.

Referring to flow diagram 3a, the aforementioned procedure is described in better detail.

Data is loaded at step 44; this can be a seismic section or a digitized representation of a seismic section. Then the procedure goes directly to step 47 for storage. The paper or document 26 is then registered as step 48 by setting the 0,0 location by step 49, the 0,x location at step 50 and the z,0 as step 51.

The seismic section is then displayed by step 52 having the dimensions as determined by steps 49 through 51 and the depth in feet is entered through step 53 with the internal velocity set in step 54. The process is now ready for drawing the cross sectional plot on paper 26.

As previously discussed paper 26 has been over laid on tablet 20 with the locations 0,0, 0,x and z,0 set out on paper 26. Generally, the horizontal axis will be marked off in feet on paper 26 and the vertical axis will be marked off in feet illustrating the depth of the various layers to be located on paper 26.

CROSS SECTIONAL PLOTTING METHOD The steps of performing the cross sectional plot are clearly illustrated in Figs. 4, 5 and 6.

Initially a color is selected as illustrated in step 55. This color can be any color the monitor is capable of producing and, if desired, the layer to be plotted is named in step 56. Once step 55 and 56 are completed, cursor 21 is placed over the possible point where the first layer will be located on paper 26 both vertically and horizontally which will represent the first point on a layer to be plotted. Such a process is illustrated in Fig. 6 where, for example, the first point will be illustrated as point A. Button 41 is then depressed which transmits a signal through tablet 20 to digitize the location of cursor 21 as step 57 and communicates this information to micro-processor unit 23 through cable 22 and processed data to computer 10 through cable 24 (See Fig. 1). The x,z coordinate is then processed in computer 10 by referring to the flow diagram shown in Fig. 5.

Computer 10 will check to see if the point x,z is above or below any previous layer already selected as illustrated in step 58. If the point that is picked is above a previous layer, computer 10 will automatically shift the point down so that it is at least one foot (300mm) below the previous layer selected as illustrated in steps 59 and 60. Computer 10 will next determine if this is the first point selected in step 61. If it is the first point selected, then computer 10 must simulate a point to the left since the balance of the program functions on having two points for determining the ray path tracing portion of the program. If it is the first point selected then computer 10 through step 62 will automatically generate a point a fixed distance from the selected point.Computer 10 will place a simulated point at the same depth as the point selected and to the left of the point selected providing the selection of points is from left to right. Once computer 10 generates a point (if it is a first point) then the slope is calculated at step 63. The slope is calculated between the point selected and the last previous point selected (or the computer generated point). Computer 10 will then calculate the equation of a ray line which is perpendicular to the slope at the point selected and toward the surface of the earth through step 64. It will then calculate the inersection of the ray with a previous layer or the surface of the earth which ever is first at step 65.

If the layer being determined is below the first layer, (referring to Fig. 5b), it will through step 66 and utilizing Snell's law, calculate the angle of incidence, the angle of transmission in step 67 and then calculate the equation of a transmitted beam through step 68. Snell's law is well known to those skilled in the art and has been used to calculate the ray paths through acoustical discontinuities. Such a law is set out below.

Sin o1 V1 Sin 02 V2 Where 01=angle of incidence between the ray path 81 and the normal 82 to a layer 83, 02=angle of transmission from layer 83 of ray path 84.

In the example illustrated in Fig. 5b, a ray from point 80 is reflected at 90 degrees to layer 3 along path 81 to layer 2. The angle of incidence 01 between ray paths 81 and the normal 82 to layer 2 is calculated. Using Snell's law, the angle of 02 between transmitted ray 84 and normal 82 to layer 2 is calculated.

Computer 10 will then, through step 69, determine if the surface of the earth has been reached, if not, a layer will be subtracted in step 70. Using Snell's law, a new 02 is calculated using the ray path 84, v2 and v1. As ray path 84 passes through layer 1 to a new velocity v 1, it will be transmitted as ray path 85 to the surface at the angle 02 to the normal 86 to layer 1. Once ray path 85 intersects the surface at point x, (the distance from 0,0 to the intercept point) as required in step 71, then X is recorded and travel time T calculated in step 72, where T=2(T1+T2+T3---+Tn); where T1 is the time it required for the ray to travel ray path 85, etc.

Returning to Fig. 4, computer 10 then plots a point or mark (x,T) in step 90 on the seismic display 31 with X as the horizontal coordinate and T as the vertical coordinate, such as A on display 31 in Fig. 6. Depending upon the interpreter's selection, computer 10 will either draw a line 91 to the last plotted point or mark without the line. The interpreter in step 92, must decide if the mark or the plotted line falls on the event. If it falls above or below the event, then cursor 21 is backed up as in step 93, usually to the last previously plotted point. The line in step 94, just drawn, may then be deleted if desired by pressing a button on cursor 21 (such as button 73).

Whether the line is or is not deleted, the next step is to determine if more detail is needed.

If more detail is needed, then a window may be created which expands the seismic displayed section to an area defined by the interpreter as set out in step 95. The expansion is accomplished (with reference to Fig. 6) by moving cursor 21 to a location 74 and depressing a data button on cursor 21. Then cursor 21 is moved to a second location, for example, 75 and the. location entered in computer 10. Computer 10 will then display, in step 96, a expanded view or window as illustrated in Fig. 7. Thus, for example, points B, C and D have been plotted and difficulty is being exhibited in the determination of the proper location of point E. The window, as illustrated in Fig. 7, can be used. If the plotted point or line is falling on E', then cursor 21 will be moved down and a new point selected by pressing button 41.If the new location falls on E", then cursor 21 will be raised a slight amount and button 41 again depressed.

Computer 10 will then instantly display point E, which will probably fall on the event 76 being plotted. Thus, once the window, as illustrated in Fig. 7, has been displayed then the procedure of following through steps 57, 90, 91, 92 and if necessary 93, 94, 95 and 96 can be followed until the plotted mark E falls on mark 76.

The seismic display can be returned to the normal display at any time by merely pressing button 91 whereby computer 10 will then regenerate the original seismic section on the monitor.

If the plotted point falls on the seismic event, then it is necessary to determine if the end of the event 76 has been reached. If it has not been reached, then through step 98 again a window selection may be made. Like the previous discussion of step 95, if a window selection is made, then the window will be reset upon proper position of cursor 21 as set out in step 99, whereby the additional steps then 57, 90, 91 and 92 will be performed. Of course, if no window is desired, then the steps 98 and 99 will be omitted.

Once the end of the layer is reached as questioned in step 97, then it is determined if all layers have been done in step 100. If additional layers are to be modeled through step 101, a new layer is selected, a new interval velocity is entered in step 102 and a new color in step 55 is entered with the event being named if desired in step 56. The process then proceeds exactly as previously described through the additional steps down through 100.

Once all of the steps have been completed, the program is termined through step 103.

Since the plot has been drawn on a piece of paper on the digitizing tablet, no portion of the output need be recorded. It is also obvious, of course, that any data entered or the results of any of the processing can be recorded if desired by the geophysicist.

MIGRA TION This procedure will function as well on migrated dta as unmigrated data. Normally, the modeling process is done on nonmigrated data. However, if migrated data is available the interpreter need only insert the velocities he would like to use for the migration of the computer generated points. Once the migrated data velocities are inserted, the function of the program will be identical to that as previously described for the unmigrated data.

SOURCE OF ERROR Referring to Fig. 8, the program may possibly generate one source of erroneous information. In Fig. 8 are illustrated the surface of earth 110, a first layer 111 and a second layer 112. If first layer 111 is being processed to the point 113, a ray 114 being traced to the surface of the earth 110 as described will actually pass through layer 111 between layers 111 and 112 having a velocity v2. The ray would normally reflect as illustrated by ray path 118 through layer 112 into a portion of the earth having a velocity v3 and then passing along ray path 119 until striking layer 112 again where it changes velocity back to v2.

The ray will then travel along ray path 115 to layer 112. When the ray reaches layer 112 it will travel again through v1 along ray path 116 intercepting the surface of the earth at a distance x which is different from the distance x', the intercept cordinate generated by the program. Such an error can not be corrected when plotting layer 111, since layer 112 has not yet been plotted nor has the velocity v2 nor v3 been entered into com, putter 10. This type of error, however, is obvious to the skilled geophysicist and when such an error is noticed, particularly after the sharp dip illustrated by point 117 is noted, the geophysicist can determine that ray 113 had to pass back through layer 111, thereby causing an erroneous point X1 and travel time T1, The geophysicist, after he has completed layers 112 and entered velocities v2 and v3 and subsequent layers, may go back and redo the region where a ray such as 114 will be reflected erroneously, thereby correcting the intercept location x' to its proper location x.

Claims (17)

1. A method of producing a cross sectional plot having a synthetic time response compatible with real seismic data comprising: (a) displaying seismic data as distance vs.

time on a monitor; (b) establishing a field with a work area having time as a z axis and distance as an x axis; (c) registering said field so that said work area and said displayed seismic data corre spond as to distance on said x axis and time on said z axis; (d) entering a depth value corresponding to the maximum time on said z axis; (e) selecting a first event of said displayed seismic data and entering a velocity v1 for the earth between the surface of the earth and said selected event; (f) plotting on said work field a first point having an x, z, coordinate; (g) digitizing said x, z coordinate; (h) calculating the slope between said digi tized coordinate and a last plotted point and generating a ray toward said surface of the earth, and perpendicular to said slope at said point;; (i) calculating an intersection of the path of said ray with the surface of the earth, said intersection point X, and the two way travel time T of said ray; (j) plotting a mark having coordinates (X,T) on said displayed seismic data; (k) comparing said plotted mark on said dis played seismic data with said selected event; (I) repeating steps (g) through (k) if said plotted point on said work area did not cause said displayed mark to fall on said selected event; (m) continuing steps (g) through (I) for addi tional plotted points spaced within said work area whereby said plotted points on said work area will form a cross sectional plot of said selected event.

2. A method as claimed in claim 1 including the step of forming a visible line connecting said plotted mark and a previously plotted mark on said displayed seismic section.

3. A method as claimed in claim 1 further including the step of deleting said plotted mark prior to step (I).

4. A method as claimed in claim 2 further including the step of selecting a color for said visible line.

5. A method as claimed in claim 3 further including the step of deleting said formed visible line prior to step (I).

6. A method as claimed in any preceding claim wherein subsequent events (1 +a), where "a" is an integer representing the position of said event sequentially deeper than said first event are plotted by: (a) selecting a next deeper event (1 +a); (b) entering a velocity v(1 +a) which is the velocity of the earth between event (1 +a) and event (1+a)-1; (c) plotting on said work area a point having an x, z cordinate; (d) digitizing said plotted point; (e) determining the slope between said point and said last plotted point; (f) generating a ray path perpendicular to said slope at said point, said ray path tra veling toward the surface of the earth; (g) passing said ray path through each sub sequent layer above said (1 +a) layer while bending said ray path in accordance with Snell's law until said ray path intersects said surface of the earth;; (h) determining the intersection point X on said surface of the earth and the two way tavel time T; (i) plotting a mark having coordinates (X,T) on said displayed seismic data; (j) comparing said plotted mark on said dis played seismic data with said selected next event (1+a); (k) repeating steps (c) through (i) when said plotted mark does not coincide with said selected next event; and (i) continuing steps (a) through (k) for all events desired to be plotted as layers on said field.

7. A method as claimed in claim 5 when said plotted mark is deleted prior to repeating steps (c) through (j).

8. A method as claimed in claim 6 additionally including the step of forming a visible line connecting said plotted mark and a previously plotted mark location on said displayed seismic data.

9. A method as claimed in claim 6 wherein said visible line formed on said displayed seismic data is deleted prior to repeating step (e).

10. A method as claimed in claim 7 further includes selecting a color for said plotted mark formed on said displayed seismic data.

11. A method as claimed in claim 8 including the further step of selecting a color for said visible line formed on said displayed seismic data.

12. A method of producing a cross sectional plot having a synthetic time response compatible with real seismic data comprising: (a) displaying a seismic section as distance vs. time on a monitor; (b) estblishing a field having time as a z axis and distance as an x axis; (c) setting with reference to said field, the 0 (zero) time location and said 0 (zero) dis tance location on said field; (d) setting with reference to said field sec tion, the 0 (zero) time location and said maximum distance on said x axis on said field; (e) entering with reference to said field sec tion, a depth value corresponding to the maximum time on said axis of said field; (f) selecting a first event on said displayed seismic section and entering a velocity for the earth between said surface and said se lected first event; (g) plotting on said field a first point having an x, z, coordinate;; (h) digitizing said x, z coordinate; (i) calculating the slope between said digi tized coordinate and a last plotted point on said field and generating a ray path perpen dicular to said slope at said point toward the surface of the earth; (j) calculating an intersection of said ray path with the surface of the earth, said in tersection point X, and the two way travel time T, and displaying said point X dis placed in time T on said displayed seismic sectio as a location X, T; (k) formig a mark at said location; (I) comparing said mark line on said dis played seismic section with said selected first seismic event; (m) repeating steps (g) through (i) if said plotted point did not cause said mark line to fall on said selected event; (n) continuing steps (g) through (m) for addi tional plotted points spaced along said field; ; whereby said plotted points in said field will form a depth vs. distance plot of said se lected first event.

13. A method as claimed in claim 12 including the step of forming a visible line connecting said mark and a previous mark on said displayed seismic section.

14. A method as claimed in claim 12 further including the step of deleting said plotted mark prior to step (k).

15. A method as claimed in claim 13 further including the step of selecting a color for said visible line.

16. A method as claimed in claim 13 fur ther including the step of deleting said formed visible line prior to step (I).

17. A method of producing a cross-sectional seismic plot, substantially as hereinbefore described with reference to the accompanying drawings.