GB2090409A - Method of seismic exploration - Google Patents

Abstract

In seismic exploration, the amplitude of the gathered CDP traces representing a reflection anomaly is plotted as a function of offset. The plotted contours (upper part of Figure) are displayed contigously with the portion 24 of the seismic section having an anomalous reflections. The patterns of the contours distinguish the various types of anomalous reflections. Anomalous amplitude contours caused by variations in reflection coefficients, such as a hydrocarbon indicator, lie along slanted CDP lines on the contour plot, whereas anomalies caused by other factors, e.g. source strength or receiver channel variations, have different patterns. <IMAGE>

Description

SPECIFICATION Method of seismic exploration This invention relates to seismic exploration, and more particularly to the technique of common depth point (CDP) exploration in which the resulting seismic section has reflection amplitude anomalies.

In one method of seismic exploration, the field records obtained include traces from detectors at different offsets from a source along a line of exploration to obtain multiple coverage of common reflection points in the earth's subsurface. Subsequently, the field traces representing multiple coverage of common reflection points are stacked to suppress noise, such as multiple reflections and the stacked traces are displayed as a function of distance along the line of exploration to form a seismic section. A seismic section depicts the subsurface layering of a section of the earth and is one of the principal tools used by the geophysicstto determine the nature of the earth's subsurface formations.

Various names have been given to the general process of obtaining multiple seismic coverage, for example common depth techniques, common reflection point techniques, and roll-along techniques. All of these techniques involve the general principal of recording multiple seismic traces from the same reflection point in the subsurface by employing a plurality of horizontal offsets between a seismic source and the seismic detectors. A description of such techniques is given by Lorenz Shock in an article entitled "Roll-Along and Drop-Along Seismic Techniques", published in Geophysics, XXVIII, 5, Part II, pp.831-841, (October, 1963). The data obtained are corrected for normal moveout and statics and thereafter stacked.

Common depth point seismic techniques are generally credited with producing bettter seismic data than those techniques which produce singlefield seismic data. In stacking the common depth point seismic data, the primary reflections are essentially in phase and thus are added whereas the distortions, such as multiple reflections, are out of phase and tend to be cancelled. Thus, the multiple reflections are suppressed and the primary reflections are enhanced.

Recently, an intense study has been made of the relationships between amplitude and phase anomalies on stacked seismic sections and rock type, porosity and fluid content of the reflecting zone. Certain anomalies have been referred to as "bright spots" or "HCI" (hydrocarbon indicators). Various studies of velocity, density and reflection coefficient variations and case histories have been made on these HCI in stacked seismic sections. While some anomalies in reflection amplitude have been found to indicate an interface with a hydrocarbon bearing formation, anomalous reflection amplitudes have been attributed to other causes, also. Thus, anomalies in reflection amplitude may be caused by source strength variation, receiver channel gain variation, source and receiver coupling problems associated with the earth's surface, and non-adherence to CDP points in gathering the traces.

Attempts have been made to identify HCI on stacked sections, weil logs and field records as anomalous reflection amplitudes dependent upon offset between seismic source and detector. However, difficulties have been encountered in demonstrating the reflection amplitude versus offset behaviour because the only display medium has been seismic sections having common offset.

According to the present invention, there is provided a method of seismic exploration in which there are obtained seismic field records including a plurality of traces from detectors with different offsets from a source along a line of exploration, the method comprising the steps of gathering together traces representing reflections from common reflection points; stacking the common reflection point traces to produce a seismic section; for a particular reflection on the seismic section, determining the amplitude of the gathered traces as a function of offset; and plotting the amplitude as a function of offset.

In accordance with the invention, offset dependent reflection parameters are analyzed and displayed and, more particularly, for a given reflection on a seismic section, the amplitude of each field trace is determined as a function of offset between source and detector producing that trace. Contours of equal values of amplitude are then plotted as a function of offset and distance along the line of exploration, and the contour plots show offset versus source location on the line. The present invention takes advantage of the power of common depth point processing and the large dynamic range and pattern display properties of a contoured plot.

This contour plot can be displayed contiguously with the portion of the seismic section displaying the reflection anomaly of interest. This provides a very useful tool for distinguishing between various types of anomalies. It can be shown that amplitude anomalies caused by variation in reflectivity coefficient, i.e., true HCI, will occur along the CDP lines which are tilted in the contoured plot of this invention. On the other hand, an amplitude variation due to source strength change will have a vertically oriented contour pattern. A recording channel including detector, cable, and preamplifiers having an incorrect gain will produce an amplitude anomaly which shows up on the contour plot as a horizontal anomaly. Similarly, coupling anomalies and CDP gathering point anomalies can be distinguished on the contour plot.These system errors indicated on the contour plots are corrected to produce a seismic section with a better signal to noise ratio and which better represents the seismic reflections.

The present invention will now be described in greater detail by way of example with reference to the accompanying drawings, in which: Figure 1 depicts a four-fold common depth point field procedure; Figure2 is a flowsheet depicting the method of the invention; Figure 3 shows an output plot produced by the method of the invention; Figure 4 shows the points on an output plot which depict the appearance of a reflection anomaly along tilted lines; Figures similar to Figure 4 and explains the appearance of a source anomaly along vertical lines; Figure 6 is similar to Figure 4 and depicts the appearance of a recording channel anomaly along a horizontal line; Figure 7 is similar to Figure 4 and depicts the appearance of a coupling anomaly along a vertical line and a tilted line;; Figures 8A-8C depict a source and seismic detectors in a situation which causes a coupling anomaly; Figure 9 is an example of an output plot of the invention showing a reflection anomaly together with an anomaly caused by marine streamer drift; and Figure 10 shows an alternative plot of amplitude versus offset.

Referring to the drawings, Figure 1 depicts a CDP field procedure of seismic exploration for obtaining seismic field records including a plurality of traces from detecterswith different offsets from a source.A source of seismic energy is successively energized at source locations 11, 12, 13 and 14, commonly referred to as "shot points", along the line of exploration. For ease of description, the line of exploration is shown four timers in Figure 1 with the source and detectors at different locations. For each shot, reflections from the subsurface are detected by a string of geophones including detectors 15, 16, 17 and 18.When a shot is detonated at the source location 11, the detector 15 detects a reflection from a common subsurface reflecting point 19; when the source is energized at location 12, the detector 16 detects the reflection from point 19; when the source is energized at location 13, detector 17 detects the reflection from the common reflection point 19; and when the source is energized at location 14, the detector 18 detects the reflection from the common reflection point 19. These reflections are recorded as field seismograms on magnetic tape as a plurality of field records, including a trace from earth of the detectors in the string. The offset H between the source and the detector producing each trace is recorded in a header for each field record.One common form of exploration utilizes a string of 32 detectors, thereby providing a 32-fold coverage of subsurface points. These field seismograms are indicated at 20 and 20A in Figure 2, and are gathered into sets of traces which represent common reflecting points. This step is indicated at 21 in Figure 2. Referring again to the simplified example of Figure 1, the trace in record 1 from the detector 15, the trace in record 2 from detector 16, the trace in record 3 from detector 17, and the trace in record 4 from detector 18 are gathered into a set.

These traces are stacked as indicated by the step 22 in Figure 2 to form a single composite trace which enhances the reflections and suppresses noise. The traces in all of the field records are gathered and stacked in a similar manner. When these composited traces are displayed side by side, they form a seismic section, indicated at 23 in Figure 2. A portion of such a seismic section is shown at 24 in Figure 3. Each of the vertical traces in section 24 of Figure 3 represents the amplitude of detected seismic energy reflected from a point beneath the shot points spaced along the line of exploration on the earth's surface. The portion of the seismic section shown in Figure 3 contains a strong reflection between 0.9 and 1.0 seconds of record time.

This reflection has an anomalous amplitude which makes it of great interest since, it is indicative of a possible HCI.

In accordance with the present invention, a tool for determining the cause of this amplitude anomaly is provided. As shown at the top of Figure 3, the amplitude of each field trace is plotted as a function of the offset for that trace and the shot point location along the line of exploration and values of equal amplitude are then connected by contour lines to form the contour plot shown at the top of Figure 3. This can be better understood with reference to Figure 4 which is a simplified representation of the points on the contour plot of Figure 3. Points in the horizontal direction represent shot points and common depth points. Points in the vertical direction represent field records and traces in the field records.For example, assuming that the reflection being contoured is from the reflecting interface which includes the point 19 in Figure 1, the shot location 11 of Figure 1 is represented by the point 25 and the shot locations 12, 13 and 14 are represented respectively by the points 26, 27 and 28 in Figure 4, then at the point 29 the amplitude of the reflection in the trace from record 1 produced by detector 15 is plotted; at the point 30 the amplitude of the same reflection in the trace from the detector 16 in record 2 is plotted; at the point 31 the amplitude of the same reflection from the trace produced by detector 17 in record 3 is plotted; and at the point 32 the amplitude of the same reflection in the trace from detector 18 in record 4 is plotted.

Equai values of amplitude are then connected with contour lines. When the reflection coefficient changes along a reflecting interface, all traces with that reflection point will display the reflection coefficient change.

Therefore, the change will occur along a CDP line projected from the CDP point over the subsurface change.

As an example, changes overthe reflection point 19 in Figure 1 will occur aong the tilted line 33 in Figure 4.

The contour patterns in these plots will always follow the titled CDP direction. Reflection coefficient anomalies tilt along these CDP lines opposite to the direction of shooting.

Referring again to the flowsheet of Figure 2, the field seismograms 20 are gathered into sets of CDP traces as indicated at 21 and stacked at 22 to form the seismic section 23. A reflection, such as the reflection shown at the bottom af Figure 3, is digitized as indicated at 35 in Figure 2. This digitizing is accomplished by tracing the peak on a digitizing tablet so that the coordinates of the peak in shot position and time are recorded.

Apparatus for performing the step of digitizing includes, for example, the commercially available Talos digitizer supplied by Talos Systems Incorporated.

The output of the step 35 is a set of coordinates t,x which identify the reflection in question on each of the field traces. The field traces are played back from tape as indicated at 20A and the amplitude of the reflection on each trace is determined as indicated at 36. This procedure is performed by conventional seismic processing techniques which determine the amplitude of a field trace from location x at the record time t.

The procedure draws each CDP set from tape one at a time, enters at the coordinates t,x specified by shot point number and time. The nearest peak is located in a range selected by the window for each trace of the CDP set. Then, the next CDP set is processed in a similar manner. Each peak amplitude or reflection parameter is stored in a digital word tagged by shot point, CDP number, and offset distance. These are denoted SP, CDP, and H respectively and amplitude is denoted A. These tagged words are stored as indicated at 37 in Figure 2. Tagged words can be sorted on many possible gathers including shot point, CDP, or offset. As indicated at 38, equal values of amplitude are selected. These equal values of amplitude are plotted as indicated at 39 to form the contour plots.

Figure 5 shows the contour pattern of an amplitude variation due to source strength change. Such variations will occur on all traces common to the source in question. Therefore, these anomalies have contour patterns which are vertically oriented along the field record.

Figure 6 depicts the contour pattern of a recording channel anomaly. The recording channel includes the detector, cables, preamplifiers, analog to digital converter and amplifier. If any of these have an incorrect gain, the amplitude anomaly will appear on the contour plot as an anomaly passing through the offending channel in common with all field records. This is a horizontal contour pattern.

A coupling anomaly results from a surface or near surface effect which results in an inadequate coupling of the source and detector. Such an anomaly will appear on the contour plot on all traces in shot point/offset space that pass over this surface location. As an example, Figure 7 depicts the points on a contour plot wherein a coupling anomaly will appear in a vertical pattern 40 and in a pattern 41 tilted at a 45" line. This may be explained by the example of Figures 8A-8C which depict a source S and a string of detectors being moved across a mud-filled stream. In the position of Figure 8A one of the detectors is over the mud-filled stream, in the position of Figure 8B another detector is over the mud-filled stream, and as shooting progresses other detectors are over the stream.This accounts for the 45 anomaly pattern 41 in Figure 7.

When the source itself is over the mud-filled stream as shown in Figure 8C, all of the traces in the field record will have the amplitude anomaly. This accounts for the vertical contour 40 in Figure 7.

Figure 9 is another example of actual field data represented in the portion of the seismic section at the bottom of the figure. The top of the figure shows the reflection amplitude contours for the particula reflection shown in the seismic section. The contour plot shows the reflection coefficient change as the recording system pases over a reflection anomaly. The contour pattern is along CDP lines but leads them slightly. This is caused by the drift of a marine streamer which produced the field records.

The seismic section portion of Figure 9 shows a reflection coefficient ridge at both ends of the reflection.

The center of the anomaly has a low reflection coefficient. The contour map at the top of the figure shows that the traces along this line are improperly stacked producing a smeared version of the true reflection. The traces should therefore be regathered along paths parallel to the reflection coefficient. By stacking these regathered traces, a representation of the reflection with better signal to noise ratio will be obtained. Other errors in the system for obtaining and processing the traces can be corrected in a similar manner to compensate for these errors as indicated on the plot of amplitude as a function of offset.

Figure 10 shows an alternative plot of amplitude versus offset. Typically, the amplitude of the reflections decreases until the critical angle a/c is reached, and thereafter the amplitude increases as is shown by the broken line plot. The continous line plot is atypical for reflections from a gas saturated sand. The broken-continuous line plot is typical of a reflection from a gas saturated sand where the source produces pressure waves which are converted to shear waves at the reflecting interface.

Many commercially available computing systems are suitable for practice of the invention. By way of example, the following computing system avaiiable from Control Data Corporation is particularly suitable for processing seismic traces and producing the plots of the present invention: Central Processor CDC Cyber 175 Extended Core Storage ECS Array Processor Map Ill Disk 844-21 Mass Storage Tape Drives CDC Electrostatic Plotters Statos 41 Digitizer Tablet Talos Systems, Inc.

The programming will be apparent to those skilled in the art from the user's manuals forthe particular system being used and from known seismic gathering, stacking and plotting procedures.

Claims (8)

1. A method of seismic exploration in which there are obtained seismic field records including a plurality of traces from detectors with different offsets from a source along a line of exporation, the method comprising the steps of gathering together traces representing reflections from common reflection points; stacking the common reflection point traces to produce a seismic section; for a particular reflection on the seismic section, determining the amplitude of the gathered traces as a function of offset; and plotting the amplitude as a function of offset.

2. A method according to claim 1, which includes the further step of selecting equal amplitude values, and wherein the plotting step includes plotting contours of equal value of amplitude as a function of offset and distance along the line of exploration.

3. A method according to claim 2, which includes the further step of plotting the portion of the seismic section containing the reflection contiguously with the plot of amplitude contours.

4. A method according to any one of claims 1 to 3, which includes the further step of correcting the method to compensate for errors indicated by the plot of amplitude as a function of offset.

5. A method according to claim 4, wherein the correction step includes regathering the traces on common reflection points as indicated by the plot and restacking the regathered traces.

6. A method according to any one of claims 1 to 5, wherein the step of determining the amplitude comprises digitizing the particular reflection to produce coordinates representing time and horizontal distance and, for each of the coordinates, determining the peak amplitude of the trace in the field records.

7. A method according to claim 6, which includes the further step of storing the peak amplitude in a digital word tagged with distance along said line of exploration as represented by the horizontal location of the shot point and/or the common reflection point for the trace, and with the offset of the shot point from the detector producing the trace.

8. A method according to claim 7, which includes the further step of plotting equal amplitude points having an ordinate and an abcissa specified by the horizontal location and offset from the digital word, the equal amplitude points being connected by contour lines.