Reconnaissance profiles with WLP in complex geological regions

Journal of Appfied Geophysics, 29 ( 1993 ) 285-300 285 Elsevier Science Publishers B.V., Amsterdam

Reconnaissance profiles with WLP in complex geological regions

Dominique Michon Compagnie Gbnbrale de Gbophysique, I rue Lbon Migaux, 91341 Massy, France

(Accepted after revision September 30, 1992 )

ABSTRACT

Michon, D., 1993. Reconnaissance profiles with WLP in complex geological regions. In: R. Cassinis, K. Helbig and G.F. Panza (Editors), Geophysical Exploration in Areas of Complex Geology, I. J. Appl. Geophys., 29: 285-300.

3-D has now become an essential technique for subsurface imaging. Until recently, it was predominantly employed for reservoir description, but is now starting to be used for exploration. However, in crustal seismic, it would seem that such a tool is still far from being cost-effective. WLP, or Wide Line Profiling, developed by CGG in 1973, is an attractive alternative which we shall examine.

First, we shall examine why 3-D is so difficult to apply, for both financial and technical reasons. Next, we shall study the principle of WLP, and emphasize the fundamental differences between WLP and 3-D. We shall then study the resulting data processing and parameters. Using a set of examples, we shall also examine the special characteristics of the method and the advantages that it can offer:

( 1 ) 3-D analysis of reflections on a 2-D section. (2) Avoiding interference between reflections. (3) Highlighting faults or folds. (4) For continuous horizons, positioning reflection points. (5) Possibility of mapping in depth, whether in migrated time or not, the swaths explored by the seismic line. (6) Improving the quality of the standard time section by noise reduction.

Introduction

It is barely conceivable nowadays to per- form a seismic survey in complex zones with- out using the 3-D method, but how do we op- erate if the depth of investigation is in the range of 30 to 50 kin, and we want to carry out reconnaissance: in other words, financing is difficult if not precarious. The purpose of my paper is to demonstrate that, even if 3-D may be too expensive, we should not abandon the idea of positioning in 3 dimensions the origin of the seismic arrivals that we may record on a profile. This is the aim of WLP, wide line profding.

First of all, let us examine why 3-D is too expensive:

When we want to determine possible reflectors which are located vertically below the source, and if these reflectors are horizontal, a source-receiver array with zero offset is ade- quate. However, if the reflectors are dipping, this no longer applies. A simple geometrical diagram illustrates that, if we want reflectors with a possible dip of 45 ° located at 30 km depth, we have to cover a circular surface with a radius of 30 km (Fig. 1.

As a result, any 3-D surface coverage will lose an ineffective, or at least partially ineffective, strip of the order of 20 kin. This remark makes cost-effective 3-D acquisition impractical for deep investigation.

In addition to these financial considera-

0926-9851/93/$06.00 © 1993 Elsevier Science PublishersB.V. All rights reserved.

286 D. M I C H O N

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tions, there is also the fact that all programs which permit the positioning of reflectors are designed for relatively modest depths--a few kilometers---and their extrapolation for large

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depths is not directly possible. These algo- rithms assume that depth is increased in steps of a few hundred meters. An excessive increase in the number of levels creates a dispersion generating artefacts and noise.

Only programs using ray tracing can give ac- ceptable results, but the most efficient among

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Fig. 5. These sections represent the cross-line sections of the WLP lane. We can estimate the lateral gradient of the coher- ent events.

them assume preliminary picking of the reflectors to be positioned, a factor which consider- ably limits the advantages of 3-D because the effectiveness of its interpretation lies mainly in the achievement of a "full" volume.

In these conditions, WLP is an attractive alternative because it enables us, as we will see later on, to position in a 3-D space the origin of reflections picked on a 2-D section.

The WLP method

Let us examine how the method works: In traditional 2-D seismic, the emission cre-

ated at one point on the profile is recorded on a linear spread along the profile with geophones spread out over a distance proportional to the required investigation depth, usu-

ally of the order of 5 to 6 km, straddling the emission point. The geophones are laid out in groups (each group representing a trace) with 25 to 100 m spacing between them.

For the next emission, the whole spread is moved so that it is still straddling the next emission point. The interval between emissions is systematically equal to the interval between the groups. Each point in the subsurface is therefore examined under a whole series of incidence angles of which the number is equal to half the number of traces in the acquisition spread: this is, what we call, degree of coverage.

With such a configuration, along the profile there is a source emission point in every trace interval as schematized in Fig. 2a.

The WLP method consists in offsetting laterally either the emissions, or the receivers fol-

RECONNAISSANCE PROFILES WITH WLP IN COMPLEX GEOLOGICAL REGIONS 289

Fig. 6. Presentation of the results from the WLP processing. The vectors indicate the direction of the total gradient and their length is proportional to this gradient. An horizontal vector indicates in-line dip. A vertical vector indicates a cross- line dip.

lowing a zig-zag pattern (Fig. 2b). So, the number of emissions and the number of traces are identical for the WLP method and the traditional method. Simple geometrical consid- erations show that for the series of source points with the same lateral offset, the "nominal" reflection points (equidistant from source and receiver) are no longer vertical below the surface spread but on a line parallel to it at a distance equal to half the offset. If a series of four offsets is used, as is the case in the figure, four parallel lines of "nominal reflection points" will be obtained, forming a grid. By

contrast, each of these lines has a quarter of the number of rays reaching each point, with a source-receiver spacing that is four times the standard spacing. Depending on the reflection points, four types of distribution are possible, derived from each other by a simple transposition.

It is interesting to see how these different series are distributed on the grid (Fig. 3). The distribution of the source-receiver point distances actually makes it possible to filter all the noise created by the emission, and the filtering depends essentially on the distribution func-

290 D. MICHON

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tion of these distances• The midpoints with the same function have the same background noise, while the midpoints with different func- tions have different background noise which affects the required signal in a different way.

It is worth noting that the cross-lines are made up of four points which belong systematically to different series. Likewise, on each line, the four series alternate at regular intervals.

Each of the four lines of reflection points is processed separately, but with the same parameters. The velocity analyses are carried out by combining the four lines.

We obtain four sections which resemble each other (Fig. 4). We may also consider the cross- lines (Fig. 5), formed by only 4 traces; these traces resemble each other to a sufficient degree to detect a lateral gradient at a glance.

The specific processing method with WLP

(1) Measuring the gradient

The processing method which is specific to WLP involves calculating the dip. This calculation may be carried out by using automatic picking; the gradient is then deduced directly

from this pick and the corresponding values are linked to a given horizon. This lateral gradient is measured on the small four trace cross-lines, the longitudinal gradient on one of the lined- up sections or on the stack of the four sections. The total gradient is the vectorial composition of these two gradients. The values are gener- ally averaged over a length of several hundred meters.

The calculation may also be made by using cross-correlation. The values are therefore plotted as time series and are represented in constant times.

As a rule, the results are presented in the fol- lowing way. On the time section (Fig. 6), the representation of which is entirely standard, vectors are superimposed; the length of these vectors is proportional to the calculated time gradient and they are oriented in the downward direction of the dip. Conventions are such that a horizontal vector represents a dip in the vertical plane of the profile, a vertical vector oriented downwards, indicates a gradient perpendicular to the profile and dipping in the direction of the observer. When the vector is di- rected upwards, this indicates a dip in a plane perpendicular to the profile and rising up to- wards the observer. Of course, when the vector is vertical, the event appears horizontal on the section• Finally, an oblique vector indicates an oblique direction. Note that if we lay the section out on the location map in such a way that the horizontal is superimposed on the line of the profile, the vector indicates the direction of the dip. The time contours are therefore perpendicular to this direction.

In this way, with practice, we can obtain the 3-D view of the subsurface for visual inspec- tion by separating the horizons: those that are located at the rear of the vertical plane of the profile (vector component downwards), those that are situated at the front (component upwards), and those which are in the vertical plane of the profile (horizontal vectors).

Furthermore, we may also trace a lane of t ime contours for seismic horizons which are

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sufficiently continuous. Of course, the process must begin, as is customary, by picking the horizon, reading the times and plotting these values on the location map. When this is com- pleted, the dip information must be plotted. The simplest way is to reproduce on the location map either the vectors, or better still a lit- tle element perpendicular to the vector. The

time contours are then traced in the normal way, and should be perpendicular to the vectors. A certain amount of tolerance, depending on the quality of the horizons, must be permit- ted; we can gain a fair idea of what we can tol- erate by examining any oscillations in the longitudinal gradient which are visible on the section (Fig. 7).

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Knowledge of the total gradient is especially useful when the profile intersects with a fault. In this case, closure by a loop is no longer ver- ified, whereas the t ime contours must be drawn accurately as a number of structures close on the fault.

There may be seismic events on the section which are not of a sufficient extension to jus- tify the tracing of a map. Often these events are so short that they are only visible on one profile. The interpreter is then reluctant to use them because there is considerable risk of er- ror. Let us take, for example, the subhofizontal levels El which are located on profile 3 at around 4 s (Fig. 8 ): do they correspond to very deep sedimentary levels in conflict with the overburden? Are they reflections in the base- ment, are they due to lateral arrivals? So many questions are left unanswered if we do not know the lateral gradient; therefore, unless a profile crosses over just at the right spot, the interpreter is compelled to ignore them. On our section, we can see that the total gradient is large, very high even. Comparison of the length of the vectors shows that the gradient is two to three times larger than that of the shallower horizons. There is no further doubt that these arrivals come from diffraction points and correspond quite probably to reflected refrac- tions. By knowing the velocities, we can position these diffraction points about 3 km north of the profile (Fig. 9). We can put these dif- fractions in direct correspondence with fault Fl intersected again by profile 1 at point 165 (Fig. 10). The direction of this fault is then ensured and we can safely trace it up to profile 2 at point 81 (Fig. 11 ). We can see on Fig. 7 that the re- flectivity of this fault varies along the profile, presunably due to a variation in its regularity; we can see again at around shotpoint 60 reflec- tivity which we lost between points 100 and 60.

(2) Selecting arrivals according to their lateral gradient

When we add the traces corresponding to a cross-line, the amplitude of the sum depends

on the shift between these traces, which is to say that it depends on the lateral gradient (Fig. 12).

If we repeat the stack after application of a series of dip corrections, the amplitude of the result varies. It is max imum when the correc- tion applied corresponds to the existing dip. This max imum may be interpolated between two consecutive stacks.

If we apply weighting to each of the traces according to their similarity with the average trace, it is possible to render the amplitude more sensitive to the dip. The result is that each stack only retains the events with a dip similar to the selected dip. The successive stacks, corresponding to a series of dips, make a visual analysis of the section possible. In particular, the interferences can be clarified (Fig. 13 ).

In Fig. 13, the interference is obvious, but it may be less obvious as in Fig. 14, where discordance or interference may be imagined. This phenomenon is more likely to occur if the reflections originate from deep layers.

AMPLITUDE

Lateral time gradient

Fig. 12. Response curve for the lateral gradient of a normal stack and weighted stacks for determining the lateral gradient. The WLP special stack is the sum of weighted stacks.

296 D. MICHON

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Fig. 14. (a) Standard section. We do not know the lateral gradients, and we do not know therefore whether there is discordance between A and B. (b, c) Event A originates from one side of the line, event B from the other. They originate from the same geological surface which presents a synclinal form oriented along the profile.

(3) Positioning 3-D reflectors (migration)

When the velocity function is known, con- struction of chart curves facilitates this positioning. The vector gives the direction and on the chart curve, from the value of the total gradient and the reflection time, migration offset and migrated time are determined. Figure 15 shows this operation: when we look along the profile, the reflection point follows the sinuous curve from A to B. The migrated time contours are perpendicular to the migration vectors.

It is easy to see the effect of a change in the velocity function on the migration. This is par- ticularly effective when it is a question of focusing a diffraction hyperbola (Fig. 16). We retain the function which offers the best con- vergence to the migration vectors.

(4) Improving the signal-to-noise ratio

We have seen that during dip investigations we select the corresponding reflections for each

gradient. If we add the different selections, we can see that we only retain seismic events which have coherence, whether dipping or not, along the line. The noise, which by definition has no coherence from line to line is cancelled. The section becomes clearer. A typical example is given in Fig. 17. Not only do we have more confidence in the visible reflections after a WLP analysis, but we can see that they break down into two groups situated on different planes (Fig. 18 ).

Conclusion

When for financial or technical reasons, as is often the case for crustal studies (plate tec- tonics), 3-D seismic is not possible, the WLP method is highly desirable.

Data acquisition costs about the same as for a standard linear profile, data processing is slightly more complex if we want to obtain the full benefit of the potential offered by this

Fig. 13. (a) Section from Wyoming, USA. The reader can differentiate between the two systems of reflection thanks to the lateral component of the dip. (b) On this figure, only the events coming from horizons located behind the vertical plane of the section have been retained (vectors with a downward component on (a). (c) On this figure, only the events coming from horizons located in front of the vertical plane of the section have been retained [vectors with an upward component on (a) ].

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Fig. 15. (a) Section obtained in C24 in southwestern France using WLP method. (b) Data appearing on the interactive screen. At the top, the picked horizon as it was preserved. In the centre, the values of the lateral gradient. At the bottom, the migration vectors. These vectors join the recording position to the vertical projection point of the depth point on the surface• Thus when the recording point moves in a straight line from point 32 to point 242, the reflection point follows a curved line joining point A to point B.


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300 D. MICHON

method. However, standard processing is still an option.

The advantages of the method may be listed as follows: (1) Measuring the lateral gradient of any

event which can be picked on the profile. (2) 3-D positioning of the corresponding re-

flector or diffractor. (3) Separating interference, inherent to all in-

line acquisitions. (4) Improving the signal-to-noise ratio.

All these advantages lead to more reliable

interpretation and answer most of the questions raised by interpretation of a linear profile.

References

Michon, D., 1972. Wide Line profiling offers advantages. Oil Gas J., 70 (4, November 27).

Michon, D., 1973. Seismic survey by Wide Line profiling tackles 3D problems. Oil Gas J., February 18: 64-66.

Michon D. and Bourdaire J.M., 1973. Nouveau moyen d'interpr6tation sismique: le profil large. P6t. Techn., Juillet 73: 21-29.

Reconnaissance profiles with WLP in complex geological regions

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