Understanding Seismic Propagation through Triangle

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Chapter 3

Understanding Seismic Propagation through Triangle Zones Anne Jardin · Rosina Chaker · Piotr Krzywiec

Abstract.  Getting a geologically realistic seismic image still remains a great challenge in foothills exploration. When geological structures are complex, seismic sections show distorted images and become difficult to be interpreted. Seismic processing tools, like time and depth migrations, have been developed to deal with this. Furthermore, when lateral velocity variations are strong, depth migration is definitely required. However, for a correct application of this processing, the construction of a detailed depth model describing lateral and vertical velocity variations is essential. This study aims at showing that, in triangle zones, the structural features can be better retrieved if the seismic wave propagation through the complex subsurface is better understood. This is obtained by application of a workflow combining direct seismic modelling and depth migration processing. The seismic modelling gives a consistent and accurate velocity model used as input in the seismic depth migration. To illustrate the benefits of this approach, we present a synthetic case study based on a schematic model and a real case study from the frontal part of the Polish Carpathians.

1 Introduction The development of new prospects for oil and gas exploration will rely on our ability to detect reservoirs in complex structural settings (Aamir et al., 2006). In these areas, seismic imaging difficulties are mainly due to highly contrasting velocities, steep structural dips and inverse faults. These limitations lead to erroneous interpretations especially when only time seismic sections are interpreted. To reduce the risks of exploration drilling, seismic modelling is the welladapted method for assessing the ambiguities of interpretations. Various modelling techniques exist to simulate the seismic responses through a subsurface model. Kinematic modelling computes seismic reflection traveltimes and simulates seismic ray paths. This is the method most applicable to structural problems and is used to easily compare synthetic and real data by simulating the real seismic acquisition. In this paper, an advanced seismic modelling and migration processing workflow will be proposed. The

objectives of this combined method are, on one hand, to understand the complex ray paths of the seismic waves and, on the other hand, to compute a seismic image representative of the real structures. This approach will be applied on synthetic and real data sets. The synthetic example will aim at understanding the seismic path distortion within a schematic triangle structure and at analysing the variability of the depth migrated images. The real case of the Polish Carpathians will be used to evaluate the efficiency of the combined modelling and migration studies for structural interpretation.

2 Geological and Seismic Characteristics of Triangle Zones 2.1 Geological Settings Triangle zones are particularly prominent at the foothills margin. It appears in areas where conjugate thrusts are developed as a result of the compressive stress propagation onto the foreland. A schematic model of triangle zone has been proposed (Jones, 1996) illustrating the triangle structures of the Canadian Rocky Mountains (Fig. 1). A triangle zone or passive roof duplex is composed of a sequence of dipping autochthonous rocks juxtaposed against opposite-dipping strata contained in

Fig. 1.  A schematic model of a “Triangle Zone” or “Passive Roof Duplex” proposed by Jones (1996). The geometrical features are characterized by a sequence of dipping autochthonous layers overlain by rocks dipping in the opposite direction.

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imbricate thrust structure. These sequences are usually underlain by relatively underformed rocks. The upper detachment separates the autochthonous rocks on its hanging-wall from the allochthonous rocks in its footwall. The lower detachment is a surface common to the entire thrust and fold belt. It separates allochthonous from autochthonous rocks. The roof and floor of the thrust have opposite vergence and merge at depth to form a frontal tip line that marks the extremity of the intercutaneous wedge. The wedge will continue thickening and advancing into the foreland basin until a critical point along the lower detachment.

2.2 Seismic Imaging Difficulties In triangle zones, several factors reduce the effectiveness of seismic methods. The topography in the central part of the triangle zone is usually rugged and is associated with near-surface velocity inversions which degrade the quality of the seismic image. These characteristics lead to low signal-to-noise ratio, inadequate penetration of energy through overburden, poor geophone coupling with the surface and wave scattering. Because of the limited assumptions of time processing, the quality of the time sections is usually deteriorated and seismic interpretation often leads to erroneous structural interpretation. Seismic processing workflow must be selected according to the characteristics of the velocity variations. Therefore, depth migration which takes into account lateral velocity variations is the adapted technique for depth seis-

mic imaging in complex areas. But, in order to deliver a reliable depth image, this processing needs a depth velocity model for accurately focussing the diffracted energy and shifting the events in their true position. Thus, the approach described in the next paragraph will aim at better imaging the triangle zone features by an improvement of both seismic propagation understanding and depth migration processing.

3 Combined Seismic Modelling and Migration Eorkflow We propose to apply a combined workflow of two main modelling and migration steps (Fig. 2). Firstly, an initial velocity model is built based on an a priori geological knowledge from regional studies and well data (especially measured depths and sonic velocities) associated with seismic velocity analysis and time section interpretation. This model is then iteratively refined by modelling. IFP has developed a model builder and ray tracer software appropriate to model complex geological features and simulate seismic ray travel paths. The flexibility of this technique allows using both block and smoothed parameterised models through which ray paths and traveltimes are computed for various seismic acquisition patterns. The ray shooting modelling is performed by 2-D and 3-D two-point paraxial ray tracing (Clarke, 1997) which is the kernel of many kinematic applications such as quality control of velocity models, tomography and Kirchhoff migration. A blocky model is made of blocks within which the velocity is constant or variable with a vertical gradient.

Fig. 2.  Combined modelling and migration workflow allowing reliable velocity model building and depth seismic imaging. A priori information from geology and geophysics is used to build an initial depth model iteratively refined by comparison between synthetic and real seismic data.

Chapter 3 · Understanding Seismic Propagation Through Triangle Zones

Once the blocky model has been built, a smoothing is introduced in order to replace sharp interfaces with smooth velocity variations. Secondly, synthetic seismic travel times computed for zero and non-zero offset patterns are compared with picked traveltimes from real shot points and stacked traces. A satisfactory matching ensures a reliable kinematic computation and validates the estimated velocity model. Then this model is introduced in the depth migration processing. In presence of rough topography and complex overburden, innovative migration algorithms using wave equation or Kirchhoff formalisms have been developed by IFP to accurately migrate 2D and 3D seismic data (Duquet et al., 2003), (Rousseau et al.; 2000). Finally the depth seismic image is interpreted to give a depth structural model.

4 Synthetic Model Study Two models based on the geometrical features of the schematic triangle structure of the figure 1 are built with high and low velocity contrasts within the duplex formation. Each reflector is described by one interface and velocities are assigned to each block (Fig. 3). The constant velocity value (V) and the vertical velocity gradient (K) for each unit of the two models are summarised in Table 1. Smooth versions of these blocky velocity models are computed to overcome the propagation effects related to non realistic abrupt velocity contrasts. For kinematic ray-tracing modelling, various 2D acquisition patterns were used to model zero-offset sections (Fig. 4) and shot-point gathers (Fig. 5). The sources and receptors are located on the first interface which is used to simulate the curved topography. The green dots shown at this interface indicate a selection of geophone positions. Fig. 6 shows the zero-offset time seismic sections obtained with the two models. Dynamic modelling is also performed to obtain synthetic traces. These traces will be used to test the sensiFig. 3.  Synthetic blocky velocity model with velocity values indicated in Table 1.

Table 1.  Velocity parameters for the two synthetic blocky models with low (MODEL 1) and high (MODEL 2) velocity contrasts. Structural features of the model are given on Figure 3. Velocity indexes

Depth (km)

Velocity (km/s)

Velocity gradient (s-1)

Model 1

Model 2

V0

0

1.5

1.5

0

V1

0.65

3.35

3.35

0.1

V2

3.5

5.88

5.88

0.2

V3

1.9

3.9

6

0

V4

2.3

3.52

4

0.1

V5

2.2

3.52

3.52

0.1

V6

1.98

3.52

5.5

0.1

V7

1.98

3.52

3.52

0.1

V8

2.8

3.52

5.5

0.1

V9

1.5

3.52

3.52

0.1

V10

4.6

5.7

5.7

0.1

V11

1.0

3.52

3.52

0.1

V12

1.26

3.52

3.52

0.1

tivity of depth seismic images to velocity variations by performing pre-stack depth migration (Fig. 7).

4.1 Time Seismic Artefacts Various effects of complex seismic-ray propagation are detected:

 Shadow zones at the deep interfaces are noticeable

on the 2D ray-tracing results (Figs. 4, 5 and 6). The analysis of these results indicates that these reflector discontinuities are generated either by the geometrical features of the triangle zone or the velocity distribution within the duplex units. Figures 4 and 5 show that the rays are distorted when they crossed the ramp or travelled through the duplex. As the

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Anne Jardin · Rosina Chaker · Piotr Krzywiec Fig. 4.  Zero offset ray tracing modeling for the synthetic models with low (top-MODEL1) and high (bottom-MODEL2) velocity contrasts showing ray path distortion through backthrust and duplex layers and possible shadow zones at the deep and horizontal interface.

geological structure is common for both models, the impact of strong variations in the velocity distribution is analyzed on the seismic responses. We can say that this phenomenon of shadow zone extent is more important for the model with the duplex presenting high velocity contrast.  Particular attention is paid to the deep horizontal interface. The complicated travel paths of the seismic waves yield pull up effect giving a geometrical deformation of the seismic event (Fig. 6). This may lead to errors in the seismic interpretation because a pull-up could be associated with an anticline. Fig. 6, we do not notice significant kinematic variations on the time zero-offset sections computed with the high or low velocity contrast models, demonstrating that the pull-up effect of the deep reflector is mainly due to the velocity contrast between the dipping layers of the backthrust and the overburden gentle structure. We can conclude that the velocity variations of the duplex do not strongly modify the presence of this effect; they just change the size of the pull-up phenomenon.

4.2 Sensitivity of Depth Migrated Images to Velocity Models Pre-stack depth migration was applied on synthetic traces to analyze depth-migration failures related to the triangle-zone geometry and velocity model uncertainties. Several depth models used in migration processing were computed by producing increasingly smoothed versions of the exact blocky models. The sensitivity of depth seismic images to these smoothed models could then be assessed. The results of depth imaging using three smoothed velocity models are shown in Fig. 7. To judge how good these depth images are, we pick the deepest reflector and compare its depth to the exact value. From this comparison, it is clear that in the depth image A (top of Fig. 7), all elements of the model are correctly positioned and imaged. The depth images B and C (respectively middle and bottom parts of Fig. 7) show a slight degradation of the image as this deep reflector is not positioned at the true depth. Thus, the degradation of the image with increasing smoothing is gradual. This image degradation is strongest in

Chapter 3 · Understanding Seismic Propagation Through Triangle Zones Fig. 5.  Different shot point tracing modeling through the synthetic models with low (topMODEL1) and high (middle and bottom-MODEL2) velocity contrasts showing raypath distortion through backthrust and duplex layers.

Fig. 6.  Synthetic zero offset time sections for the synthetic models with low (top-MODEL1) and high (bottom-MODEL2) velocity contrasts showing pull-up effect and non illuminated or shadow zones at the deepest interfaces.

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Anne Jardin · Rosina Chaker · Piotr Krzywiec Fig. 7.  Sensitivity analysis of depth seismic images to smoothness degrees of depth velocity models introduced in pre-stack depth migration of synthetic seismic traces computed by dynamic modelling. Depth accuracy of the target deep interface has changed with the increase of the smoothing parameters (increase from A to C).

Fig. 8:  Geological map and location of the study area in the Polish Carpathian foredeep basin.

Chapter 3 · Understanding Seismic Propagation Through Triangle Zones

the deeper complex zone where the smoothing has the largest effect on wave propagation. If depth migration is performed via a wave-equation algorithm, a blocky representation of the velocity model could be used. However, the Kirckhoff migration, the most often applied algorithm in complex area, requires a smooth velocity model. Thus, we propose to build a first velocity model with a blocky representation that allows the integration of geological information in the velocity model. Then this model is smoothed and its kinematic coherency is checked by ray tracer modelling before performing a depth migration.

5 Polish Carpathians Real Case Study The geology of the Carpathians is very complex and seismic horizons are highly deformed by tectonics movements, numerous faults and thrusts. The Carpathian foredeep and the Carpathian overthrust belong to the largest petroleum provinces of central

Europe (Fig. 8). The Outer Carpathian orogenic belt consists of several imbricated thrust sheets built-up of Cretaceous to lower Miocene Flysch deposits and imbricated Miocene foredeep deposits (Lafargue et al., 1994). The Zglobice and Stebnik units are the most external thrust sheets in the nape pile. The central part of the Zglobice Unit is a passive-roof duplex defining a Miocene triangle zone. Fig. 9 (top) shows the time migrated section of the 2D seismic line 3-02-02K which was acquired across the triangle zone (Krzywiec, 2001), (Krzywiec and et al., 2004). Above the duplex, in the roof of the triangle zone, major synclinal folds are recognised in the Miocene deposits. Some of the inferred tectonic structures are classified as synsedimentary structures that developed during deposition of the post evaporitic Miocene siliciclastic foredeep infill. An important detachment level within the foredeep is defined by the Badenian evaporitic deposits (rock salt, anhydrites, and gypsum). This evaporitic horizon forms an excellent marker for the analysis of seismic reflection data. Beneath the floor thrust of the Zglobice unit, there is a deep buried erosional val-

Fig. 9.  The time migrated stack section 3_02_02K (top) and the vertical depth converted stack section with a constant velocity value (3500 m/s) (bottom).

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Anne Jardin · Rosina Chaker · Piotr Krzywiec Fig. 10  The depth seismic image (top) obtained by poststack depth migration. Residual hyperbolic events remain visible below the incised valley (Lower Badenian Deposits) and could be interpreted as real reflectors or non-migrated diffractions (top seismic image). The a priori constant velocity model (bottom) used in migration was built from stacking velocity mean values.

ley cuts into the Meso-Paleozoic basement. The vertical depth conversion of the time migrated stack using constant velocity values is presented Fig. 9 (bottom). The geometrical features of several geological interfaces have been clearly modified after this operation on the left and right parts of this seismic image. An initial depth migration of the stack time section (i.e., post-stack depth migration) was performed using a priori velocity model derived from seismic stacking velocity analysis and velocities from projected well logs (Fig. 10). After this processing, several ambiguities in structural interpretation below the triangle structure remain – e.g., real reflected event or non migrated diffraction. Structural model improvement still requires a more accurate velocity model for seismic depth imaging and will be obtained by application of the combined modelling – migration approach.

5.1 Accurate velocity model building A 2D subsurface model is built based on the interpretation of the seismic stack 3-02-02K depth-convert-

ed using the a priori velocity model (Fig. 11). The surfaces selected for modelling are those which delimit the triangle structure: Flysch and Miocene surfaces in the deformed and undeformed Miocene foredeep infill, the evaporitic formation, the incised valley, the fault and the main surfaces that composed its footwall and hangingwall (Cretaceous, Jurassic and Carboniferous, Precambrian formations). Using interval velocity computed from vertical seismic profile measurements, P-velocity values are assigned to the different blocks of the horizontal extent of the 2D depth model (Table 2). The depth of the model is calibrated against the interpreted depth seismic line 3-02-02K. Thus, the 2D model is defined in practice over 16 km is x-direction; 4.6 km is z-direction. Data on CMP elevation and floating datum static corrections written in the trace header do not indicate great variations, implying that we have, in this case, a relatively gentle topography. The datum plane estimated around 200 m is taken into account in the model description. A kinematic modelling was performed using this initial velocity model. Seismic acquisition parameters used for modelling are derived from real acquisition

Chapter 3 · Understanding Seismic Propagation Through Triangle Zones Table 2:  Layer velocity variations for the geological units of the Polish Carparthians case study. Geological units

Layer velocity variations (m/s)

Carpathians Flysch

3500–3800

Deformed Miocene units

2700–3300

Undeformed Miocene units

2700–3300

Miocene evaporites

3800–4200

Sub-evaporitic lower Badenian siliciclastics (including infill of the erosional valley)

3400–3800

Cretaceous

3700–4600

Jurassic

4500–5000

Carboniferous

5500

Precambrian

5500

Fig. 11.  The depth seismic image (top) obtained by poststack depth migration using a more detailed velocity model (bottom). This model was computed by applying the combined modelling and imaging approach of the figure 2 and contains lateral and vertical gradient variations. A better focussing of the diffraction events below the incised valley is clearly visible due to the velocity model improvement.

parameters. Travel times are calculated by both shotgather and zero-offset ray-tracing modes through the velocity model for the horizons picked on the 3-0202K section: Flysch formation limits, interface base of deformed and undeformed Miocene units, incised valley interface base, Cretaceous and Jurassic interface bases. As the zero-offset mode simulates a time stack section, these travel times are superimposed on the 302-02k real stack section to check the kinematic coherency between the real and synthetic data (Fig. 12). The aim is to assess the depth velocity model. Compared to the a priori velocity model (Fig. 10), this second velocity model is more sophisticated and showed velocity variations that could be correlated with structural and geological variations (Fig. 11).

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Anne Jardin · Rosina Chaker · Piotr Krzywiec Fig. 12.  Comparison of real times from direct interpretation of stack time section and synthetic times (in black curves). Synthetic times were computed by kinematic modelling through the depth velocity model of the figure 10. This analysis is done to check the kinematic consistencies of all the data used in seismic imaging processing.

Fig. 13.  Comparison of depth images using the a priori velocity model (top) and the detailed velocity model (bottom). Improvement of diffraction focussing and fault delineation are clearly visible. This is due to the better estimation of velocity variations in the overburden triangle zone.

Chapter 3 · Understanding Seismic Propagation Through Triangle Zones Fig. 14.  Comparison of depth (top) and time (bottom) migrated seismic images. Depth migration has improved the layer delineation (no crossing reflectors) and the geometry of the deformed Miocene deposits (passive roof duplex and backthrust).

5.2 Depth migration

6 Conclusion

The blocky velocity model is slightly smoothed before performing the depth migration whatever the algorithms used. The depth seismic images using the two velocity models in the area of the triangle structure are compared (Fig. 13). The seismic event located around 2000-3000 m is better migrated and reflector endings are more visible when the kinematically consistent velocity model has been used. Thus the quality of the seismic image is improved. This will help in reducing the ambiguities in the seismic interpretation, especially those below the core of the triangle zone. To complete this analysis, the time migrated stack section and the depth migrated stack image are compared (Fig. 14). Even if time migration could also give a good diffracted energy focussing, layer termination and local pinch outs are better resolved on depth images. The dips of the geological units are also more accurately and directly estimated. Further improvements can still be obtained by application of an efficient pre-stack depth migration on the pre-processed shot point gathers followed by the stacking of partial angle traces. The checking of seismic event flatness on depth migrated trace gathers ensures the reliability of the velocity data as an additional quality control of depth imaging processing.

In fold and thrust belt areas, structural features named „triangle zones” are characterized by complex layer geometry and velocity variations. The poor quality of the seismic image encountered in such geological settings is due to the complex propagation of seismic energy through the overburden structure associated with strong velocity distribution. It results in shadow zones and in geometrically deformed seismic events, which make any interpretation difficult. In order to select the seismic processing parameters like migration velocities and reduce the ambiguities in structural interpretation, it is important to better understand the real seismic wave propagation for these specific seismic events. In this study it was performed using adapted seismic modelling and the application of a combined modelling and migration workflow. Two examples, a synthetic example based on a schematic triangle zone and a real case from the Polish Carpathians, have been addressed. Velocity models were built and introduced in the seismic modelling software. Using non-zero and zero-offset ray-tracing modelling, seismic interpretation has been carried out to analyse specific seismic travel paths encountered in these areas. We have demonstrated that these events could be generated by complex seismic wave propagation which leads to ambiguities in time seismic in-

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terpretation because specific artefacts like pull-up and shadow-zone effects are present. Based on geophysical and geological framework, a velocity model was built using an interpretative approach. This depth model was next introduced in the seismic modelling software in order to assess its kinematic accuracy. The aim was to compare the synthetic and the real data. When the two agreed to within an acceptable level of accuracy, the depth velocity model has been used for depth migration. These two case studies illustrated the practical use of ray-tracing modelling to:

 Simulate and understand seismic wave propagation

and analyse seismic processing and interpretation difficulties in this complex geological context.  Check the kinematic accuracy of the velocity model built by an interpretative and iterative approach. The conclusion is that the combined analysis of real and synthetic gathers computed by adapted seismic modelling can provide the key elements for the determination of the structural origin and of the seismic nature of particular events which can not be clearly interpreted on seismic time sections. In case of complex seismic propagation, their efficient identification will guide the geologist interpreter for properly analysing the picked reflections and the geophysicist processor for applying the adapted imaging processing with appropriate parameters.

Acknowledgements The authors would like to thank P. Aleksandrowski from the University of Wroclaw Poland - Institute of

Geological Sciences, R. Florek and J. Siupik from Polish Oil & Gas for providing the real data and for their contribution in the interpretation of the Polish Carpathians seismic images, and also T. Perdrizet from IFP for its efficient and patient help in the use of modelling software.

References Aamir M., Maas Siddiqui M. (2006), Interpretation and visualisation of thrust sheets in a triangle zone in eastern Potwar, Pakistan, The Leading Edge, 1 : 24 –37. Clarke R.(1997), Modelling and inversion of 3D complex kinematic data, Ph.D thesis, Université de Pau et des pays de l’Adour. Duquet, B., Xu S., Lambaré G. (2003), 3D multi arrival Kirchhoff versus wave equation migration, Application to the 3D SEG/EAGE salt Model, The Leading Edge, 10 : 969–972. Jones P.B. (1996), Triangle zone geometry, terminology and kinematics, Bulletin of Canadian Petroleum Geology, 2 : 139– 152. Krzywiec P. (2001), Contrasting tectonic and sedimentary history of the central and eastern parts of the Polish Carpathian foredeep basin-results of seismic data interpretation, Marine and Petroleum Geology 18: 13–38. Krzywiec P., Aleksandrowski P., Florek R., Siupik J. (2004), The structure of the Outer Carpathian orogenic front: an example of the Miocen Zglobice unit between Brzesko and Wojnicz – new data, new models, new questions, Przegload Geologiczny, 11 : 1051–1059. Lafargue E., Ellouz N., Roure F. (1994), Thrust controlled exploration plays in outer Carpathians and their foreland (Poland, Ukraine and Romania), First Break, 2: 69–79. Rousseau V., Nicoletis L., Svay-Lucas J., Rakotoarisao H. (2000), 3D true-amplitude migration by regularization in the angle domain, 62nd EAGE Conference, Expanded Abstracts.