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An Introduction to Seismic InterpretationChapter 1: Introduction

1.1 Introduction: Purpose of this TextThis text discusses how active-source, refl ection seismic surveying is used to study the Earths interior. The focus is on the geologic interpretation of seismic data, although topics related to wave propagation, data acquisition and processing, and quantitative rock property prediction are also discussed. Most seismic datasets are collected by the petroleum industry in its quest to explore for and develop hydrocarbon reserves, and so this text will explore many topics of relevance to the petroleum industry. However, seismic data have long been used in other branches of Earth Science and are seeing increasing use in other fi elds. Accordingly, this text is intended to also benefi t a non-petroleum readership. In a general way, it should be of interest to:

Geologists with minimal training in geophysics Geophysicists with minimal background in geology Engineers or others who use seismic data, or might be exposed to the results of a

seismic interpretation, but have little/no background in either geology or geophysics.

Seismology is a branch of earth science that focuses on the study of seismic waves, elastic disturbances that propagate through the Earth (based on Sheriff, 2002) after earthquakes (passive-source seismology) or in response to man-made perturbations (known either as active- or controlled-source seismology). Applications of seismology include using seismic waves to probe the deepest parts of the Earths interior (e.g., the core-mantle boundary; e.g., van der Hilst et al., 2007) or the upper few meters of a sediment column (e.g., Baker et al., 1999). Lay (2009) described some of the topics currently being addressed by the seismology community.

For simplicity, the term seismic data is used in this text to describe seismic refl ection data collected using sources and receivers at the Earths surface (sometimes referred to as surface seismic data). Borehole seismic methods involve putting receivers, sources, or both down boreholes. Selected topics in borehole seismology are discussed in CHAPTER 5. Seismic refraction surveys, that analyze a type of seismic wave known as a head wave, are also used to the study the Earths interior (e.g., refraction methods are generally considered to be more useful than refl ection methods to study the upper few meters of the subsurface; Burger et al., 2006), but will not be discussed here.

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Various aspects of the historical development of seismic technology have been discussed by others (e.g., Sheriff and Geldart, 1995; Chopra and Marfurt, 2005) and will not be repeated here. Suffi ce it to say that improvements in technology (e.g., the advent of digital recording and continuous development of computer graphics capabilities) in the 20th century and since have signifi cantly changed the speed and accuracy with which interpreters can defi ne subsurface structure, stratigraphy and rock properties (e.g., FIGURE 1.1; ANIMATION 1).

The seismic interpretation problem is a diffi cult one. Seismic waves are used to remotely image geologic features of the Earths interior. In some respects, the method has similarities to the way magnetic resonance imaging (MRI), CT scans or X-ray imaging are used to image the human body. However, the medical imaging problem differs from seismic imaging in at least two fundamental ways:

The MRI data are collected in carefully controlled environmental conditions whereas the seismic data are collected in uncontrollable fi eld conditions that can be quite variable (e.g., onshore versus offshore seismic surveying).

Human bodies are all more-or-less the same (knees, brains, and other body parts are all about the same size and shape, and made of the same materials), whereas the targets of seismic imaging can be extremely variable. Some seismic data are collected to provide high-resolution images of the upper several meters of unconsolidated sediment over relatively small areas. Other seismic data are collected to image large-scale structures in igneous and metamorphic rocks.

Accordingly, each seismic project can be quite different in terms of how the data are acquired and the processing that needs to be applied in order to generate interpretable images. Seismic data quality can be variable, depending on acquisition and processing parameters (e.g., weather conditions during acquisition, choice of processing algorithms). The seismic interpreter is faced with imperfect images of geologic features that can have largely unknown dimensions, orientations, internal structures and physical

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An Introduction to Seismic Interpretation Chapter OneIntroduction

FIGURE 1.1:Comparison of different vintages of seismic imaging

FIGURE 1.2:Magnetic resonance imaging (MRI) slice through the authors knee.

Figure 1.2: Magnetic Resonance Imaging (MRI) slice through the authors knee

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The slice from a 3-D volume that was collected in the controlled environment of the medical imaging facility. The shapes, sizes and physical properties of features can be determined from this type of image, but knowledge of anatomy is clearly needed to understand it. Scale bars in cm.

Figure 1.1: Comparison of diff erent vintages of seismic imaging Back to Chapter

A

C

B

C) Three-dimensional seismicvisualization of mounded structures in Pliocene deposits of the eastern Mediterranean sea. Refl ections marked fl at spot represent probable hydrocarbon-water contacts. From Frey-Martinez et al. (2007).

A) Seismicinterpretation in the 1940s. Key refl ection events are identifi ed in two unprocessed paper copies of shot records collected several to many kilometers/miles apart. Reproduced with permission from Stommel (1950).

B) Seismic imagingin the early part of the 21st Century. Three-dimensional visualization of a Pleistocene submarine channel system. From Labourdette and Bez (2010).

ANIMATION 1Volume-rendering of a 3-D seismic cube showing igneous intrusions. Courtesy CGG-Veritas.

Animation 1: Volume-rendering of a 3-D seismic cube showing igneous intrusions. Courtesy CGG-Veritas .

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properties. Furthermore, those images generally contain artifacts related to how the data were acquired and processed.

Knowledge of geology is clearly needed to interpret seismic images, in the same way that knowledge of anatomy is needed to interpret medical imagery (FIGURE 1.2). However, other knowledge is also needed in order to maximize the benefi t obtained from a seismic interpretation, including:

The physical controls on seismic wave propagation through the subsurface, including how refl ections are generated and limits on seismic resolution.

The relationships between acquisition and processing parameters and seismic image quality.

The ways in which seismic data are displayed and analyzed. How seismic data are tied to other sources of subsurface information, such as wireline

logs, in order to calibrate the seismic images.

These topics are addressed in Chapters 2 5 respectively. Geologic aspects of seismic interpretation are covered in Chapters 6 (structural geology) and 7 (sedimentary geology), whereas CHAPTER 8 focuses on quantitative methods for rock property prediction.

1.2 What is a Seismic Interpretation?Doing a seismic interpretation means different things to different people. Seismic data are collected and used by many types of people to study portions of the Earth that are inaccessible to direct observation. Seismic datasets can consists of a few 2-D seismic lines (e.g., Snyder et al., 2005), integrated grids of 2-D and 3-D seismic data that cover large areas (e.g., Hsiao et al., 2004), small (1 square mile or less; ~1.6 km2) 3-D surveys collected onshore by small oil companies (e.g., Hart et al., 1996), or mega 3-D surveys that cover nearly 8000 square miles (~20,000 km2; e.g., Fugelli and Olson, 2007). As discussed below, variable amounts of other data might be incorporated into the interpretation.

Geophysicists, geologists, engineers and potentially others all collect and analyze seismic data, and for different purposes. Although most seismic datasets are collected by the petroleum industry in its quest to fi nd and exploit hydrocarbon accumulations, the technology is an indispensable part of many other branches of applied and fundamental geoscience and engineering. An incomplete list of non-petroleum applications of seismic technology includes:

Exploration and development in the mining industry. Seismic methods have been tested for mineral exploration (e.g., Milkereit et al., 1997; Eaton et al., 2003) but are relatively uncommon (Milsom, 2006), partly because of cost but mostly because the lack of coherent layering in igneous and metamorphic rocks makes processing and interpretation diffi cult. Applications for coal mining are more common (e.g., Gochioco, 2000; Pu and Xizun, 2005; Zuo et al., 2009).

Seismic data have been collected by research groups for many years to image deep crustal structure (e.g., Allmendinger et al., 1987; Milkereit and Eaton, 1998; Snyder et al., 2005; FIGURE 1.3) and to plan ocean drilling projects (ODP/IODP) and interpret the data (core, wireline logs) collected (e.g., McIntosh and Sen, 2000; Tobin et al., 2009).

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Petroleum-industry seismic data are commonly donated to research groups used to address a wide variety of fundamental geoscience questions. Applications to structural and sedimentary geology are well established, but even igneous processes are being studied using petroleum industry seismic data (e.g., Hansen et al., 2008; FIGURE 1.4).

Seismic data are acquired in civil engineering studies for a variety of reasons, including detection of faults or other zones of weakness, determining depth to bedrock, locating cavities and voids, delineation of different lithologies (e.g., natural soils versus fi ll), etc. (e.g., Burger et al., 2006; Schuck and Lange, 2008).

Oceanographers have begun using seismic data to study the thermohaline structure of the ocean (e.g., Holbrook et al., 2003; Nandi et al., 2004).

Seismic data are used to map and characterize aquifers (Cardimona et al., 1998; Louis et al, 2002; Sharpe et al., 2004; FIGURE 1.5). The distribution of water in aquifers is most commonly controlled by stratigraphic features that determine the distribution of porous and permeable strata, but fractures can be important in some cases.

Methods and concepts of seismic data acquisition, processing, and interpretation have been adapted to ground-penetrating radar studies (e.g., Bristow and Jol, 2003; Moysey et al., 2003) and so it is hoped that this text will be of interest to GPR interpreters.

Despite this wide range of potential applications, it is possible to identify three main purposes for collecting and interpreting seismic data:

To defi ne subsurface structure. This might entail looking for faults, folds, igneous intrusions or other features. A petroleum geologist might be interested in faults and folds because they can trap hydrocarbons.

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FIGURE 1.3:Perspective view showing integration of crustal-scale seismic profi les with surface geologic maps

FIGURE 1.4:Application of petroleum-industry 3-D seismic data to study igneous intrusions from the North Sea

FIGURE 1.5:Seismic transect across Quaternary and Holocene strata from eastern Ontario

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From Snyder et al. (2005).

Figure 1.3: Perspective view showing integration of crustal-scale seismic profi les with surface geologic maps

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A

C

B

C) Zoom in on a portion of the top of basalt horizon. From Thomson (2007).

A) Arbitrary sectionthrough a 3-D seismic volume showing various types of igneous intrusions.

B) The same profi le withan illuminated horizon showing top of a basalt layer (TB in part A).

Figure 1.4: Application of petroleum-industry 3-D seismic data to study igneous intrusions from the North Sea

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The seismic profi le has been integrated with aerial photography and (not shown) outcrop exposures of glacial deposits.

From Cummings and Russell, 2007.

Figure 1.5: Seismic transect across Quaternary and Holocene stratafrom eastern Ontario, showing a channel incised into Paleozoic bedrock and associated glacial deposits



To a civil engineer, faults could be potential planes of weakness, or possible signs of modern seismic activity. Faults, folds and other structures can be used to reconstruct the tectonic history of an area in an academic study. These themes are discussed in CHAPTER 6.

To defi ne subsurface stratigraphy. Stratigraphic features act as hydrocarbon reservoirs, baffl es that retard subsurface fl uid fl ow, or seals (caprocks) that trap hydrocarbons. At shallower depths, stratigraphic features form aquifers, aquitards, and aquicludes that control the movement of groundwater. Sedimentary deposits host important information about Earth history, including essentially all of what is known about the history of life on this planet. The stratigraphic interpretation of Seismic data is discussed in CHAPTER 7.

To defi ne subsurface physical properties. It may be desirable or necessary to make quantitative predictions of porosity, the nature of pore-fi lling fl uids (e.g., hydrocarbons or water), subsurface pressure or other variables. These topics are discussed in CHAPTER 8.

In some cases, such as in the petroleum industry, an interpreter or an interpretation team might pursue all three of these objectives. In other cases, seismic data are collected and analyzed to answer specifi c questions that can be broadly related to one of the three topics described above.

The Geologic Interpretation ProblemThere have traditionally been two main approaches to seismic interpretations. Physics-based interpretations (e.g., Avseth et al., 2005; CHAPTER 8) are often preferred by geophysicists who integrate seismic measurements with physical laws, laboratory measurements and other data and concepts in an effort to directly predict the physical properties (porosity, lithology, fracture spacing and orientation, etc.) of features imaged seismically. On the other hand, geologic interpretations (examples in Chapters 6 and 7) integrate qualitative and quantitative observations of seismic data with geologic principles (commonly non-mathematical constructs), analogs (outcrops, small-scale physical models, etc.), physical and chemical laws, and other data and concepts in order to reconstruct the geologic history of an area and often to make qualitative inferences about the physical properties of features imaged seismically. The physics-based problems tend to be more tractable (provided that adequate seismic measurements can be obtained), whereas the geology-based problems are beset with all of the problems that have traditionally plagued geologists: ambiguous data, incomplete and inadequate data (i.e. the problem is underdetermined), and complex issues that are not readily addressed by mathematics or physics.

Frodeman (1995) suggested that geology has aspects of both an interpretive science and an historical science. The interpretive aspect of geology implies that how geologists perceive a seismic display (or an outcrop, a thin section, etc.) will be based on the toolsets, concepts, expectations and values that are brought to the interpretation. Different levels of importance are assigned to various aspects of a seismic image based on the geologists experience, the prevailing paradigms, and other factors. This type of interpretation is analogous to the way in which medical doctors diagnose a health problem (e.g., if a patient has stomach problems, what are the important symptoms to look for and/or the important tests to make?). The historical aspect of geology means that geologists commonly deal with temporal (and spatial) scales that are not amenable to laboratory experiments. Although we can squeeze layers of modeling clay in order to partially duplicate the forms of seismic refl ections

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thought to be the product of compressive tectonic deformation, we can change base level and sediment supply in giant fl ume tanks to partially duplicate the stratigraphic character of seismic sequences, or we can build numerical models of structural or depositional systems, the results of these experiments do not (at least yet?) fully reproduce the effects of processes that shaped large portions of the Earths surface over periods of millions to hundreds of millions of years. Accordingly, geologists are left to reason by analogy (e.g., the present is the key to the past), formulate hypotheses that are consistent with all the available evidence (seismic or otherwise), and use inductive logic to eliminate alternative theories.

Geologic interpretations of seismic data draw upon fundamental principles of geology, some of which date back to the 17th century, to identify periods of deposition, erosion and tectonic disturbance, and to establish their relative timing. For example:

The Principle of Uniformitarianism states that the laws and processes (gravity, erosion, etc.) active in the past are those that operate today.

The Principle of Original Horizontality states that sediment is deposited in essentially horizontal layers (FIGURE 1.6A). It follows that steeply inclined stratigraphic units must have been tilted sometime after deposition and lithifi cation (FIGURE 1.6B). Although there are exceptions to this general rule, for example some types of sedimentary deposits (e.g., coarse-grained alluvial fans or submarine talus slopes) can support steep depositional dips, it is a good starting point. It should also be pointed out that a seismic interpreter can manipulate the vertical and horizontal scales of a seismic display to make even very gentle dips (e.g., a few degrees) appear to be quite steep (see CHAPTER 4).

The Principle of Superposition states that in an undisturbed succession of sedimentary rocks, the oldest layers will be at the bottom and the youngest layers will be at the top (FIGURE 1.6A,B).

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FIGURE 1.6:Outcrop photos illustrating various basic fundamental geologic concepts

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Figure 1.6: Outcrop photos illustrating various basic fundamental geologic concepts

C) The brown-coloredigneous dyke in the middle of this photo (from Montreal) crosscuts the layered, grey sedimentary rocks, indicating that it formed sometime after deposition of the sedimentary rocks.

A) The bedding in these sedimentary rocks from southeastUtah is essentially horizontal, indicating a lack of structural disturbance. The oldest rocks are on the bottom.

B) The bedding inthese sedimentary rocks from Alberta is no longer horizontal, indicating a period of structural deformation at some point after the rocks were deposited and lithifi ed. Sedimentary structures of the rocks have been used as way-up indicators to infer the relative ages of the strata exposed in this outcrop.

D) The fault in the center-left of this image crosscuts, and so postdates,deposition of the sedimentary rocks. The timing relationship between the tilting of the sedimentary rocks and their faulting cannot be determined from this outcrop in eastern Spain.



The Principle of Cross-Cutting Relationships states that the body of an intrusion (perhaps an igneous body or salt) or a fault must be younger than the strata which it intrudes or displaces (FIGURE 1.6C,D). An erosion surface must also be younger than the surfaces it erodes.

These and other basic geologic principles are generally taught in introductory level geology courses (FIGURE 1.7)but they are used in seismic interpretation to address complex geologic problems.

A simple example of the application of these principles is illustrated in FIGURE 1.8A, a portion of a seismic profi le collected from the offshore area of Canadas west coast. As indicated in FIGURE 1.8B, this image suggests at least six different geologic episodes that can be put in relative geologic order:

A period of relatively continuous sedimentary deposition during which the sediments in the lower portion of the image (Package 1) were deposited. The Principle of Superposition indicates that the oldest deposits should be at the base of the succession because it does not appear to have been overturned.

A period of erosion, during which an approximately U-shaped erosion surface formed. The Principle of Cross-Cutting Relationships states that the erosion occurred sometime after deposition of the youngest eroded strata.

Sedimentary deposition in the eroded area (Package 2) postdated formation of the erosion surface. Again the oldest units are on the bottom.

Normal faulting affected both the eroded strata and the fi ll of the erosional hollow (Principle of Cross-Cutting Relationships both Packages 1 and 2 are affected by faulting, therefore the faulting postdates deposition of these two units).

Erosion of the fault at the sea fl oor, followed by

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FIGURE 1.7:Introductory geology level example of how fundamental geologic concepts are used to reconstruct geologic histories

FIGURE 1.8:Simple example showing how fundamental geologic principles are used to reconstruct the geologic history of a seismic image

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Figure 1.7: Introductory geology level example of how fundamental geologic concepts are used to reconstruct geologic histories

1. Emplacement of granite.2. Uplift and erosion.3 deposition of shale.4 deposition of limestone.5 deposition of sandstone.6 deposition of shale.7 deposition of sandstone.8 Faulting.9 Intrusion of dyke.10 uplift and erosion.11 deposition of shale.12 Intrusion of dyke and lava fl ow.13 deposition of sandstone.14 deposition of shale.15 uplift and erosion (modern)

Upper image shows an imaginary geologic cross section. In the lower image, geologic features are numbered in the order that they formed.

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A) Seismic profi le from thePacifi c continental shelf of Canada (image courtesy Julie Halliday; see also Halliday et al., 2005).

B) Animated reconstructionof the geologic history represented by the seismic image in A. See text for discussion.

Click image to play animation.

Figure 1.8: Simple example showing how fundamental geologic principles are used to reconstruct the geologic history of a seismic image

A

B



Deposition of a relatively thin veneer of sediments at the seafl oor (Package 3; Principle of Cross-Cutting Relationships the youngest strata do not appear to be affected by faulting, therefore their deposition must post-date the fault movement).

Some issues remain: We do not know what the lithologies are.

The stratifi ed appearance to the seismic image suggests sedimentary deposits, but are they sand, sandstone, mud, shale, limestone or some other type of deposit? Direct sampling, through coring, could be very helpful. Alternatively, information about how fast the seismic waves propagated through the sedimentary deposits might help us to put useful constraints on the possible lithologies.

We do not know the absolute ages of the deposits or how much time is missing at erosion surfaces, although we have established a relative timing for the geologic features imaged in the data. Fossils, ash beds, or other datable materials from core or drill cuttings could be useful for constraining the timing of events.

We do not know the origin of the U-shaped erosion surface. Perhaps information about its 3-D shape could be helpful (e.g., is it long like a channel or more bowl shaped?), but that information cannot be determined from this 2-D image.

We do not know how thick the succession is. The vertical scale at left is in time units (seconds, two-way traveltime) and we will need some information about seismic wave propagation velocities in order to convert from time to depth.

It is clear that, by themselves, the seismic data do not provide us with unique solutions to these and other important questions. The seismic data, and interpretations based thereon, need to be integrated with other data types in order to reduce the number of plausible interpretation


FIGURE 1.9:Venn Diagram depicting how integration of various data types helps to reduce interpretation uncertainty

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Each circle represents the range of possible interpretations that is consistent with data and concepts of geophysics, geology and engineering. The Truth is probably found in the intersection of these three diff erent sets.

Figure 1.9: Venn Diagram depicting how integration of various data types helps to reduce interpretation uncertainty

Each circle repreof possible inteconsistent withof geophysics, gengineering. Thfound in the int



options. This concept can be depicted using a Venn Diagram from Set Theory as illustrated in FIGURE 1.9. Three sets are shown, each of which illustrates the range of possible subsurface interpretations based on the geological, geophysical or engineering data alone. Commonly, geologists, geophysicists and engineers seek to reduce the uncertainty (i.e. circle size in FIGURE 1.9) by undertaking advanced interpretations in their fi eld of expertise. In reality, The Truth will be found in the intersection set that is consistent with all three types of data and concepts.

The need to integrate seismic and other data types will be a recurring theme throughout the rest of this text. The type(s) of data incorporated into a seismic interpretation will depend on the type of project. For example, an environmental marine geology project might combine high-resolution seismic profi ling with side-scan sonar imaging, multi-beam bathymetry, electromagnetic surveying, and some type of coring (Evans et al., 2000; FIGURE 1.10).Deep structural imaging might incorporate seismic images with gravity data (FIGURE 1.11; Bayer et al., 2002) or tomographic velocity analyses (FIGURE 1.12) to help constrain the results of the interpretation. In the petroleum industry, seismic analyses are commonly integrated with well data, such as borehole geophysical logs, micropaleontological or lithology data from cuttings or core, etc. (FIGURE 1.13; Sturrock, 1996). A similar approach is used in the Ocean Drilling world, where drilling locations are defi ned using seismic data, and then borehole logs and core are ultimately tied back to the seismic data (FIGURE 1.14). The seismic interpreter needs to have some degree of familiarity with the other data types included in the interpretation because information gleaned from those datasets can be used to constrain seismic interpretation possibilities. Knowledge of the limitations and capabilities of these other methods is useful for integrating them and weighing their importance and limitations when, almost inevitably, different datasets suggest different interpretations.

Defi ning the Purpose of the InterpretationThe purpose of a seismic interpretation project is likely to change from project to project, and may change with time. In the petroleum industry, an interpreters main duty on one project may be to quantitatively defi ne a distribution of potential reservoir sizes in an exploration area, and


FIGURE 1.12:Integration of a crustal-scale seismic profi le with tomography-derived velocity overlay

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Figure 1.12: Integration of a crustal-scale seismic profi le (variable-area display) with tomography-derived velocity overlay (color)

Velocities are useful in this context for defi ning lithology of the shallow crust, and for migrating the seismic data (e.g., FIGURE 2.23). Full seismic profi le extends to approximately 18 km depth (see FIGURE 5.13). From Snyder et al. (2005).

FIGURE 1.10:Integration of side-scan sonar data with high-resolution profi ling data in an environmental marine geology study of the Fraser Delta, Canada

FIGURE 1.11:Use of gravity data to test the validity of a seismic interpretation

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B) Deep-tow profi lerimage that includes the area shown in part A. This image provides information about the stratigraphy, but is less helpful than the side-scan sonar image for mapping purposes. Depth scale in part B assumes a water velocity of 1450 m/s. Modifi ed and used with permission from Hart and Barrie (1996).

Figure 1.10: Integration of side-scan sonar data with high-resolution profi ling data in an environmental marine geology study of the Fraser Delta, Canada

A) A side-scan sonar imageof the seafl oor. Dark areas represent hard seafl oor (Pleistocene till), and white areas are softer seafl oor (Holocene sediments). This image is useful for mapping the distribution of the diff erent substrate types, but provides no information about their thickness.

B

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Figure 1.11: Use of gravity data to test the validity of a seismic interpretation

The seismic data is shown at top, with a color overlay of stacking velocities. A simplifi ed geologic interpretation of the seismic data is shown at the bottom. The profi le and rock densities are used to predict the gravity fi eld. Close correspondence between predicted and measured gravity (middle) indicates that the seismic interpretation is at least possible, although conceivably other combinations of thickness and density could produce identical gravity profi les. From ODonnell et al. (2002).



establish the risks associated with each potential reservoir or reservoir type. The next project may have him/her using seismic data to defi ne the distribution of porosity in a specifi c oil or gas fi eld. Although interpreters sometimes develop specialties (e.g., pre-stack inversion, fracture detection, turbidite fan systems) they are also often called on to be generalists.

From the outset, an interpreter needs a clear understanding of the purpose of the project in order to help defi ne how much time and effort to devote to specifi c aspects of the interpretation, what sorts of data and/or outside expertise need to be included, and what fi nal products are expected. The objectives of the study, including the nature of the product(s) that will be generated from the seismic interpretation (maps, reports, digital outputs, etc.), are identifi ed and subsequent work focuses on attaining those objectives. It is possible to do a lot of work only to fi nd out that some, most or even all of that work was not useful for the project at hand. For example, if the main focus of a project is to defi ne infi ll drilling locations in a small, densely drilled area by mapping out channels using 3-D seismic data and wells, using those data to address questions of global eustasy (sea-level change) would probably not be a good use of an interpreters time. Maintaining this kind of tight focus might not seem appropriate from an academic perspective, where sometimes great new ideas spring from unexpected places. However, from an Industry (petroleum, mining or engineering) perspective it can be a necessity. Even in the world of Academia, projects (such as Ph.D. dissertations) usually have budgetary or time limits and promises made to a funding agency about what the deliverables will be.

Having established the purpose of the interpretation project, a seismic interpreter needs to become familiar with: 1) the other data types to be included in the interpretation (wireline logs, potential fi eld data, reservoir pressures, etc.), 2) the type of geologic features being imaged (perhaps facies models for the sedimentary deposits being imaged, or a particular type of structural model such as listric growth faults), and 3) aspects of the geologic history of the study area that are relevant to the interpretation.

Hydrocarbons and groundwater are usually found in sedimentary rocks, and so a seismic interpreter looking for those fl uids should have a working knowledge of sedimentary geology. This type of knowledge can be gathered in a variety


FIGURE 1.13:Biostratigraphic interpretation from an offshore well in the Gulf of Mexico

FIGURE 1.14:Integration of seismic profi le with core data from Ocean Drilling Program coring, offshore Costa Rica

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Figure 1.13: Biostratigraphic interpretation from an off shore well in the Gulf of Mexico

Gamma ray and spontaneous potential (SP) logs at left help to defi ne lithology. Biofacies (middle) help to defi ne water depth (SIN Shallow Inner Neritic, UPPB Upper Bathyal, etc.). Foraminiferal abundance helps to identify condensed sections and sequence boundaries. Paleontology of microfossils used to identify Faunal Discontinuity Events and Foram Bioevents (right). Synthetic seismogram used to tie biostratigraphically defi ned horizons and zones to seismic data. From Armentrout (1991).

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Figure 1.14: Integration of seismic profi le with core data from Ocean Drilling Program coring, off shore Costa Rica

From Gettemy and Tobin (2003). See that paper for description of abbreviations used in this fi gure.



of ways, including looking at outcrops, visiting modern depositional systems, looking at satellite imagery, taking courses, etc. (FIGURE 1.15). There are two primary reasons for gaining this knowledge. First, it must be recognized that there can be stratigraphic features (shale drapes, cemented zones, etc.) that can signifi cantly affect reservoir performance but cannot adequately be detected using seismic data. Second, we need to be able to associate refl ection patterns to geologic features (faults, channels, etc.). The disconnected blobs of color seen in an amplitude extraction might be meaningless to one interpreter, but might be recognizable as the components of a submarine fan to another (FIGURE 1.16). Although depositional systems show much variability in terms of stratigraphic architecture (e.g., are the channels in a submarine fan system straight or sinuous, are they fi lled with sand or shale?) and size, an interpreter needs to be able to put some constraints on the acceptable range of possibilities.

Similar logic applies to structural interpretation. Faults and folds can set up traps and faults can compartmentalize reservoirs. Sub-seismically detectable faults and fracture systems can have a signifi cant impact on reservoir performance. An interpreter needs to be able to draw upon his/her knowledge of structural geology (or perhaps seek assistance from an expert in the fi eld) in order to predict the location, dimensions, orientation and hydraulic properties of these features. Knowledge of structural geology (e.g., relationships between tectonic settings and structural styles) will help to guide a fault interpretation through areas of poor data quality. Small-scale analogs seen in outcrops can help an interpreter to understand seismic-scale structures(FIGURE 1.17). Knowledge of the tectonic history of an area can help to constrain the types of structures that are visible in an area, and their ages.

A seismic interpretation is commonly just one aspect of a project. Depending on data availability (e.g., the number of seismic surveys and wells) and the objectives of the project, the amount of work and knowledge needed to integrate all data types into an integrated interpretation can be overwhelming. Many companies and research groups therefore divide the work amongst interdisciplinary teams. In such a team, geologists, geophysicists and perhaps engineers feed off of each others work. For example, the geologists log-based interpretations are used by the


FIGURE 1.15:Becoming a better seismic interpreter by studying geology

FIGURE 1.16:Comparison of a seismic image with an outcrop-based depositional model

FIGURE 1.17:Similarity in structural features across fi ve orders of magnitude difference in spatial scale

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Figure 1.15: Becoming a better seismic interpreter by studying geology

C) Examining core, Calgary. These types of activities canhelp an interpreter to understand relationships between physical properties and depositional environment, the geometry and dimensions of sub-seismic features, etc.

A) Measuring outcropsof Cretaceous clastics, Book Cliff s, Utah

B) SCUBA diving on a modern reef, Red Sea

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Figure 1.16: Comparison of a seismic image with an outcrop-based depositional model

A) Vertical view of an opacity slab(cf. FIGURE 4.34). High amplitudes probably represent sandy deposits, and transparent areas represent shales. Modifi ed from Kidd (1999).

B) Submarine fan models based onobservations of the Permian Brushy Canyon Formation in West Texas. From Beauboeuf et al. (1999). Knowledge of the elements found in submarine channel and fan systems helps guide the interpretation of the seismic data.

A

B

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Figure 1.17: Similarity in structural features across fi ve orders of magnitude diff erence in spatial scale

A) En echelon series of fractures(white lines; cm-scale ) in Cretaceous sandstones of the San Juan Basin. Lens cap for scale.

B) En echelon series of faults (green;km-scale) in Paleozoic carbonate strata of Ohio derived from 3-D seismic mapping (Sagan and Hart, 2006).



geophysicist working the seismic data and vice versa. In other organizations, the interpreters are integrated geoscientists who are expected to work with both seismic and log data. There are advantages and disadvantages to both approaches. An interdisciplinary team that works closely together can generate an integrated interpretation that covers all aspects of a problem in greater depth than can be achieved by generalist geoscientists. On the other hand, a team can be dysfunctional if it consists of groups of specialists whose interests and knowledge do not allow them to effectively communicate and reach a common interpretation.

1.3 The Seismic InterpreterThe results of a seismic interpretation are infl uenced by many factors, including the seismic interpreters previous experience and the techniques used to undertake the interpretation (Rankey and Mitchell, 2003; Bond et al., 2007, 2008). Geologists and geophysicists both commonly undertake seismic interpretations and, given the differences between geological and geophysical training, it is not surprising that they tend to have different focuses, at least in the petroleum industry (Sternbach, 2002):

Geophysicists tend to be pay fi nders, i.e. people who focus on quantitative methods to directly detect hydrocarbon accumulations or other physical properties of interest.

Geologists tend to be play fi nders, i.e. people who integrate pattern recognition skills with an understanding of geologic models to prepare plausible models (e.g., structural reconstructions, depositional histories) that integrate different data types.

Sternbach (2002) described most interpreters as integrated geoscientists who combine training in geology and geophysics with computer skills and knowledge of engineering concepts. University programs typically focus on either geology or geophysics, and so integrated geoscientists only emerge after several years of industry cross-training. Hart (1997) discovered that most 3-D seismic interpreters learned to use computer systems and interpretation software through on the job training. Despite the abundance of generalists, some interpreters are specialists, perhaps in the structural interpretation of salt-tectonics features, amplitude-variation-with-offset, depth conversion, or some other topic.

Herron (2003) listed some of the technical and personal skills needed to survive and thrive as a seismic interpreter in the petroleum industry. Some of these included:

Natural curiosity about the Earth Enough mental fl exibility to handle multiple interpretive possibilities for projects Ability to visualize geology in 3-D without advanced interpretation software (i.e. uses

computer visualization to enhance, not replace, fundamental visualization skills) Highly developed pattern recognition skills Understanding that interpretation skills are complemented by knowledge of seismic

acquisition and processing methods Knowledge of the difference between a model-based interpretation and a model-

guided interpretation Knowledge of the difference between accuracy and precision




Understanding that there are two types of interpretations: those that have been revised, and those that need to be

The last point is particularly important. Given the imprecise and ambiguous data (seismic and otherwise) available to an interpreter, the most honest and ultimately most benefi cial approach to the interpretation problem involves:

Generating multiple working hypotheses that explain the available data and capture the range of possible interpretation alternatives. Inevitably, someone (a manager, an investor, The Public, the interpreter, etc.) will want the answer. Part of an interpreters diffi cult task is to balance that request for clarity with the need to convey the inherent ambiguity of the interpretation problem.

Developing, testing, modifying and excluding hypotheses as new data (seismic or otherwise) or theories become available.

Seldom does a single individual have all the necessary background and aptitude to be able to address all aspects of a complete seismic interpretation. There are many aspects of seismic interpretation that are amenable to quantitative data manipulation and interpretation (e.g., velocity analyses, or amplitude-variation with offset studies, described in CHAPTER 8). There are other aspects of seismic interpretation that must necessarily remain descriptive (e.g., reconstructions of structural or depositional histories, described in Chapters 6 and 7 respectively). Unfortunately, interpreters having good quantitative skills are not always good at making intuitive, descriptive interpretations, and vice versa. As such, complete seismic interpretations are often best undertaken by integrated teams (geologists and geophysicists) rather than by individuals. In this case, it is important that all members of the team (geophysicists, geologists, engineers, and potentially others) share a common basic vocabulary in order to ensure effective communication.

Herron (2009) summarized some of the technical, social, political and other problems faced by seismic interpreters in the petroleum industry.


FIGURE 1.18:A simplifi ed generic seismic interpretation workfl ow

Back to Chapter

Figure 1.18: A simplifi ed generic seismic interpretation workfl ow

See text for description. The quote (inset) refers to the diffi culty associated with simplifying complex workfl ows.

See text forThe quote the diffi cultwith simpliworkfl ows.



1.4 A Generalized Seismic Interpretation Workfl owAlthough different interpretation projects have different objectives, FIGURE 1.18 presents a generalized workfl ow that summarizes the main ingredients of a seismic interpretation. This fi gure attempts to capture the general interpretation workfl ow but unfortunately cannot include many of the important details1 and needs to be adapted on a project-by-project basis. More detailed descriptions of individual elements of this workfl ow are provided in other chapters.

The fi rst step in this workfl ow is to defi ne the purpose of the project and to decide whether seismic data can be helpful or not. That decision might be based on seismic modeling experiments, examination of previous reports/papers, or other considerations. Knowledge of rock properties and seismic wave propagation (CHAPTER 2) is necessary at this point.

The next task is to assemble and load different types of data into a common database. Seismic data may need to be collected (which means designing, acquiring and processing the data; CHAPTER 3), purchased, downloaded or otherwise obtained.

The next step is to make an initial tie between the seismic data and any other data types. In the petroleum industry, this typically means tying the seismic and well data, usually by generating synthetic seismograms but also potentially including checkshot data and vertical seismic profi les (CHAPTER 5).

A common approach is to follow the well tie by generating a structural framework that includes any faults, folds, or other structures (CHAPTER 6).

The structural framework is subsequently used to constrain the stratigraphic framework (CHAPTER 7), although usually there is typically feedback between structural and stratigraphic interpretations.

Physical properties prediction (CHAPTER 8) should begin only once the structural and stratigraphic frameworks have been established. Structural, stratigraphic or physical properties analyses may indicate a need to revise the ties between seismic and other data types.

If, following the interpretation, new data are collected (wells are drilled, new seismic lines are acquired, etc.), they should be loaded into the database and used to update the interpretation.

Ultimately some type of report, with associated maps, digital volumes, etc., will be prepared based on the work completed.

Geologic, geophysical and other types of knowledge are needed at all stages of the interpretation.

1.5 Online Sources of Data and InformationVarious sources of information and data are available online to seismic interpreters. In no particular order, these include various websites:


__________________________________________________________________

1 In the words of the French philosopher Paul Valry, All that is simple is false, all that is complex is useless. Thisquote will be included on all major fl ow diagrams in this text as a caveat about the problems of simplifying complex workfl ows.



The Virtual Seismic Atlas is an online repository that shows many examples of seismic images and their interpretations. Website: http://www.seismicatlas.org/

The Marine Seismic Data Center makes seismic refl ection and refraction data available from seismic cruises worldwide. Website: http://web.ig.utexas.edu/sdc/

Reports and other publications of the Integrated Ocean Drilling Program (IODP), a successor to the Ocean Drilling Program (ODP), present many excellent images of seismic data and show how those data can be integrated with other data types. Seismic data collected during site surveys are available for download for some expeditions. Website: http://www.iodp.org/index.php.

The Consortium for Continental Refl ection Profi ling (COCORP) was the fi rst group to use seismic refl ection profi ling to study the continental lithosphere. Information about the groups activities, data availability and links to other similar programs in other countries are available through its website:http://www.geo.cornell.edu/geology/cocorp/COCORP.html

Wayne Pennington maintains a list of publicly available seismic datasets. Website: http://www.geo.mtu.edu/spot/SeismicData/

The Woods Hole Seismic Center (United States Geological Survey) has online descriptions of various acoustic-based methods for sea-fl oor mapping. Website: http://woodshole.er.usgs.gov/operations/sfmapping/index.htm

The Center for Wave Phenomena at the Colorado School of Mines, distributors of Seismic Un*x, an open-source seismic utilities package (primarily processing). Website: http://timna.mines.edu/cwpcodes/

Various types of seismic data viewers are available as freeware. Data are generally viewable when in SEG-Y format. Several web pages provide links, including: http://gsegyview.sourceforge.net/index.php?option=com_weblinks&catid=13&Itemid=30

OpendTect is a free, open-source seismic interpretation system for 2-D and 3-D seismic data. Free datasets are available through the sites Open Seismic Repository link. Website: www.opendtect.org

The Schlumberger Oilfi eld Glossary has defi nitions of many terms used in seismic exploration and the petroleum industry in general:http://www.glossary.oilfi eld.slb.com/

Several professional societies have online material, publications, or meetings that should be of relevance to seismic interpreters. A non-exhaustive list includes:

The American Association of Petroleum Geologists (AAPG) is a professional geological society that works to advance the science of geology, especially in regard to exploration for and production of petroleum. Publications of the AAPG contain many excellent examples of seismic data and interpretations, particularly from the perspective of geologic interpretations. Articles from the Geophysical Corner of the AAPG Explorer discuss many topics of interest to seismic interpreters and are freely available online. Website: www.aapg.org

The Society for Exploration Geophysics (SEG) promotes the science of applied




geophysics and the education of geophysicists, with most of its members being from the petroleum industry. The society has a search engine (Digital Cumulative Index) that enables users to search publications of the SEG and other geophysical societies. Website: www.seg.org

The Canadian Society of Exploration Geophysicists (CSEG) serves the professional needs of geophysicists in Canada (and abroad), with a primary focus on advancing the science of geophysics, especially as it applies to exploration. The CSEGs monthly publication, The Recorder, contains many articles of interest to seismic interpreters and is freely available online. Website: www.cseg.ca

The European Association of Geoscientists and Engineers (EAGE) is a professional society whose mission is to promote the development and application of geosciences and related engineering subjects. Many of the societys activities and publications are of relevance to seismic interpreters. Website: www.eage.org

Many publications of the Geological Society of London deal with issues of relevance to seismic interpreters. Website: http://www.geolsoc.org.uk/index.html

The Environmental and Engineering Geophysical Society (EEGS) focuses on the application and use of near-surface geophysical technologies (including seismic methods) for engineering and environmental applications. The EEGSs monthly publication, fastTIMES, contains many articles of interest to seismic interpreters and is freely available online. Website: www.eegs.org

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Chapter 1: Introduction1.1 Introduction: Purpose of this Text 1.2 What is a Seismic Interpretation?The Geologic Interpretation ProblemDefining the Purpose of the Interpretation

1.3 The Seismic Interpreter1.4 A Generalized Seismic Interpretation Workflow1.5 Online Sources of Data and Information1.6 References

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