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Al-Azhar University Assiut Branch Faculty of Science Geology Department

Research on

The Geological Interpretation of Reflection Seismic Data

Prepared by

1. Abdelrhman Mohammed Abdelftah

2. Ahmed Abdelhamed Ali

3. Ahmed Mahmoud Abdelsalam

Essay Submitted for Partial Fulfillment of

Requirements for B.Sc in Geology

Under supervisor

Dr. AbdelSattar A. Abdellatief

Lecturer of Applied Geophysics

2008/2009

ii

Acknowledgements

Firstly thanks to "ALLAH" who give us the health and life to

finish up this work.

We’re grateful to the people who helped with this Research,

and to the students and colleagues who have, whether

advertently or inadvertently, introduced us to many new

learning experiences. We’re particularly grateful to

Dr. AbdelSattar for his help.

Amid such a mass of small letters, it will not seem surprising

that an occasional error of the press should have occurred. But

I hope that the number of such errors is small. And special

thanks to my parents and my uncle Rabih Elabasere.

(Abdelrhman)

Contents Acknowledgements ii List Of Figures v List Of Tables and Boxes vii Abstract viii

Chapter 1 Introduction 1 1.1 Introduction 1 1.2 Milestones in Seismic Industry 1 1.3 Principle of Seismic Survey 2

1.4 Modern Seismic Data Acquisition 3 1.4.1 Land Data Acquisition 3

1.4.2 Marine Data Acquisition 3 1.4.3 Transition – Zone Recording 4

1.5 How Are The Seismic Data Collected? 4 1.6 What Is The Interpretation Mean? 5

Chapter 2 Fundamentals Of Seismic 7 2.1 Introduction 7 2.2 The Seismic Wave 7 2.3 Types Of Waves 7

2.3.1 Compressional Waves (P-waves) 7 2.3.2 Shear Waves (S-waves) 8

2.4 Characteristics of Seismic 8 2.4.1 Reflections 8

2.4.2 Critical Reflection 8 2.4.3 Refractions 8 2.4.4 Diffractions 9

2.4.5 Multiples 9 2.4.6 Seismic Noise 9

2.5 Seismic velocities 10 2.6 Seismic Receivers 12

2.6.1 Geophones 12 2.6.2 Hydrophones 13 2.6.3 Dual Sensors 13

2.7 Seismic Data Processing 13 2.7.1 Migration 14

Contents 2.8 Seismic Sections 14

Chapter 3 Steps Of Interpretation Seismic Data 17 3.1 Introduction 17 3.2 Identification Of Reflections (Tracing) 18 3.3 Picking And Correlation Of Reflections 18

3.3.1 Correlations 19 3.4 Continuity 20 3.5 Unconformities and Seismic Facies Patterns 21 3.6 Naming 22 3.7 Fault Pattern Determination 22 3.8 Hydrocarbon Indicators 22 3.9 Mapping 24

3.9.1 Construction Of Two-Way Time Map 24 3.9.2 Construction Of Structural Cross-Sections 24 3.9.3 Contour Maps 24 3.9.3.1 Construction Of Geo-Seismic Structural

Contour Map 24

3.9.3.2 Construction Of Isopach Maps 24 Chapter 4 Reflection Data Over Geologic Structures 25

4.1 Introduction 25 4.2 Anticlines 26 4.3 Faults 27 4.4 Diapirism And Salt Domes 29 4.5 Basement Structure 31 4.6 Pitfalls In Structural Interpretation 32

4.6.1 Velocity Pitfalls 33 4.6.2 Geometrical Pitfalls 35

Chapter 5 Example Study 36 Evaluation Of Matruh Basin 36

5.1 Geological Background 37 5.2 Basin Analysis 38 5.3 Seismic Data 43 5.4 Structural Interpretation 47

5.4.1 Fault Pattern Interpretation 47 5.5 Summary And Conclusions 50

Bibliography 52

v

List of Figures Fig.

1.1 Sketch of rays reflected from a bed to receiver 5

2.1 Histogram of seismic wave velocities of various classes of rocks (after Grant and West, 1965)

12

2.2 Terminology of wiggles 16

2.3 Vertical trace 16

3.1 Parallel other reflection 21

4.1 Most type of reservoirs and traps 26

4.2 (a) Anticline from San Joaquin Valley, Calif.: (a) section as automatically migrated by computer;

26

4.2 (b) (b) immigrated section. The migration has collapsed the many diffraction patterns that concealed the actual structure. (Geocom,Inc.)

27

4.3 Pattern of faulting 28

4.4 Example of Salt Dome. 30

4.5 Basement effect in a deep-sea area 31

4.6 Identification of basement surface from diffraction patterns on section along traverse In deep water. (United Geophysical Corp, proprietary data.)

32

4.7 Distortion of sedimentary layers due to forces associated with salt-dome buoyancy. Some of the structures shown, e.g., those below the piercement-type salt dome and salt pillows (like the deep one on the right) are not real but result from velocity effects. (Exxon, Inc.)

34

4.8 "Anticline" cased by thrusting of high-velocity material over monoclinal layers. Markings on lower section indicate interpreted structure. (From Tucker and Yorston.8

34

)

4.9 Bow-tie effect observed over sharp syncline in the Adriatic Sea. Apparent anticline is actually a diffraction feature.(Geocom, Inc.)

35

vi

List of Figures

5.1 Location map of the study area 36

5.2 Basment tectonic map of the north Western Desert of Egypt (Modified after Sultan and Halim, 1988).

37

5.3 The subbasins constituting the northren Western Desert Basin (Modified after Sultan and Halim, 1988).

37

5.4 Isopach map of Bahariya Formation. Matruh Basin, North Western Desert, Egypt (C. I.=50 ft)

40

5.5 Isopach map of Kharita Formation. Matruh Basin, North Western Desert, Egypt (C. I.=200 ft)

40

5.6 Structure contour map of Bahariya Formation. Matruh Basin, North Western Desert, Egypt. Arrows refer to possible hydrocarbon migration pathway (C. I.=200 ft)

41

5.7 Structure contour map of Kharita Formation. Matruh Basin, North Western Desert, Egypt. Arrows refer to possible hydrocarbon migration pathway (C. I.=200 ft)

41

5.8 Sand to shale ratio map of Bahariya Formation. Matruh Basin, North Western Desert, Egypt.

42

5.9 Sand to shale ratio map of Kharita Formation. Matruh Basin, North Western Desert, Egypt.

42

5.10 Location the seismic lines 43

5.11 Line "q" in the original 3D seismic volume (before applying any intrpretation steps).

44

5.12 Line "f" in the original 3D seismic volume (before applying any intrpretation steps).

45

5.13 Line "k" in the original 3D seismic volume (before applying any intrpretation steps).

46

5.14 Interpreted line number "f" 48

5.15 Interpreted line number"k" 49

5.16 Interpreted part of line number"q" 50

vii

List of Tables Table. Page

2.1 Average values approximating measurements on polycrystalline bodies

10

2.2 velocities in non-porous sedimentary rocks 11

2.3 Velocity in Porous Rock filled by fluids 11

5.1 Stratigraphic column of matruh basin, Egypt (Modified after Medoil, 1983).

38

5.2 Lithologic constituents of Bahariya and Kharita formations. Matruh Basin, North Western Desert, Egypt.

39

List of Boxes Box. 2.1 Why Processing? 13

Box. 3.1 Tips for Correlation 19

viii

Abstract

This research consists of all these aspects in a brief. Starting from the

introduction to the seismic methods in Chapter 1, in Chapter 2, I tried to

give a brief introduction about the basics of the different types of

waves, and fundamental of seismic data and it's definition like multiples,

processing, etc. The next Chapter 3 deals with the steps of geological

interpretation to seismic data. Chapter 4 is about the structure

interpretation especially the common traps and reservoirs. Where

I described pitfalls in structure interpretation. In Chapter 5 i give example

study of the interpretation technique from Ph.D explained basic data and

aid data used to complete the interpretation.

The geophysical method that provides the most detailed picture of

subsurface geology is the seismic survey. This involves the natural or artificial

generation and propagation of seismic (elastic) waves down into Earth until

they encounter a discontinuity (any interruption in sedimentation) and are

reflected back to the surface. On-land, seismic “shooting” produces acoustic

waves at or near the surface by energy sources such as dynamite, a “Thumper”

(a weight dropped on ground surface), a “Dinoseis” (a gas gun), or a

“Vibroseis” (which literally vibrates the earth’s surface). Electronic detectors

called geophones then pick up the reflected acoustic waves. The signal from the

detector is then amplified, filtered to remove excess “noise”, digitized, and then

transmitted to a nearby truck to be recorded on magnetic tape or disk. In the

early days of offshore exploration, explosive charges suspended from floats

were used to generate the necessary sound waves. This method is now banned

in many parts of the world because of environmental considerations. One of the

most common ways to generate acoustic waves today is an air gun. Air guns

contain chambers of compressed gas. When the gas is released under water, it

ix

makes a loud “pop” and the seismic waves travel through the rock layers until

they are reflected back to the surface where they are picked up by hydrophones,

the marine version of geophones, which trail behind the boat. The data recorded

on magnetic tape or disk can be displayed in a number of forms for

interpretation and research purposes; including visual display forms

(photographic and dry paper), a display of the amplitude of arriving seismic

waves versus their arrival time, and a common type of display called variable

density. The variable-density display is generated by a technique in which light

intensity is varied to enhance the different wave amplitudes. For example, low

amplitude waves are unshaded and higher amplitude waves are shaded black,

thus strong reflections will show up as a black line on the display. Seismic

waves travel at known but varying velocities depending upon the kinds of rocks

through which they pass and their depth below Earth’s surface. The speed of

sound waves through the earth’s crust varies directly with density and inversely

with porosity. Through soil, the pulses travel as slowly as 1,000 feet per second,

which is comparable to the speed of sound through air at sea level. On the other

hand, some metamorphic rocks transmit seismic waves at 20,000 feet

(approximately 6 km) per second, or slightly less than 4 miles per second. Some

typical average velocities are: shale = 3.6 km/s; sandstone = 4.2 km/s; limestone

= 5.0 km/s. If the subsurface lithology is relatively well known from drilling

information, it is possible to calculate the amount of time it takes a wave to

travel down through the earth to a discontinuity and back to the surface. This

information is used to compute the depth of the discontinuity or unconformity.

However, the only way of accurately determining depth is by correlating

seismic sections to wireline logs. Reflections are generated at unconformities

because unconformities separate rocks having different structural attitudes or

physical properties, particularly different lithologies. These principles form the

basis for application of seismic methods to geologic study.

1

Chapter 1 Introduction

1.1 Introduction

The seismic method is one of the geophysical methods of prospecting It has three important/principal applications: a. Delineation of near-surface geology for engineering studies, and coal

and mineral exploration within a depth of up to 1km: the seismic method applied to the near surface studies is known as engineering seismology.

b. Hydrocarbon exploration and development within a depth of up to 10 km: seismic method applied to the exploration and development of oil and gas fields is known as exploration seismology.

c. Investigation of the earth’s crustal structure within a depth of up to 100 km: the seismic method applies to the crustal and an earth quake study is known as earthquake seismology.

Definition by Robert E. Sheriff: Seismic survey is a program for mapping geologic structure by observation of seismic waves, especially by creating seismic waves with artificial sources and observing the arrival time of the waves reflected from acoustic impedance contrasts or refracted through high velocity members. 1.2 Milestones in Seismic Industry

As the search for oil moved to deeper targets, the technique of using reflected seismic waves, known as the “seismic reflection method”, became more popular during World War II, because it aided delineation of other structural features apart from simple salt domes.

During 1960’s the so-called digital revolution ushered in what some historians now are calling the Information Age. This had a tremendous impact on the seismic exploration industry. The ability to record digitized seismic data on magnetic tape, then process that data in a computer, not only greatly improved the productivity of seismic crews

2

but also greatly improved the fidelity with which the processed data imaged earth structure. Modern Seismic Data Acquisition could not have evolved without the digital computer. The late 1970’s saw the development of the 3D seismic survey, in which the data imaged not just a vertical cross-section of earth but an entire volume of earth. The technology improved during the 1980’s, leading to more accurate and realistic imaging of earth. In 1990’s depth section preparation got focused from the prevailing time section preparation after processing the data. In 2000’s data is being acquired with an additional parameter of “time” as the 4th

This is called an event. By measuring precisely the difference in arrival time of a given event from the nearer and further receivers groups, the velocity of the rock material can be measured. The seismic measurements are made in time, so if the velocity and time are known, geophysicists can work out the depth of the event. In seismic surveys,

dimension of the existing 3D data acquisition system. This is called 4D data acquisition.

As the seismic industry made one breakthrough after another during its history, it also created new challenges for itself. Now we record not just p-waves but also converted s-waves for a wide range of objectives. Using the multi-component seismic method, commonly known as the 4-C seismic method, we are now able to see through gas plumes caused by the reservoir below. We are able to sometimes better image the sub-salt and sub-basalt targets with the 4C seismic method. Using the converted s-waves, we are able to detect the oil-water contact, and the top or base of the reservoir unit that we sometimes could not delineate using only p-waves.

1.3 Principle of Seismic Survey

Seismic wave are used to give a picture of deep rock structures. The seismic wave travels through the water and strikes the seafloor. Some of the energy of the wave is reflected back to the receivers. The rest of the wave carries on until it reaches another rock layer. The time taken for the waves to travel from the source to the receivers is used to calculate the distance traveled - hence the thickness of the rock layers. The strength of the reflected wave gives information about the density of the reflecting rock. Each time the seismic pulse meets a change in rock properties, for example from a shale to a sand layer, part of the pulse will be reflected back to the surface.

3

reflected sound waves, called signals, are combined and interpreted electronically or reproduced on graphic paper recorders. This data gives information on the depth, position and shape of underground geological formations that may contain crude oil or natural gas.

1.4 Modern Seismic Data Acquisition

Subsurface geologic structures containing hydrocarbons are found beneath either land or sea. So there is a land data-acquisition method and a marine data-acquisition method. The two methods have a common-goal, imaging the earth. But because the environments differ, so each required unique technology and terminology.

1.4.1 Land Data Acquisition In land acquisition, a shot is fired (i.e., energy is transmitted) and

reflections from the boundaries of various Lithological units within the subsurface are recorded at a number of fixed receiver stations on the surface. These geophone stations are usually in-line although the shot source may not be. When the source is in-line with the receivers at either end of the receiver line or positioned in the middle of the receiver line – a two-dimensional (2D) profile through the earth is generated. If the source moves around the receiver line causing reflections to be recorded form points out of the plane of the in line profile, then a three-dimensional (3D) image is possible (the third dimension being distance, orthogonal to the in-line receiver-line).

The majority of land survey effort is expended in moving the line equipment along and / or across farm fields or through populated communities. Hence, land operations often are conducted only during daylight thus making it a slow process. 1.4.2 Marine Data Acquisition

In a marine operation, a ship tows one or more energy sources fastened parallel with one or more towed seismic receiver lines. In this case, the receiver lines take the form of cable called Steamer containing a number of hydrophones. The vessel moves along and fires a shot, with reflections recorded by the streamers. If a single streamer and a single source are used, a single seismic profile may be recorded in like manner to the land operation. If a number of parallel sources and/or streamers are towed at the same time, the result is a number of parallel lines recorded at

4

the same time. If many closely spaced parallel lines are recorded, a 3D data volume is recorded. More than one vessel may be employed to acquire data on 24-hour basis, since there is no need to curtail operations in nights. 1.4.3 Transition – Zone Recording

Because ships are limited by the water depth in which they safely can conduct operations, and because land operations must terminate when the source approaches the water edge, or shore lines, transition-zone recording techniques have been developed to provide a continuous seismic coverage required over the land and then into the sea. Geophones that can be placed on the sea bed or used with both marine and land shots fired into them. Techniques have been developed to use both Geophones and hydrophones in the surface area where the shore line / water edge is likely to migrate towards land and sea depending on the tide of sea a day. The combination of such hydrophone / geophones is called a “Dual Sensor”. The advantage of why this is to see that either of the receiver of Dual Sensor pickups the surveyed from the slots recorded using a land or marine source and data gaps all along the coast within the area of prospect. 1.5 How Are The Seismic Data Collected?

As the vessel moves along the line, computers control the simultaneous discharge of seismic waves from the sound sources, usually every 10 seconds. The waves travel down through the rock formations. When they encounter a boundary between different formations, some sound waves are reflected back to high-capacity computers, check and store the data collected. The collected data go through several processing steps to improve the quality of the signals and filter out background noise. Geophysicists then interpret the information to develop a detailed picture of the structures and rock formations.

5

1.6 What Is The Interpretation Mean? The word interpretation has been given many different meanings by geophysicists who handle seismic reflection records and by geologist's who put the information from them to use. To some it is virtually equivalent to data processing and is tied inextricably to computer software. To others it consists of all of the operations we considered as the mechanical transformation of seismic reflection data into a structural picture by the application of corrections, time-depth conversion, and migration. Interpretation can be all of these things, subject to the one inviolable condition that it involves some exercise of judgment based on geological criteria. By this conception, interpretation can begin with planning and programming a seismic reflection survey if they are guided by the geology of the area and by the economic or scientific objectives of the survey. It can involve the choice of field parameters, such as the kind of seismic source to be used, the geometry of source and receiver patterns, and the settings on the panels of the recording instruments, as long as such choices are governed by the geological information desired. The selection of processing procedures and parameters is also an important part of the Interpretation if it is supported by the same considerations.

Fig. 1.1: Sketch of rays reflected from a bed to receiver

6

Any purely mechanical operations not requiring discretion on the part of the geophysicist would come under the category of reduction, not interpretation. It is possible to make a seismic map, particularly one in time, without carrying out any real interpretation at all if every stage of its preparation is routine or automatic and no decisions have to be made that involve geological considerations. After a seismic map is constructed, an important part of its interpretation is integrating the seismic data on it with geological information from surface and subsurface sources, e.g., fault traces or geologic contacts. This involves identifying reflections and making ties to wells or surface features. The extent to which this can be done depends on the amount of geologic information available. The computer has made it feasible to use previously unexploited characteristics of seismograms to obtain geological information. Under favorable circumstances, interval velocities can be determined from reflection records with enough precision to permit them to serve as a basis for identifying lithology.

Another property of seismic waves that has been employed for studying rock composition is attenuation of seismic-wave amplitudes between successive reflectors, a parameter that can now be measured because of the high dynamic range in modern recording equipment. Thanks to the computer technology.

52

Bibliography

Bacon, M., Simm, R., & Redshaw, T. (2003). Geological interpretation.

3-D seismic interpretation. Cambridge, England: Cambridge University

Press.

Chapman, C. H. (2004). Fundamentals Of Seismic Wave Propagation.

Schlumberger Cambridge Research: Cambridge University Press.

Coffeen, J. A. (1986). Seismic Exploration Fundamentals. Tulsa, Oklahoma,

USA: PennWell Publishing Company.

Dobrin, M. B. (1960). Introduction to geophysical prospecting (3th ed.).

Singapore: McGraw- Hill Book Company.

Gamea, A.S. What do you know about seismic survey? (2009, February).

Petroleum Journal: Petroleum Ministry, Egypt.

Goulty, N. R. (1997). Lateral resolution of 2D seismic illustrated by a real data

example. First Break, 15, 77-80

McQuillin, R., Bacon, M., & Barcly, W. (1979). An Introduction to seismic

Interpretation. Graham & Trotman press

Othman, W.M. (2007). Evaluate Oil potentiality for Matruh Basin. Applied

Geophysics. Ph.D, Faculty of Science: Suez Canal University, Egypt.

Talagapu, K. K. (2004). 2D and 3D Land Seismic Data Acquisition and

Seismic Data Processing. Andhra Pradesh, India: Geophysics Andhra

University.

Taner, M . T., Koehler, F & Sheriff R. E. (1979). Complex seismic trace

analysis Geophysics, 44, 1041-63.

Telford, W. M., Sheriff, R. E. (1980).Reflection Interpretation. Applied

Geophysics. pp 260-264

1

الملخص العربي

التفسير الجيولوجي للبيانات السيزميه األنعكاسيه

هو الخطوه األولي في عمليات األستكشاف والتنقيب ”Seismic Survey“يعتبر المسح السيزمي عن البترول ويعتبر حجر الزاويه في تلك العمليات، وفكره المسح السيزمي هي رؤية ما تحت

" مصائد بتروليه، حيث انها تحتوي Oil Trapسطح األرض من تراكيب جيولوجيه تعرف بـ "علي الزيت أو الغاز.

تقوم فكره المسح السيزمي علي اربع خطوات :

: إنشاء موجه صوتيه قويه فوق سطح األرض بأستخدام الديناميت أو العربات الخطوه األولي" وهي األكثر شيوعا . Vibroseisالزلزاليه "

interface : استقبال تلك الموجات بعد ان تنعكس عند الحد الفاصل بين الطبقات الخطوه الثانيهحيث األختالف في الكثافه (كثافه الصخور) للحوض الترسيبي ويتم رصدها باستخدام اجهزة

حيث تكون متصله مع بعضها البعض وأيضا متصله مع عربه Receiversحساسه تسمي " . Recording Unitالتسجيل "

: تسجيل تلك الموجات المرتده الي السطح مره اخري باستخدام اجهزه التسجيل الخطوه الثالثه" ومن ثم Digital Processingوإرسالها الي مركز المعالجه إلجراء عمليات رقميه عليها "

يمكن تحميلها علي اجهزه الكمبيوتر المعده لذلك باستخدام برامج متخصصه .

: هي التفسير حيث يقوم الجيوفزيائي والجيولوجي بعمل تفسيرات سيزميه لتلك الخطوه األخيرهالمقاطع لتحديد المصائد البتروليه وعمل خرائط كنتوريه عليها لتحديد مكان حفر البئر.

يحتوي هذا البحث علي األتي بأختصار:

U الفصل األول

مقدمه تعريفيه بالمسح السيزمي وكيفيه جمع البيانات ومعني التفسير السيزمي.

2

U الفصل الثاني

محاوله لتقديم التعريفات والمفاهيم األساسيه المستخدمه في التفسير بصوره مبسطه مثل انواع الموجات والفرق بين الموجات األنعكاسيه موضوع البحث والموجات التشتتيه والتطرق الي

انواع المستقبالت.

U الفصل الثالث

تقديم خطوات للتفسير الجيولوجي للبيانات السيزميه األنعكاسيه مع اقصاء المعادالت الرياضيه المعقده والنظريات الفزيائيه المطوله والتركيز علي المفهوم الجيولوجي للتفسير وهدفه، حيث

ألخ. ..………تضمنت الخطوات المضاهاه ورسم الخرائط

U الفصل الرابع

دراسة األنعكاسات السيزميه فوق التركيب الجيولوجيه المختلفه المعهود تواجد البترول بها وكيفيه تفسير صوره الموجات فوق تلك التراكيب مع ذكر األخطاء التي يجب ان تتجنب اثناء تفسير تلك

التراكيب.

U الفصل الخامس

تقديم نموذج لدراسه كامله علي منطقه حوض مطروح كنموذج لتطبيق خطوات التفسير الجيولوجي علي البيانات السيزميه األنعكاسيه وتقييم األحتماالت البتروليه بها.

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سورة الزلزلة

جامعه األزهر كليه العلوم بأسيوط قسم الجيولوجيا

بحث في

التفسري اجليولوجي للبيانات السيزميه األنعكاسيه

إعداد الطالب:

أحمد عبدالحميد علي عبدالحافظ .1

أحمد محمود عبدالسالم حسن .2

عبدالرحمن محمد عبدالفتاح سليم .3

العلوم الجيولوجيه بكالورياألستكمال الحصول علي 2008/2009للعام

تحت إشراف

د/عبدالستار عبدالنعيم عبداللطيف محاضر الجيوفزياء