Atlas of Structural Geological Interpretation from Seismic Images (eBook)

Achyuta Ayan Misra, Soumyajit Mukherjee (Herausgeber)

eBook Download: EPUB

2018
John Wiley & Sons (Verlag)
978-1-119-15835-6 (ISBN)

Infos

This comprehensive book deals primarily with reflection seismic data in the hydrocarbon industry. It brings together seismic examples from North and South America, Africa, Europe, Asia and Australia and features contributions from eleven international authors who are experts in their field. It provides structural geological examples with full-color illustrations and explanations so that students and industry professionals can get a better understanding of what they are being taught. It also shows seismic images in black and white print and covers compression related structures.

Representing a compilation of examples for different types of geological structures, Atlas of Structural Geological Interpretation from Seismic Images is a quick guide to finding analogous structures. It provides extensive coverage of seismic expression of different geological structures, faults, folds, mobile substrates (shale and salt), tectonic and regional structures, and common pitfalls in interpretation. The book also includes an un-interpreted seismic section for every interpreted section so that readers can feel free to draw their own conclusion as per their conceptualization.

• Provides authoritative source of methodologies for seismic interpretation
• Indicates sources of uncertainty and give alternative interpretations
• Directly benefits those working in petroleum industries
• Includes case studies from a variety of tectonic regimes

Atlas of Structural Geological Interpretation from Seismic Images is primarily designed for graduate students in Earth Sciences, researchers, and new entrants in industry who are interested in seismic interpretation.

ACHYUTA AYAN MISRA, Exploration, Reliance Industries Ltd., Reliance Corporate Park, Thane Belapur Road, Ghansoli, Navi Mumbai, India.

SOUMYAJIT MUKHERJEE, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India.

Textauszug

1
Introduction to Seismic Data

Achyuta Ayan Misra* and Ashok Yadav

Reliance Industries Ltd, Mumbai, India

* [e-mail]; [e-mail]

1.1 Seismic Reflection Method

Whenever a reflection seismic section is mentioned, something similar to Figure 1.1 comes to mind. The process leading to the generation of such a section is briefly discussed in this chapter. There is a large volume of literature detailing all the processes and their variations (e.g., Sheriff and Geldart, 1995; Yilmaz, 2001; Liner, 2004; Ashcroft, 2011; Herron and Latimer, 2011; Onajite, 2014). Only a brief account is given here to build the platform for the following chapters.

Figure 1.1 A seismic section showing reflections from sedimentary boundaries.

Seismic data courtesy Reliance Industries Ltd. Reproduced with permission from the Directorate General of Hydrocarbons (DGH), India.

Seismic waves propagate through the Earth at velocities that depend on the acoustic impedance and density of the medium through which they travel. The acoustic impedance, Z, is expressed by (Liner, 2004):

(1.1)

where V is the seismic wave velocity and ρ is the rock density. If the rock varies in density in several directions, one can work with the “effective density” deduced in Mukherjee (2017, 2018, in press).

When a seismic wave propagating through the Earth encounters a boundary between two materials of different acoustic impedances, a part of the energy reflects off the interface while the remainder refracts through it. Seismic reflection prospecting involves generating seismic waves at the surface, which propagate into the subsurface, and capture the reflected wavefronts from the different interfaces while propagating. At each layer most of the energy is transmitted or refracted and a part reflects back (Sheriff and Geldart, 1995; Yilmaz, 2001; Liner, 2004; Ashcroft, 2011; Herron and Latimer, 2011; Onajite, 2014).

To generate the disturbance, a ‘shot’ or a vibration is made on the sea surface or on Earth’s surface in onland. As the wave propagates into subsurface, each layer reflects the wave at multiple incidence angles and these reflected waves are measured at the surface by receivers, which are hydrophones on water and geophones on land (Figure 1.2). The distance between the source and the receiver is termed the ‘offset’. The data from receivers near the source are called ‘near offset’ and those far away as ‘far offset’. The near receivers receive the reflected signal quicker than those further away from the source, so the response of the same boundary will appear progressively later (Figure 1.3).

Figure 1.2 (a) Schematic diagram showing layout of transmitted and reflected energy from the shot point and to the individual receivers. At every boundary, a part of the energy is transmitted and another part is reflected. The latter reach the receivers at the surface. Note there are two reflectors, numbered (1) and (2). Rn: receiver number; (b) shows the corresponding simplified seismic wriggle traces.

Figure 1.3 Schematic seismic response of reflector (1) on the receivers as shown in Figure 1.1.

There are two types of seismic waves: (i) P‐waves (longitudinal /compressional /body waves), where the particle motion is parallel to the direction of wave propagation, and (ii) S‐waves (shear /transverse waves), where particles move perpendicular to the wave propagation direction. P‐waves convert into S‐waves and vice versa when they transmit or reflect across a boundary, where there is a phase change i.e. solid to liquid/gas or liquid/gas to solid. Pore spaces have liquid/gas and thus this conversion is very common. Both P and S waves follow Snell’s law of reflection and refraction (Yilmaz, 2001). The angle of incidence equals the angle of reflection; the incident ray, the reflected ray, and the normal to the plane of incidence are co‐planar (Figure 1.4). The refracted seismic waves also follow Snell’s law, which states:

(1.2)

where θ1 is the angle of incidence, VPR the velocity of reflected P‐wave, θ2 the angle of transmitted P‐wave, VPT the velocity of transmitted P‐wave, ϕ1 the angle of the reflected S‐wave, VSR the velocity of the reflected S‐ (converted from P‐) wave, ϕ2 the angle of the transmitted S‐wave and VST the velocity of the transmitted S‐wave.

Figure 1.4 Schematic diagram showing mode conversion of incident P wave in P‐ and S‐waves at a boundary of two lithologies with different velocities V1 and V2. The refracted (transmitted) waves follow Snell’s law. SR: Reflected mode converted S‐wave; PR: reflected P‐wave; ST: Transmitted mode converted S‐wave; PT: transmitted P‐wave.

1.2 Seismic Data Acquisition

Earth’s interior can be imaged by reflection seismic data much like remote sensing satellites image the Earth surface. Rock layers, for example sands and shales, differ in density. The acoustic impedance (Equation 1.1) to seismic wave velocity passing through the layers thus differs and it is reflections of the wave that are imaged. These layers appear as reflections in a seismic record and are interpreted to reconstruct the geological architecture. This reconstruction is challenging and takes time. To understand the data properly, the data acquiring process has to to be understood.

Seismic data acquisition parameters are guided as per requirements. For example, if a large NE trending anticline of 100 km2 area at 3000 m below sea level is to be imaged, the spacing between individual lines in a 2D seismic survey should be 10–20 km. On the other hand, when a 50–100 m long reservoir fault demands mapping, a high resolution 3D volume will be acquired (see Section 1.3.4 for a discussion on resolution). The trend of the structure will determine the orientation (azimuth) of the receiver lines, for example the NE trending structure will need NW oriented receiver lines. Again, for crustal architecture studies (Misra et al., 2015, 2016) a larger record length (Box 1.1), a high‐energy source, a larger offset etc. yield a good image.

Box 1.1 Important terms in seismic acquisition and processing.

1. Acoustic impedance (AI): a product of density and velocity. The greater the AI, the stronger the reflection. Acoustic impedance, Z = Vρ.
2. Amplitude: reflection strength.
3. Fold: not the fold in structural geology! For the one in structural geology, see Chapter 4. The geophysical fold is the numbers of traces in a CMP gather, so if data has 10 traces, the “foldage” of the data is 10 and when it has 60 traces, the foldage is 60. The more the foldage of the data, the clearer will the imaging, due to a higher signal‐to‐noise ratio.
4. Migration: restoring a dipping reflector to its correct subsurface position.
5. Minimum phase wavelet: front loaded energy, that is at time zero minimum energy and elsewhere maximum.
6. Offset: Distance between the source and the receiver.
7. Receiver: geophone (on land) or hydrophone (offshore); devices to measure ground movement or sound from the shot.
8. Record length: predetermined time after which the receiver measures the reflected seismic waves. A larger record length indicates greater depth of imaging.
9. Reflection coefficient: type and size of acoustic impedance change.
10. Reflections: acoustic waves reflected from an interface of contrasting lithologies. If the boundaries are gradational, the reflections may be chaotic or comprise a number of reflections, depending on the frequency of the data.
11. SEGY: or SEG‐Y, is a popular format for storing seismic data. Controlled by the Society of Exploration Geophysicists. Associated file extensions: .segy and .sgy.
12. Seismic reflector: boundary across which the competence changes. Also called acoustic‐impedance boundary.
13. Shot: initial disturbance/sounding/explosion; generated by explosives or seismic vibrators on land and by pressurized‐air guns offshore.
14. Shot point: geographic location of the shot, measured precisely by positioning systems.
15. Source pulse or wavelet: Resulting sound wave from the shot. This wavelet is commonly used for zero‐phasing the seismic data (see “Zero‐phase wavelet”).
16. Streamer feathering: only in marine acquisition; the deviation of the streamer array away from the linear towing direction. This may happen due to water currents or change in towing direction.
17. Streamer length: length of the cable on which the hydrophones are attached. The length of the streamers depends on the depth of the objective. Typically, the streamer lengths equal the target depth.
18. Trace: stream of reflections recorded by geophone.
19. Two‐way time:...

Produkt-Details

• Erscheint lt. Verlag: 26.2.2018
• Sprache: englisch
• Themenwelt: Sachbuch/Ratgeber ► Natur / Technik ► Natur / Ökologie
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Geologie
• Themenwelt: Naturwissenschaften ► Geowissenschaften ► Geophysik
• Themenwelt: Naturwissenschaften ► Physik / Astronomie
• Schlagworte: Angewandte u. ökonomische Geologie • Atlas of Structural Geological Interpretation from Seismic Images • atlas to seismic data • atlas to seismic imaging • earth sciences • Economic & Applied Geology • fault planes • Geological Concepts • Geology • geophysical concepts • Geophysics • Geophysik • geoscience • Geowissenschaften • guide to seismology • Physics • seismic faults • Seismic Imaging • seismic imaging guide • Seismology • Structural Geology • Structural Geology & Tectonics • Strukturgeologie, Tektonik • study of seismic activity • Tectonics • tectonic study
• ISBN-10: 1-119-15835-4 / 1119158354
• ISBN-13: 978-1-119-15835-6 / 9781119158356