Atlas of Structural Geological Interpretation from Seismic Images

Achyuta Ayan Misra, Soumyajit Mukherjee, Achyuta Ayan Misra, Soumyajit Mukherjee

• Publisher: Wiley-Blackwell
• Year: 2018
• ISBN:
  9781119158356
• Edition: 1
• Topic: Naturwissenschaften
• Subtopic: Geophysik

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.

1
Introduction to Seismic Data

Achyuta Ayan Misra* and Ashok Yadav

Reliance Industries Ltd, Mumbai, India

* [email protected]; [email protected]

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). R_n: 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, V_PR the velocity of reflected P‐wave, θ₂ the angle of transmitted P‐wave, V_PT the velocity of transmitted P‐wave, ϕ₁ the angle of the reflected S‐wave, V_SR the velocity of the reflected S‐ (converted from P‐) wave, ϕ₂ the angle of the transmitted S‐wave and V_ST 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. S_R: Reflected mode converted S‐wave; P_R: reflected P‐wave; S_T: Transmitted mode converted S‐wave; P_T: transmitted P‐wave.

1.2 Seismic Data Acquisition

Earth’s interior can be imaged by reflection seismic data much lik...