Seismic stratigraphy

GROUP MEMBERS

FARHAD JAFFAR CHISHTI MUHAMMAD AZHAR MUSHTAQ USMAN ALI SHUJAH NASIR WAJID MANZOOR NASEER AHMAD Muhammad HUSSAIN

Types of STRATIGRAPHY Lithostratigraphy Biostratigraphy Seismic stratigraphy chronostratigraphy Sequence stratigraphy Magnetostratigraphy

Seismic stratigraphy

1. Introduction2. Seismology3. Seismic methods4. Parameters used5. Depositional environment

Introduction

An English seismologist, Robert Mallet. Was the first scientist to measure the velocity of seismic waves. He initiated experimental seismology in 1848.

Seismic stratigraphy is a new science born in early 1960’s.

Seismic Stratigraphy

It is the combination of 2 words1. seismic2.stratigraphy

1.Seismic Seismology:

Study of earthquakes and the structure of earth on the basis of characteristics of seismic waves.

Seismic waves:These are waves of energy caused by sudden breaking of rocks with in the earth. Seismic waves have two main types:

1. body waves2. surface waves

BODY WAVES

1. p-waves2. S-waves

P- Waves These waves travel in

all medium like solids, gases and liquids.

They travel at 1.7 times the speed of s-waves.

rock vibrate parallel to the wave propagation.

S-Waves Rock vibrate

perpendicular to the wave propagation.

They can not pass through the liquid surfaces.

It’s velocity ranges from 3.5 to 7.5 km/sec.

Surface waves

It has two types1. Love waves2. Rayleigh waves

Love waves The first kind of surface waves in love

waves. Which is named after A.E.H Love a British

mathematician. It is the fastest surface wave and moves the

ground from side to side. It moves the surface forward and

backward,left and right at the same time.

Rayleigh waves The second type of surface waves is

Rayleigh wave. Which is Named after J.W.S.Lord Rayleigh, in

1885. These waves are combination of P , S-waves. It moves the ground up and down,side to

side at the same time in the direction that the wave is moving.

StratigraphyStratigraphy is a branch of

geology which studies rock layers (strata) and layering (stratification).

Seismic StratigraphyThe study of seismic data for the

purpose of extracting stratigraphic information is called seismic stratigraphy.

It is widely used on land and at sea because of it’s applicability to subsurface.

Seismic MethodsThere are two methods:

1.refraction seismic method2.reflection seismic method

Refraction : when a wave enters from rare medium (air) to denser medium (glass) then it bends towards normal. This phenomenon is called refraction.

Refraction Seismic Method Refraction seismic method for determine the

structure of sub-surface formations.

Principle: Artificially generated seismic waves are refracted or bent at discontinuity surfaces as they travel downward below the surface.

Process When waves meet a discontinuity at critical

angle( angle of Incidence for which angle of reflection is 90*), some of the waves are refracted or bent.

The refracted waves will travel along the top of the layer underlying the discontinuity surface with greater velocity then that at which they passed downward through the overlying layer because velocity of seismic waves increases with depth.

Reflection

When a ray of light passes from a rare medium ( air) to a denser medium (glass) a part of it is reflected from the surface back into the air. This phenomenon is called reflection.

Reflection Seismic Method Refraction method did not work well for deeper structures because of

the excessive distances required between short points (source of seismic waves) and detectors (geo phones)

In reflection method, waves created by an explusion or reflected back to the surface directly from sub-surface rock without being refracted and travelling along discontinuity surfaces.

Reflection method are relatively more used than refractive techniques because detectors can be located at relatively short distances from short points.

Refracted waves are also generated but due to closer location of detectors, are not picked up.

principle The seismic waves travel at known velocities

through rock materials, velocities vary with the type of rocks.

Where the sub-surface lithology is known relatively well from the drill whole information, it is possible to make accurate calculations of time required for a seismic signal to travel from the surface to a given depth and then be reflected back to the surface.

Application of reflection seismic method The science of seismic stratigraphy was developed

largely by petroleum companies out of pragmatic (practicable) necessity to locate petroleum deposit in deep.

It is also used to study deep structures and features like folds, faults etc…

Seismic reflection are used to identify and map the structural attitudes sub-surface sedimentary layers.

This method is also used to find the depth of ocean.

Parameters used in seismic stratigraphic interpretation

Seismic reflections are critical to entire concept of seismic stratigraphy because seismic reflections occur in response to the presence of density-velocity changes at discontinuities.

The important parameters of Seismic interpretation are:a) Reflection configurationb) Reflection continuityc) Reflection amplituded) Reflection frequencye) Interval velocity


a) Reflection configuration It refers to the large no. of stratification patterns identified on seismic

records.

Types of Reflection configuration are:1. Parallel patterns2. Divergent configuration3. Prograding reflection configuration4. Chaotic reflection configuration


1. Parallel patterns Parallel patterns, including sub-parallel and

wavy patterns are generated by strata that were deposited at uniform rates.


2. Divergent configuration They are characterized by a wedge shaped unit in which lateral thickening of

entire unit is caused by thickening of individual sub-units within the main unit.

they are studied to signify variation in rates of deposition or tilting of sedimentary surface during deposition.


3. Prograding reflection patterns They are reflection patterns generated by strata that were deposited

by lateral outbuildings. e.g clinoform, sigmoid(super-posed S-shaped reflectors), oblique or hummocky patterns etc.

4. Chaotic reflection pattern They are the disordered arrangements of reflection surfaces, showing

soft-sediments deformation.

Depositional environments There are three Depositional environments in relation to wave base:1. Undaform Flat surface above the wave base where sediments are moved by

waves.2. Clinoform The sloping surface extending from wave base down to the flat floor.3. Fondoform The flat surface In the depth of wave base. The rock units which are formed in each of these environments are

called UNDATHEM, CLINOTHEM and FONDOTHEM.


b) Reflection continuity It depends upon the continuity of the density-

velocity contrast along bedding surfaces or unconformities.

It is closely associated with continuity of strata. It provides information about depositional process

and environment.


c) Reflection amplitude it is measured as the distance from the mid-position of a wave to the extreme position.d) Reflection frequency No of vibrations of seismic wave per second.e) Interval velocity it is the average velocity of seismic waves between reflectors.

Education

Seismic stratigraphy