reflection seismic

7/28/2019 Reflection Seismic

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Seismic Reflection

Speakers:

-Atok Yuliantono -Intan Dewi Meutia SariMuslihudin

-Rizky Gustiansyah -Intan Widya


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OUTLINE

1 Introduction

2

Acquisition

3 Processing

4 Interpretation

5 Case Study


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Introduction

Seismic Reflection is a method of

exploration geophysics that uses the

principles of seismology to estimatethe properties of the Earth's subsurface

from reflected seismic waves.


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Introduction


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Basic Concept

Fermats Principle


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Basic Concept

Huygens Principle


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Basic Concept

Snells Law


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Medium earth consists of several layers of rock, whichis between the layers of rock with another rock layers

can be different density and wave speed response.

According to Snell's law, can seismic waves change

direction when passing through the boundary between

the layers because of refraction and reflection.


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Acqusition


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Planning

Select and describe primary and secondary targets.

Estimate potential production and profits.

Budget acquisition costs.

Specify and document program objectives and priorities.

Establish data quality standards. Set reasonable schedules and deadlines.

Locate desired lines of survey on maps (survey design).

Select specific methods and equipment to be used.


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Permitting

Determine who all these owners are

Gain permission for seismic work for them, and

Communicate to the field crew any restrictions imposed by the

owners.


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Acquisition Requirements

Surveying/navigation system: precise locations of source and

receiver positions must be known.

Energy sources: all about appropriate amplitudes & frequency

spectra.

Receivers

Cables

Recording system


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Energy source

Desirable characteristics of seismic sources include:

Signal high amplitude, broad frequency bandwidth produced

Safety hazard in use, storage and maintenance can be

managed without excessive precautions

Cost total cost of equipment

Operation relatively simple, efficient, and fast operation

generally preferred

Environment minimal physical and biological damage to the

surroundings.


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Energy Sources


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Energy Sources

Explosive

Most often loaded at the bottom of a drilled holes to avoid

the low velocity zone.

Those holes are drilled in a geometrical patter or array to

enhances the signal and attenuates surface waves at the

source.

The charge is usually dynamite or ammonium nitrate

fertilizer mixed with diesel fuel.


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Energy Source

Explosive

Principal advantage:

o produce high energy and a broadband signal.

o A direct measure of time through low-velocity zone

can be obtained when the explosives are shot in

drilled holes.


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Energy Sources

Explosives

Disadvantages:

o Much energy lost in blow.

o Produce high amplitude horizontal noise

o

Expensiveo Strict safety regulations are imposed and tight security

is required.


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Energy Sources

Vibrator

A vehicle that uses hydraulic energy to produce a signal.

Usually using 2 to 4 vibrator trucks are positioned at source

points within source array.

Vibrators allow the selection of signals frequency content.

Available frequencies range from 5 Hz to 511 Hz.


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Seismic Receivers


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Seismic Array

A group of two or more elements (source or receivers)

arranged in a geometrical patter.

The function is to do spatial filtering.

An array response depends upon wavelength or wavenumber

of seismic energy produced or received.


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Seismic Array


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Acquisition Method

2-D Acquisition Method



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2-D Acquisition Method (Land)

Line configuration (depends on target depth)

Off end spread

Pulling the spread

Pushing the spread

Split spread Symmetrical

Asymmetrical


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2-D Acquisition Method (Marine)

Using off end spread with pulling the

spread movement.


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2-D Acquisition Method (Marine)

In 2005, In 2005, Ocean Bottom

Nodes/Seismic (OBN / OBS) - an

extension of the OBC method that uses

battery-powered cableless receivers

placed in deep water.

This method is called Ocean Bottom cablesystem.

Usually used in shallow marine & transition

zone (


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General Acquisition Parameter

Line parameters

Number and orientation of lines

Line spacing

Line length


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Source parameters

For explosives

Size (e.g., pounds of dynamite)

Number of holes

Hole depth

Pattern


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Source parameters

For vibrators

Number and layout of source positions per source point

Number of units

Sweep type

Number of sweeps

Sweep length

Initial and final frequencies


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Source parameter

For airguns

Number and sizes of guns

Array designs

Number of arrays

Depth at which array is towed


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Spread parameters

Spread types

Off end

Source pulling or pushing spread

Split spread Gap

Symmetric or assymetric

Number of groups

Group interval Maximum and minimum offsets


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Fold

Each spread provides spread of subsurface coverage.

Moving the spread spread length between shots thus

provides continuous coverage of the subsurface below the

line.

Common depth point (CDP) and CMP concept.


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Because of the shortcoming of 2-D, such as:

1. Distortion of the image of geologic structure

2. Inadequate subsurface sampling to define small-scale

geologic features


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Procedure (Based on Figure 4.41):

1. Eight receiver lines are laid but only six are active at a time.

This total length of the six lines is called a swath.

2. Patch is the receiver groups used for the active source.

3. The patch and source are moved up along the active

swath.

4. When the first swath is completed, one or more receiver

lines are moved laterally (rolled) such that there is overlap

in surface and /or subsurface coverage.

5. This continues until all sources have been shot and the

entire survey area covered.

3 D A i i i M h d


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Processing

S i i D t P i


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Seismic Data Processing

Is to process the raw seismic data whichreceived from field seismic acquisition to extract

a good quality and quantity final product as an

input to interpretation step on the line of seismic

exploration.

Fl Ch t


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Flow Chart

DEMULTIPLEX


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DEMULTIPLEX

REARRANGE DATA FROM FIELD TO PROCESSING

ORDER.

CONVERT FROM FIELD FORMAT (MANY, VARIABLE) TO

INTERNAL FORMAT.

PROVIDE FIRST LOOK AT THE RAW DATA.

G t


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Geometry

G t S ifi ti


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Geometry Specification

Source Geometry



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Receiver Geometry


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Data Editing


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Data Editing

BAD RECORDS

BAD TRACES

- ISOLATED, RANDOM

- NOISY GEOPHONE GROUPS

- MISSING GROUPS (ENDS OF LINES)

NOISY TIME ZONES

- SPIKES

- NOISE BURSTS

Editing Option


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Editing Option

Killing Traces


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Killing Traces

before after

Top Mute


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Top Mute

before after

Datum Correction


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Datum Correction


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Gain (Amplitude Variations)


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Gain (Amplitude Variations)

Geophone Output (Ungained Recorder Trace)


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Geophone Output (Ungained Recorder Trace)

Common Gain Problems


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Common Gain Problems

Shot Records


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Shot Records

Objectives of Gain


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Objectives of Gain

(AGC or TRACEWISE BALANCE)

- Best continuity

- Events visable at all times

- Bright spots visible

- Amplitudes proportional to reflectioncoefficients

Signal and Noise


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Signal and Noise

NOISE IS WHAT DO NOT WANT

NOISE IS WHAT IS NOT IN THE MODEL

SIGNAL IS WHAT WE DO WANT

SIGNAL IS DESCRIBED BY THE MODEL

Split- Spread Field Record


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Split Spread Field Record

Types of Noise


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Types of Noise

Causes of Poor Signal to Noise


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Causes of Poor Signal to Noise

NON-OPTIMAL FIELD PROCEDURES

STRONG COHERENT NOISE

SCATTERING OR ABSORPTION

NEAR-SURFACE PROBLEMS

IMPROPER PROCESSING (STACK)

Filtering


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Filtering

Types of Filter

Bandpass Filters

Deconvolution

Wave shaping

F-K Filters

Bandpass Filter


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a dpass e

Spectrum Amplitude after Bandpass Filter


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p p p

One Dimensional Filter


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Two Dimensional Filter (FK Filter)


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( )

FK Filter


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before after

Deconvolution


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NMO


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NMO : Natural curvature of reflection events onField records & CMP gathers

NMO Corrections : Time shifts in the computer to

change the curvature (to flat)

NORMAL MOVEOUT

The variation in reflection arrival time with offset distance between

source and receiver.

MOVEOUT AT OFFSET X:

Surface

Reflector

S R


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WITHOUT NMO CORRECTION WITH NMO CORRECTION


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Picking Velocity


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Residual Static


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elevation static correction put the shot pointand geophone at the same datum level so that

the influence of different elevation can be

eliminated.


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CMP GATHERS BEFORE AFTER RESIDUAL STATICS

Stacking


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Stacking is the sum of traces in one gather that aims to

enhance the signal to noise ratio (S / N).

STACK BEFORE & AFTER RESIDUAL STATICS

Migration


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Migration is to move the position of the visible reflectors on

seismic data recorded into the actual position according to the

position below the surface.


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Interpretation

Flow ChartData Collecting &

Verification- Regional Geology

- Bouguer Map


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Loading Seismic and Wells Data

Loading Supporting Data(Checkshot, Formation Market, etc.)

Seismic Well Tie

Seismic Interpretation /Fault Reconstruction

Picking Horizon

Structural Map, Isopach

& Isochron

Lead & Prospect

Risk Analysis

Iteration

Y/N

Reflection Pattern


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Reflection Pattern


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The purpose of interpretation is to obtain depth map (structural

map) of The surveyed area. We can divide interpretation into two parts

The interpretation ofstructure using the geometry of the beds

The interpretation oflithology using seismic signatures and

seismic attributes.


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Structural interpretation is relatively straightforward and is

largely visual.

the internal geometry of layered strata is revealed

sediment packages can be identified

erosion surfaces can be identified channelling can be identified

We must remember the various scale distortions that may exist

in a seismic record.


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Offshore sparker survey

timescale lines 40ms apart.


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It is possible to estimate the lithology (sediment type) from a

seismic record, although this is less precise than determiningthe structure.

The key is the seismic signature of the material. This is the

internal appearance of a bed, arising from the composite effect

of numerous small reflectors within it.


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A key issue concerns the sound source, since this influences

the signature as well as does the sediment type. The signatures obtained in marine surveys in particular are

very sensitive to the sound source in use.

Thus, in a given material, a boomer may produce a different

signature from a sparker. This is due to the differing frequencyspectra and resolving power of the two sources.

This is less of a problem in terrestrial surveys since the higher

frequencies (=details) are usually lost.


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False layering produced by non-lithological features

These signatures are both

from identical lithologies


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Signatures are broadly characteristic of the parent materials

(with the above proviso). This leads to the idea of a seismicfacies.

A seismic facies is a unit of sediment that has a consistent

seismic appearance. It is often assumed that this implies a

consistent lithology. The full geophone record can be analysed statistically as a

time series to obtain eg its frequency content, average

amplitude, autocorrelation etc.

These are known as seismic attributes and can becharacteristic of particular layers.

93


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Acoustic Impedance & Reflection Coefficient


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Acoustic Impedance & Reflection Coefficient

VV

VV

AIAI

AIAI

122

1122

12

12

RC

AI1

AI 2

AI 3

AI 4

RC1

RC2

RC3

AI = V

Model Seismic Responses - Input


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Courtesy of ExxonMobil

10%

Porosi ty

Gas

Oil

Br ine

20%

Porosi ty

30%

Porosi ty

Model Seismic Responses - Output


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10% Po rosi ty

Offset OffsetOffset

30% Po rosi ty20% Poro si ty


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Synrift Example


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y p

Could you tell me where is Fluvial Environment ?

Could you tell me where is Deltaic Environment ?


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Could You Tell Me Where is Fluvial Environment (Braided Stream, Fan DelCould You Tell Me Where is Deltaic Environment ?


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Marine? Marine?

DELTAIC

FLUVIAL

TRANSITION

Lacustrine?


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Seismic DHI

s are anomalous seismic responses related to the presenceof hydrocarbons

Acoustic impedance of a porous rock decreases as hydrocarbon replacesbrine in pore spaces of the rock, causing a seismic anomaly (DHI)

There are a number of DHI signatures; we will look at a few common

ones: Amplitude anomaly Fluid contact reflection Fit to structural contours

DHI=DirectHydrocarbonIndicator

DHIs: Amplitude Anomalies


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High AmplitudeLow

Change in amplitudealong the reflector

Anomalous amplitudes

DHIs: Fluid Contacts


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L12 Data Analysis Courtesy of ExxonMobil

Hydrocarbons are

lighter than waterand tend to form flat

events at the gas/oil

contact and the

oil/water contact.

Thicker Reservoir

Fluid contactevent

Fluid contactevent

Thinner Reservoir

DHIs: Fit to Structure


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Since hydrocarbons are

lighter than water, the

fluid contacts and

associated anomalous

seismic events are

generally flat in depth

and therefore conform

to structure, i.e., mimic

a contour line

Intro to Exercise


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Goal: To map the extent of the A1 gas-filled reservoir

Courtesy of ExxonMobilFigure 1Inline 840

A1 Gas

Sand

W E

Changes in Amplitude Indicate Fluid


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Inline 840 Figure 1

Gas SandWater Sand

Traces areclipped

Fluids within the A1 Sand


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Inline 840 Figure 1

Extent of Gas

References


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Brown, A.R. 2004. Interpretation of Three Dimensional

Seismic Data. AAPG Memoir 42 SEG Investigations inGeophysics. Tulsa

Munadi, S. 2000.Aspek Fisis Seismologi Eksplorasi. Program

Studi Geofisika UI. Depok.

Munadi, S., D. Rubyanto dan B. Triharjanto. 1995. ResolusiSeismik. Lembaran Publikasi Lemigas No.2. Jakarta.

Russell, B. H. 1991, Introduction to Seismic Inversion Methods,

S.N. Domenico. Editor Course Notes Series. Volume 2 3rd

edition. USA.

References


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Sismanto. 1996. Modul 1: Akuisisi Data Seismik.Laboratorium Geofisika UGM. Yogyakarta.

Sismanto. 2006. Dasar Dasar Akuisisi dan Pemrosesan

Data Seismik. Laboratorium Geofisika UGM. Yogyakarta.

Sukmono, S. dan A. Abdullah. 2001. KarakteristikReservoar Seismik. Lab. Geofisika Reservoar Teknik

Geofisika ITB. Bandung.

Umam, M. S. 2004. Seismic Interpretation in Petroleum

Exploration and Production. Course by Chevron. Pekanbaru.

reflection seismic

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