2. Fundamental of Seismic_TM_2013.ppt

8/10/2019 2. Fundamental of Seismic_TM_2013.ppt

1/39

Oleh:

Dr. Ir. Eko Widianto, MT

Jurusan Teknik PerminyakanFakultas Teknologi Kebumian dan EnergiUniversitas TRISAKTI

2013

Fundamental of Seismic Reflection Method

Exploration Geophysics


2/39

LECTURE MATERIALS

1. INTRODUCTION (1X)a. Definition

b. Geophysical Methods and their main applicationsc. Level of Petroleum Investigation

2. REFLECTION SEISMIC (8X)a. Fundamental of Seismic Reflection Methodb. Acquisitionc. Processingd. Structural Interpretatione. Stratigraphic Interpretationf. Exerciseg. Field Trip (if possible)

1. GRAVITY (3X)a. Introduction and general application of gravity datab. Gravity data analysis for Oil and Gas Explorationc. Paradigm Shift in Gravity data utilizationd. Gravity data analysis for Oil and Gas Reservoir Monitoring (Time lapse)

2. MAGNETIC (1X)a. General Application of Magnetic Data


3/39


4/39

Seismic Methods:Refraction and Reflection

Seismic methods, as typically applied inexploration seismology, are considered active

geophysical methods. In seismic surveying,ground movement caused by some source* ismeasured at a variety of distances from thesource . The type of seismic experiment differs

depending on what aspect of the recordedground motion is used in the subsequentanalysis.


5/39

One of the first active seismic experiments was conducted in 1845 byRobert Mallet, considered by many to be the father of instrumentalseismology. Mallet measured the time of transmission of seismic waves,probably surface waves, generated by an explosion. To make thismeasurement, Mallet placed small containers of mercury at variousdistances from the source of the explosion and noted the time it took forthe surface of the mercury to ripple after the explosion. In 1909, AndrijaMohorovicic used travel-times from earthquake sources to perform aseismic refraction experiment and discovered the existence of the crust-mantle boundary now called the Moho .

The earliest uses of seismic observations for the exploration of oil andmineral resources date back to the 1920s. The seismic refractiontechnique, was used extensively in Iran to delineate structures thatcontained oil. The seismic reflection method, now the most commonlyused seismic method in the oil industry, was first demonstrated in

Oklahoma in 1921.

History


6/39

Elastic Waves

When the is Earth rapidly displaced or distorted at some point, theenergy imparted into the Earth by the source of the distortion can betransmitted in the form of elastic waves . A wave is a disturbance thatpropagates through, or on the surface of, a medium. Elastic wavessatisfy this condition and also propagate through the medium withoutcausing permanent deformation of any point in the medium. Elasticwaves are fairly common. For example, sound propagates throughthe air as elastic waves and water waves propagate across thesurface of a pond as elastic waves.


7/39

The wave is characterized by : Amplitude is the peak to trough height of the wavedivided by two.

Wavelength is the distance over which the wave goes

through one complete cycle (e.g., from one peak to thenext, or from one trough to the next).

Period is wavelength measured in time

Frequency is number of cycle in 1 second

Velocity is the speed of wave propagation


8/39

SEISMIC WAVES ARE B ODY WAVES : Elastic waves that propagate throughthe Earth's interior. In reflection and refraction method, body waves are thesource of information used to image the Earth's interior. Like the ripples on

the surface of the pond, body waves propagate away from the source in alldirections. If the speed at which body waves propagate through the Earth'sinterior is constant, then at any time, these waves form a sphere aroundthe source whose radius is dependent on the time elapsed since thesource generated the waves. Shown above is a cross section through theearth with body waves radiated from a source (red circle) shown at severaldifferent times.


9/39

Wave Type(and names) Particle Motion Other Characteristics

P (Compressional) ,Primary, Longitudinal

Alternating compressions(

pushes

) and dilations(

pulls

) which are directedin the same direction as the

wave is propagating (alongthe raypath); and therefore,

perpendicular to thewavefront.

P motion travels fastest in materials,so the P-wave is the first-arrivingenergy on a seismogram. Generallysmaller and higher frequency than the

S and Surface-waves. P waves in aliquid or gas are pressure waves,including sound waves.

S (Shear) , Secondary,

Transverse

Alternating transverse

motions (perpendicular to thedirection of propagation, andthe raypath); commonlyapproximately polarizedsuch that particle motion isin vertical or horizontal

planes.

S-waves do not travel through fluids,

so do not exist in Earth

s outer core(inferred to be primarily liquid iron)or in air or water or molten rock(magma). S waves travel slower thanP waves in a solid and, therefore,arrive after the P wave.

Seismic Body Waves


10/39

Compressional Wave (P-Wave)

Deformation propagates. Particle motion consists of alternating compressionand dilation. Particle motion is parallel to the direction of propagation

(longitudinal). Material returns to its original shape after wave passes.


11/39

Shear Wave (S-Wave)

Deformation propagates. Particle motion consists of alternating transverse motion. Particlemotion is perpendicular to the direction of propagation (transverse). Transverse particlemotion shown here is vertical but can be in any direction. However, Earth

s layers tend tocause mostly vertical (SV; in the vertical plane) or horizontal (SH) shear motions. Material

returns to its original shape after wave passes.


12/39


13/39

Rayleigh Wave (R-Wave)

Deformation propagates. Particle motion consists of elliptical motions(generally retrograde elliptical) in the vertical plane and parallel to thedirection of propagation. Amplitude decreases with depth. Material returnsto its original shape after wave passes.


14/39


15/39

1. What seismic wave type is shown here?


16/39



17/39



18/39



19/39

Wavefrontsand Raypaths

Raypaths - Raypaths are nothing more than lines that show the direction that the seismicwave is propagating. For any given wave, there are an infinite set of raypaths that could beused. In the example shown above, for instance, a valid raypath could be any radial line drawnfrom the source. We have shown only a few of the possible raypaths.

Wavefront - Wavefronts connect positions of the seismic wave that are doing the same thing

at the same time. In the example shown above, the wavefronts are spherical in shape. Onesuch wavefront would be the sphere drawn through the middle of the dark blue area. Thissurface would connect all portions of the wave that have the largest possible negativeamplitude at some particular time.

In principle and in practice, raypaths are equivalent to the directions of current flow, andwavefronts are equivalent to the equipotential lines. They are also equivalent to field directionand strength in magnetism.


20/39

Wave Interaction with Boundaries


21/39

Snell's Law

These raypaths are simply drawn to be perpendicular to the direction ofpropagation of the wavefield at all times. As they interact with the boundary, theseraypaths obey Snell's Law. Snell's Law can be derived in a number of differentways, but the way it is usually described is that the raypath that follows Snell'sLaw is the path by which the wave would take the least amount of time topropagate between two fixed points.

Consider the refracted raypaths shown above. In our particular case, v 2 , thevelocity of the halfspace, is less than v 1 , the velocity of the layer. Snell's Lawstates that in this case, i2 , the angle between a perpendicular to the boundary andthe direction of the refracted raypath, should be smaller than i1 , the anglebetween a perpendicular to the boundary and the direction of the direct raypath.

This is exactly the situation predicted by the wavefronts shown in the figureabove.


22/39

Seismic Wave Speeds and Rock Properties

It can be shown that in homogeneous, isotropic media

the velocities of P and S waves through the media aregiven by the expressions shown to the right. Where Vp and Vs are the P and S wave velocities of the medium,r is the density of the medium, and and k arereferred to as the shear and bu lk moduli of the media. Taken together, and k are also known as elastic parameters. The elastic parameters quantitativelydescribe the following physical characteristics of themedium.

Bulk Modu lus - Is also known as the incompres s ib i l i ty of the medium. Imagine wehave a small cube of the material making up the medium and that we subject thiscube to pressure by squeezing it on all sides. If the material is not very stiff, we

can image that it would be possible to squeeze the material in this cube into asmaller cube. The bulk modulus describes the ratio of the pressure applied to thecube to the amount of volume change that the cube undergoes. If k is very large,then the material is very stiff, meaning that it doesn't compress very much evenunder large pressures. If k is small, then a small pressure can compress thematerial by large amounts. For example, gases have very smallincompressibilities. Solids and liquids have large incompressibilities.


23/39

Seismic Wave Speeds and Rock Properties

Shear Modu lus - The shear modulus describes how difficult it is to deform a cube of the

material under an applied shearing force. For example, imagine you have a cube of materialfirmly cemented to a table top. Now, push on one of the top edges of the material parallel tothe table top. If the material has a small shear modulus, you will be able to deform the cubein the direction you are pushing it so that the cube will take on the shape of a parallelogram.If the material has a large shear modulus, it will take a large force applied in this direction todeform the cube. Gases and fluids can not support shear forces. That is, they have shearmoduli of zero. From the equations given above, notice that this implies that fluids andgases do not allow the propagation of S waves.

Any change in rock or soil property that causes r, m, or k to change will cause seismicwave speed to change. For example, going from an unsaturated soil to a saturated soil willcause both the density and the bulk modulus to change. The bulk modulus changesbecause air-filled pores become filled with water. Water is much more difficult to compressthan air. In fact, bulk modulus changes dominate this example. Thus, the P wave velocitychanges a lot across water table while S wave velocities change very little.

Many other factors causing changes in velocity (such as changes in lithology, changes incementation, changes in fluid content, changes in compaction, etc.). Thus, variations inseismic velocities offer the potential of being able to map many different subsurfacefeatures.


24/39

Seismic Velocities of Earth Materials

Material P wave Velocity (m/s) S wave Velocity (m/s)

Air 332

Water 1400-1500

Petroleum 1300-1400

Steel 6100 3500

Concrete 3600 2000

Granite 5500-5900 2800-3000

Basalt 6400 3200

Sandstone 1400-4300 700-2800

Limestone 5900-6100 2800-3000

Sand (Unsaturated) 200-1000 80-400

Sand (Saturated) 800-2200 320-880

Clay 1000-2500 400-1000

Glacial Till (Saturated) 1500-2500 600-1000

The P and S wave velocities of various earth materials are shown below.


25/39

Simple Earth Model:Low-Velocity Layer Over a Half space

Shown below are a few snapshots of the seismic waves as they propagate awayfrom the source at times of 65, 80, and 110 ms.


26/39


27/39


28/39

LESSONS LEARNED

Seismic method uses body waves to carry earthsubsurface information to the surface

As seismic wave hit the elastic boundary it willget reflected, refracted, and transmitted

The type of seismic methods differs dependingon what aspect of the recorded reflected or

refracted is used in the analysis Seismic Method: Refraction and Reflection


29/39


30/39

SEISMIC REFRACTION


31/39

SEISMIC REFRACTION


32/39


33/39


34/39

N e a r

O f f s e

t

Far Offset

1 st shot

2 nd shot

Distance betweenshot points n

n t R e c e

i v e r

1 s

t R e c e

i v e r

Spread Length (RL)

Distance betweenReceiver points

1st

shot2 nd shot

3 rd shot

4 th shot

1 1 2 2 3 3 4 4 4 4 4 4 4 4 4 4 3 3 2 2 1 1Fold CoverageFor four times shots Full Fold Coverage

Common Shot Points

Common Mid PointsCommon Receiver

Receiver Points

Source PointsMid Points

Multiple Coverage Seismic Reflection Survey


35/39

11

2

2

222

o NMOo NMO

o

t V

xt t

v

xt t

Hyperbolic Move Out

Reflected waves recognized by itshyperbolic shape in seismic record

Normal move out correction is a time

shift apply to seismic reflection recordto get zero offset response


36/39

SEISMIC REFLECTION RECORDS

R fl ti S i i


37/39

t t t t t t Q Source Receiver Raw data

seismic Section geology

Reflection Seismic


38/39

ZERO OFFSET SECTION


39/39

Documents

2. Fundamental of Seismic_TM_2013.ppt