Seismic Method

Principles of Geophysics (250G)

(Seismic Prospecting Methods)

Compiled byCompiled by

Prof. Dr. Abudeif A. Bakheit

Email : abakheitEmail : [email protected]@yahoo.com

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Seismic methods

Seismic methods may be classified into two major divisions depending on the energy source of the seismicdivisions depending on the energy source of the seismic waves;

O i hi h th t l h k f th kOne in which the natural shock waves from earthquakes are interior, is called earthquake seismology.

The other, in which the seismic waves are generated by artificial explosions at selected sites to obtainartificial explosions at selected sites to obtain information about regional or local structures, is called exploration seismology

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exploration seismology.

Seismic ProspectingDefinition

Seismic ProspectingDefinition

Basis of the seismic method is the timing of artificially generated pulses of elastic wave energy that propagates through the ground.g g

These pulses of elastic wave energy or seismic waves d t t d i l t ti t d ll dare detected using electromagnetic transducers called

geophones.

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Wave propagationRay- paths: y pLines that show the direction that thedirection that the seismic wave is propagating and are p p g gperpendicular to the wave front.Wave- front:Is a surface of constant wave, the leading edge of a

El ti d t f i t

wave disturbance.

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Elastic wave energy spreads out from a point source as an expanding sphere of energy

Terminology of Seismic Waves

Characteristics of Waves Measurements of a wave

Wave period the time interval between the passage of two successive crests

Frequency the number of repetitions per unit timef 1/T it i H t ( b f f = 1/T unit is Hertz (number of repetitions/sec)Velocity wave speed time it Velocity - wave speed time it takes the wave front to traverse a known distance

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a known distance

Elastic Moduli/ Elastic constants/ coefficients E l ti i l i d ith l ti Exploration seismology is concerned with elastic deformation. Velocities in earth materials depend on physical properties of earth materials such as densitiesphysical properties of earth materials such as densities and elastic moduli. Modulus = stress/strain Modulus = stress/strain The higher the value of the modulus, the stronger the

material and the smaller the strain produced by amaterial, and the smaller the strain produced by a given stress

Elastic constants include: Bulk modulus K Shear Modulus Youngs modulus E

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g Poissons Ratio

Definition of Elastic ConstantsB lk d l

Bulk modulus is the Stress/strain ratio

Bulk modulus

Bulk modulus is the Stress/strain ratio under simple compression.

It is a measure of how much force is It is a measure of how much force is needed to change the volume of the material without change in shapematerial without change in shape.

The bulk modulus (K) is a measure of the capacity of the material to becapacity of the material to be compressed or the incompressibility of the material.t e ate a

it is defined as volumetric stress over volumetric strain, and is the inverse of

P = PressureV= Volume

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volumetric strain, and is the inverse of compressibility K = P (volume stress)

v/v (volume strain)

Denoted by S or sometimes is defined as

Shear or Rigidity Modulus Denoted by S or sometimes , is defined as

the ratio of shear stress to the shear strain.

f f Taken as an indication of the strength of a material under shear forces. Shear modulus is usually measured in GPa (Gigapascals)

If a material has a large shear modulus, it will take a large force applied to deform it. Gases and fluids can not support shear forces, they have shear modulus of zero.

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Shear stress (F/A)/shear strain tan

Young's modulus (E) or modulus of elasticity

Also called elastic modulus or tensile modulus is a measure of the stiffness of a given material.

It is defined as the ratio of the uniaxial stress over the uniaxial strain

The Young's modulus allows engineers and other scientists to l l h b h i f i l d l dcalculate the behavior of a material under load.

It can be used to predict the amount a wire will extend under t i t di t th l d t hi h thi l ill b kltension, or to predict the load at which a thin column will buckle under compression

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Poisson s ratio ( , or ) When a sample of material is stretched in

one direction, it tends to get thinner in the other two directions. Poisson's ratio ( ) is a measure of this tendency. s e su e o s e de cy.

It is defined as the ratio of the strain in the direction of the applied load to thethe direction of the applied load to the strain normal to the load. For a perfectly i ibl i l h iincompressible material, the Poisson's ratio would be exactly 0.5.

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Seismic Waves

Four most important types of waves: Body waves Body waves -

Compressional (longitudinal, primary or P-waves)T ( h d S ) Transverse (shear, secondary or S-waves)

Surface waves - Love waves (transverse, horizontal) Rayleigh waves (circular, reverse of water wave motion)

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Seismic Waves

Body wavesCompressional p(longitudinal, primary or P-waves))

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B dTypes of seismic waves

Body waves Primary (P) waves

Push-pull (compress and expand) motion, changing the volume of the intervening material

Travel through solids, liquids, and gases

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Seismic Waves

Body waves :transverse (shear, (

secondary or S-waves)

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Types of seismic waves

Body waves Secondary (S) waves

Slower velocity than P waves Slightly greater amplitude than P waves Second to appear at recording station Travels through solids onlyg y

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Surface WavesSu ace a es

Surface waves propagate along the Earth's surface. Their amplitude at the surface of the Earth can be very p y

large, but decays quickly with depth. Surface waves propagate at speeds that are slower than Surface waves propagate at speeds that are slower than

S waves. They have amplitudes that decay with distance from the They have amplitudes that decay with distance from the

source more slowly than is observed for body waves

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Seismic Waves

Surface waves - Love waves:

are essentially horizontally y ypolarized shear waves (SH waves)

Love waves travel with a slower velocity than P- or S-waves, but faster than Rayleigh waves.

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Seismic WavesSurface waves :Raleigh waves

Also known as the Rayleigh-Lamb Wave or "ground roll

Particle moves in a circle or Particle moves in a circle or ellipse like water waves, but in opposite direction

If one measures particles deeper in the material, the particles move slower thenparticles move slower, then reach a "no movement" depth.

Its velocity is slower than Love waves

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Seismic wave velocities in rocks

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 (Unsat.) 200-1000 80-400

Sand (Sat.) 800-2200 320-880

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Clay 1000-2500 400-1000

Glacial Till (Sat.) 1500-2500 600-1000

Because seismic sources radiate

Snells Law & Critical RefractionBecause seismic sources radiate waves in all directions. Some ray m st hit interface at e actl the criticalmust hit interface at exactly the critical angle, icThis critically oriented ray will then travel along the interface between the two layers.If more oblique than critical, all wave q ,energy is reflected The reflected energy is useful tooenergy is useful too.

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E l ti S i lExploration Seismology

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Seismic reflectionSeismic reflection

The travel time to each geophone for the direct wave in the first layer is simplywave in the first layer is simply

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Th t l ti f th fl t d f 2The travel time for the reflected wave for a 2-layer model can be expressed as follows.

Where :t is the travel timeh1 is the depth to the first interface (thickness of the first layerV1 is the velocity of seismic waves in the first layer

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The equation in the last slide can beqre-formatted as

The second equation is in a standard format forThe second equation is in a standard format for expressing a hyperbola curve.

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Seismic refraction

Seismic refraction only consider the first arrivals soSeismic refraction only consider the first arrivals -so simple and easy to use

The detection depth is about 1/4 to 1/10 of your geophone spreadgeophone spread

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Travel Time curves

Refracted arrivalRefracted arrival

Direct wave

Determining the shape of the travel-time curves versus offset is 29

the primary task in the refraction seismic method.

Analysis of travel time curves Direct wave

CriticallyCross over distance Critically refracted wave

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At the crossover distance xcross the travel times to ati l h th f th di t dparticular geophone are the same for the direct wave and

the refracted wave, so we have

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32Reflection and refraction Travel Time curves

3 layers 2 interfaces

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35Multilayer Seismic Refraction Travel Time curves

Determining depthg1. Using intercept time time at x =0

F i t f 2 lFor one interface 2 layers

36For two interfaces 3 layers

Determining Depth

2. Using the crossover Distance

Depth Layer1

c

o

n

d

s

)

y

Layer2L2

(

m

i

l

l

i

s

e

c

yL1

e

l

T

i

m

e

L1 = Layer 1

XcoSource to Geophone DistanceT r

a

v

e

V2 -V1V2 V1

Xco2

D1 =yL2 = Layer 2V1 = Velocity of Layer 1 = 1/Slope of L1V2 = Velocity of Layer 2 = 1/Slope of L2

V2+V12

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V2 Velocity of Layer 2 1/Slope of L2Xco =Crossover Distance

Velocity Model

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Note the difference in length of the direct wave

Field ProceduresField Procedures

Equipment: Seismic energy Source - apparatus for delivering gy pp g

seismic energy into the ground Geophones devices capable of measuring Geophones -devices capable of measuring

ground motion generated by the seismic source Seismograph - stores the ground motion detected

by a number of geophones

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Field Procedures

This device consists of a mass hanging on a spring. When the ground moves, the mass (because it has inertia) remains motionless. Wrapped around the mass is a strand of wire. Surrounding the wire-wrapped mass is a magnet that is fixed to the Earth. As the earth moves the magnet moves up and down around the mass TheAs the earth moves, the magnet moves up and down around the mass. The magnetic field of this moving magnet produces an electrical voltage in the wire.This voltage can be amplified and recorded by a simple voltmeter. It is relatively

40easy to show that the voltage recorded by the voltmeter is proportional to the velocity (speed) at which the ground is moving.

Different geophones are used for different types of survey.

For refraction surveying the typical natural frequency is 14 Hz;

For detailed shallow reflection 100 Hz; ;

For surface waves 1.5 Hz

40 Hz can capture both refraction and reflection41

40 Hz can capture both refraction and reflection.

Field ProceduresField Procedures

Important notes:Important notes: Spend time to know as much as possible about the local

l f th t dgeology of the study area. Use spread length at least 3 times the target length Geophones must be well coupled with firm ground Can fill holes with water before placing geophones Can fill holes with water before placing geophones Lay cable along a line of equal elevation

A id i d d t ffi i Avoid very windy areas and traffic noise Think about the source you will be using

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Problems interpreting refraction data

High Velocity LayerO L V l it LOver Low Velocity Layer no critical refraction

l l only one layer seen

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Problems

Hidden Layers (LVZ between two high velocity zones)

Only two layers seen in the time distance curve.

No critical refraction between V1 and V2, hence V2 layer will not be seen

Depth to V3 layer will be much thicker

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ProblemsHidd L (Bli d Z Hidden Layers (Blind Zone-thin layer with high velocity)

The travel-time curve would show 2 layers only.

It is overtaken by the rapidly traveling head wave coming g gfrom the V2-V3 boundary.

Depth calculated would be Depth calculated would be too shallow

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Applications of Seismic Refractionpp

Depth and thickness of geologic strata Ground water exploration applications as aquifer Ground water exploration applications as aquifer

thickness, buried valleys, mapping water table elevationelevation

Engineering geology applications as depth to bedrock Waste disposal site evaluationbedrock, Waste disposal site evaluation

Providing velocities for seismic reflection i t t tiinterpretation

Detection of subsurface fracture system.48

Seismic Refraction;Advantages and Limitations

Advantages Determination of depth and soil/rock velocityDetermination of depth and soil/rock velocity Infer soil competency, weathering, fractures Acquisition and processing less expensive than reflection Acquisition and processing less expensive than reflection

Limitations R l ti l th fl ti Resolution less than reflection surveys

Large impact source may be required Increased rock velocity with depth required Hidden layers may be detected, but possibly not interpreted

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EARTHQUAKE SEISMOLOGYEARTHQUAKE SEISMOLOGY

What is an EARTHQUAKE?

An earthquake is the motion, shaking or tremblingof the ground produced by sudden displacement ofof the ground produced by sudden displacement ofrock in the Earth's crust.

They result from tectonics, volcanism, landslides,and collapse of cavernsand collapse of caverns.

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Surface waves

Body waves

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The effect of an earthquake may be manifested in any of h f ll i fthe following forms:

Surface faulting, landslides soillandslides, soil liquefaction, and structural damagestructural damage.

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Earthquake parameters- Focus (hypocenter)- Epicenter- Focal Depth- Epicenter Distancep ce te sta ce

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Seismograph (Seismometer)

- A seismograph, or seismometer, is an instrument used to detect and record

Earthquake Recording

instrument used to detect and record earthquakes. Generally, it consists of a mass attached to a fixed base.D i th k th b d-During an earthquake, the base moves and

the mass does not. The motion of the base with respect to the mass is commonly p ytransformed into an electrical voltage. The electrical voltage is recorded on paper, magnetic tape or another recordingmagnetic tape, or another recording medium. -This record is proportional to the motion of the seismometer mass relative to the earth, but it can be mathematically converted to a record of the absolute motion of the ground

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record of the absolute motion of the ground.

ForeshocksForeshocksForeshocks are relatively smaller earthquakes that precedethe largest earthquake (mainshock) in a series.Not all mainshocks have foreshocks.

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Earthquakes Intensity Scale and Magnitude Scale

-The severity of an earthquake can be expressed in terms ofboth intensity and magnitude However the two terms areboth intensity and magnitude. However, the two terms arequite different, and they are often confused.

I t it i b d th b d ff t f dIntensity is based on the observed effects of groundshaking on people, buildings, and natural features. It variesfrom place to place within the disturbed region dependingfrom place to place within the disturbed region dependingon the location of the observer with respect to theearthquake epicenter.q pMagnitude is related to the amount of seismic energyreleased at the hypocenter of the earthquake. It is based onyp qthe amplitude of the earthquake waves recorded oninstruments which have a common calibration. The

59magnitude of an earthquake is thus represented by a single,instrumentally determined value.

The Modified Mercalli Intensity Scale:

The effect of an earthquake on the Earth's surface is calledthe intensity. The intensity scale consists of a series of certainkey responses such as people awakening, movement offurniture, damage to chimneys, and finally total destruction.

The intensity scale of earthquakes currently used in theUnited States is the Modified Mercalli (MM) Intensity Scale.ItS ( ) y Swas developed in 1931 by the American seismologists HarryWood and Frank Neumann.

This scale, composed of 12 increasing levels of intensity thatrange from imperceptible shaking to catastrophic destructionrange from imperceptible shaking to catastrophic destruction,is designated by Roman numerals. It does not have amathematical basis; instead it is an arbitrary ranking based on

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; y gobserved effects.

I. Not felt except by a very few under especially favorable conditions.II F lt l b f t t i ll fl f b ildi D li t l d d bj t

The Modified Mercalli Intensity Scale (MMI).

II. Felt only by a few persons at rest, especially on upper floors of buildings. Delicately suspended objectsmay swing.III. Felt quite noticeably by persons indoors, especially on upper floors of buildings. Many people do notrecognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of arecognize it as an earthquake. Standing motor cars may rock slightly. Vibration similar to the passing of atruck. Duration estimated.IV.Felt indoors by many, outdoors by few during the day. At night, some awakened. Dishes, windows, doorsdisturbed; walls make cracking sound. Sensation like heavy truck striking building. Standing motor carsrocked noticeably.V. Felt by nearly everyone; many awakened. Some dishes, windows broken. Unstable objects overturned.Pendulum clocks may stop.Vl Felt by all many frightened Some heavy furniture moved; a few instances of fallen plaster DamageVl. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damageslight.Vll. Damage negligible in buildings of good design and construction; slight to moderate in well-builtordinary structures; considerable damage in poorly built or badly designed structures; some chimneysbroken.Vlll. Damage slight in specially designed structures; considerable damage in ordinary substantial buildingswith partial collapse. Damage great in poorly built structures. Fall of chimneys, factory stacks, columns,

t ll H f it t dmonuments, walls. Heavy furniture overturned.IX. Damage considerable in specially designed structures; well-designed frame structures thrown out ofplumb. Damage great in substantial buildings, with partial collapse. Buildings shifted off foundations.X. Some well-built wooden structures destroyed; most masonry and frame structures destroyed with

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X. Some well built wooden structures destroyed; most masonry and frame structures destroyed withfoundations. Rails bent.Xl. Few, if any (masonry) structures remain standing. Bridges destroyed. Rails bent greatly.Xll. Damage total. Lines of sight and level are distorted. Objects thrown into the air.

Isoseismal lines:- An isoseismal (line) is a( )contour or line on a mapbounding points of equal

fintensity for a particularearthquake.

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Earthquake Magnitude:Earthquake Magnitude: The magnitude is a number that characterizes the relative size of an

earthquake. Magnitude is based on measurement of the maximum q gmotion recorded by a seismograph.

Several scales have been defined, but the most commonly used are 1 l l it d l f d t "Ri ht it d 1. local magnitude commonly referred to as "Richter magnitude,2. surface-wave magnitude (Ms), 3 body-wave magnitude (Mb) and3. body wave magnitude (Mb), and4. moment magnitude (Mw).

All magnitude scales should yield approximately the same value for any given earthquake.

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Richter magnitude scale

The Richter magnitude scale was developed in 1935 by Charles F. Richter of th C lif i I tit t f T h l th ti l d i t ththe California Institute of Technology as a mathematical device to compare the size of earthquakes. The magnitude of an earthquake is determined from the logarithm of theThe magnitude of an earthquake is determined from the logarithm of theamplitude of waves recorded by seismographs. Adjustments are included forthe variation in the distance between the various seismographs and theepicenter of the earthquakes.On the Richter Scale, magnitude is expressed in whole numbers and decimalf ti F l it d 5 3 i ht b t d f d tfractions. For example, a magnitude 5.3 might be computed for a moderateearthquake, and a strong earthquake might be rated as magnitude 6.3. Because of the logarithmic basis of the scale each whole number increase Because of the logarithmic basis of the scale, each whole number increasein magnitude represents a tenfold increase in measured amplitude;As an estimate of energy, each whole number step in the magnitude scale

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gy, p gcorresponds to the release of about 31 times more energy than the amountassociated with the preceding whole number value.

Richter magnitude scale

Earthquakes with magnitude of about 2.0 or less are usually calledi th k th t l f lt b l dmicro earthquakes; they are not commonly felt by people and are

generally recorded only on local seismographs.E t ith it d f b t 4 5 t th lEvents with magnitudes of about 4.5 or greater--there are several

thousand such shocks annually--are strong enough to be recordedby sensitive seismographs all over the worldby sensitive seismographs all over the world.Great earthquakes, have magnitudes of 8.0 or higher. On theaverage one earthquake of such size occurs somewhere in theaverage, one earthquake of such size occurs somewhere in theworld each year.Although the Richter Scale has no upper limit the largest knownAlthough the Richter Scale has no upper limit, the largest known

shocks have had magnitudes in the 8.8 to 8.9 range.Recentl another scale called the moment magnit de scale has

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Recently, another scale called the moment magnitude scale hasbeen devised for more precise study of great earthquakes.

S i i it I E tSeismicity In Egypt

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Seismicity In the World

Earthquakes depicted on the seismicity maps are taken from the USGS/NEIC PDE catalog

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y g

Last update 28 November, 2005

Ring of Fire:The "Ring of Fire", alsoThe Ring of Fire , also called the Circum-Pacific belt, is the zone of

th k diearthquakes surrounding the Pacific Ocean--about 90% of the world's earthquakes occur there.The next most seismic region (5 6% ofregion (5-6% of earthquakes) is the Alpide belt (extends from Mediterranean region, eastward through Turkey, Iran and northern India

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Iran, and northern India.

REFERENCES-Dobrin and Savit (1986): Introduction to geophysical

prospecting4th Ed. ,McGraw Hill book company, New York, 867p.

-Reynolds, J.M., (1997): An introduction to applied and environmental geophysics John Wiley & Sons, Chichester: 796 P.

-Sheriff, R.E. and Geldart, L.P. (1995): Hand book of exploration seismology. Cambredge University Press. 592P.

-Telford,W.M., Gildart,L.P., Sheriff,R.E., and Keys,D.A. (1976):Applied Geophysics , Cambridge University Press, 860 P.

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Seismic Method