VẬT lý địa CHẤN seismic reflection

Review: Body WavesThere are two types of body wave waves which travel through the earth.. Review: Seismic Velocitiesmass e appropriat force restoring waves of velocity = The velocity de

Seismic Reflection Surveying

•The most widely used and well know geophysical technique

•A seismic section looks similar to a geologic cross-section – a trap for the unwary

•Only by understanding how the reflection method is used and seismic sections are created can geologists make informed interpretations

•Today we will discuss some background theory and methods

•Thursday we will collect some data

•Next Tuesday we will discuss time series analysis

•The following Thursday we will discuss and actually do some processing

Review: Body Waves

There are two types of body wave (waves which travel through the earth)

P-waves – Travel through the

earth in a series of dilations and

compressions Akin to sound

through air

S-waves Shear wave, do not

travel through fluids, travel at

about half the speed of P-waves

Review: Seismic Velocities

mass e

appropriat

force restoring

waves of

velocity =

The velocity depends on two main things – the restoring force (analagous to the

strength of a spring), and the mass (analagous to the mass of the spring) As the restoring force increases, the velocity increases However, as the mass increases, this will slow the spring, reducing the velocity The mass in the case of a rock is its density (mass per unit volume)

S-waves involve a change in shape – this requires a shear force The size of the

force depends on the shear, or rigidity modulus, μ A P-wave also involves a

change in size, so the compressibility modulus κ is also involved.

ρ

µ

ρ

µ κ

• Rocks differ in their elastic moduli and

densities and, hence, in their seismic

•Vertical Seismic Profiles

•In situ logging using – measuring the travel time of a high frequency acoustic pulse

i

i i

t

z

i=1i=2i=3

•Each layer is characterized by an interval

velocity

•If z 1 is the thickness of layer i and t i is the

one-way travel time through it then the interval velocity

of that layer is:

•The root-mean-square velocity of the section down to the nth interface can be

approximated by:

2 1

1 1

2

n i

i i

n i

i n

rms v t t v

2 1

1 1

2 , =   ∑ ∑  

=

=

n i

i i

n i

i n

2

1 2

2 2

s m 064

1882

13 1 21 1 14 2

) 13 1 2345

( ) 21 1 2000

( ) 14 2 1500

× +

× +

×

•If the radius of the wavefront is r, the amount of energy contained within a unit area

of the shell is E/4πr 2

•With increasing distance along a ray path, the energy contained in the ray falls

of as r 2 due to geometrical spreading of the energy

•Wave amplitude, which is proportional to the square root of the wave energy, falls of as r-1

•The ground is imperfectly elastic – energy is gradually absorbed by internal

frictional losses

•Absorption coefficient: proportion of energy lost during transmission through

a distance equivalent to a complete wavelength – (dB λ-1)

•Absorption coefficient is usually assumed to be independent of frequency

•Higher frequency waves therefore attenuate more rapidly than lower

frequency waves as a function of time or distance

•Absorption produces a progressive lengthening of the seismic pulse

1 A 10 Hz seismic wave travelling at 5 km s-1 propagates for 1000 m through a medium with an absorption coefficient of 0.2 dB λ-1 What is the wave

attenuation in dB due solely to absorption?

2 Repeat the above exercise for a 231 Hz seismic wave

3 Comment on the differences

1 λ=5000/10 = 500 m Attenuation = 1000/500 * 0.2 = 0.4 dB

2 λ=5000/231 = 21.65 m Attenuation = 1000/21.65 * 0.2 = 9.24 dB

Reflection and Transmission

•The total energy of a transmitted and reflected ray must equal the energy of the incident ray

•The relative proportions are determined by the acoustic impedance – the

product of density (ρ) and velocity (v):

ρ

v

Z =

•Generally speaking, the “harder” the rock the greater its acoustic impedance

•Acoustic impedance contrast is the important factor

•Maximum transmission of seismic energy requires a matching of acoustic

impedances

Reflection and Transmission

1 2

1 1 2

2

1 1 2

2

Z Z

Z

Z v

v

v v

R

+

−

=+

−

=

ρ ρ

ρ ρ

•Reflection coefficient R is a numerical measure of the effect of an interface on

wave propagation It is the ratio if the amplitude A 1 if the reflected ray to the

amplitude A 0 of the incident ray:

Expanding, this becomes (for a normally incident ray):

•A negative value of R indicates a 180o phase change in the reflected ray

•The transmission coefficient T is the ratio of the amplitude A 2 of the transmitted

ray to the amplitude A 0 of the incident ray:

1 2

1 0

becomesray this

incident normally

afor

Z Z

Z T

A

A T

+

=

=

Reflection and Transmission

•If R = 0, all the incident energy is transmitted.

•There is no acoustic impedance contrast

•Velocity and density of the layers may still be different

Review: Snell’s Law

2

2 1

sin

v

i v

5 sin

5

sin 4

37 sin

• Dependant on seismic wavelength

• Partly determined by distance between traces

• Parts of a reflector separated by less than the width of the Fresnel zone will not be resolved

Seismic Sources

•A spark is produced by the discharge of a high voltage capacitor bank through an underwater electrode

•Produces a rapidly expanding bubble of ionized gas

Electrical Sources: Sparker

• Aluminum plate attached via a spring loaded mount to a resin block

• A capacitor bank is discharged through the coil, setting up eddy currents in the aluminum plate

• The currents set up a secondary field that opposes the primary field, and the plate is repulsed

• Typically a high frequency source, with resolution of ~0.1 m

• Depth penetration <100 m

Electrical Sources: Boomer

High Pressure Air

Sources: The Air Gun

The lower control chamber has a top

diameter that's smaller than its bottom

diameter, so the air pressure there makes

the piston want to retract (downwards),

sealing the upper, firing chamber High

pressure air is filling the firing chamber

through the T-shaped passage, and the

firing, or actuating air passage is blocked

(solid black) by a solenoid valve

Now, full pressure has built up in the upper chamber The Solenoid has been triggered, releasing high- pressure air into the active air passage, which is now yellow The air fills the area directly below the piston, overcoming the sealing effect of the air in the lower, control chamber The piston starts to move upwards, releasing the air in the upper chamber into the surrounding water

The sound source has activated A large bubble of compressed air is expanding into the surrounding water The air in the lower control chamber has been

compressed by the upward movement of the piston The triggered air, released into the space below the piston, is fully

expanded, and can now exhaust at a controlled rate through the vent ports As this takes place, the piston rapidly but gently moves downward, re-sealing the chamber, and readying the sound source for refilling

•Bolt Air gun

•The most common marine seismic source

•Very Repeatable signal

•Airguns suspended from stowed

booms

•Single Air gun – note air ports

Air Guns

•An ideal pulse convolved with the seafloor creates a simple seismogram

The Ideal Shot

•The output seismogram is a convolution of the source signal and the earth (the seafloor)

•Sharp seafloor signal becomes “ringy”

Reality

•A single airgun creates a “ringy” signal

Tuning An Air Gun Array

•Summing the signal of multiple guns creates a more desirable signal

•Note the relative scales of the left and right plots

Tuning An Air Gun Array

Air Gun Deployment

•Airguns are suspended from buoys to maintain depth

Bubble from Air Gun Explosion

GI Gun

•Two air guns in the same body

•The generator is fired and the bubble starts to expand

•When the bubble approaches its

maximum size, the injector is fired into the bubble

•Reduces bubble oscillation

Other Sound Sources

– Similar in principal to airguns

– Detonated in shallow holes

– Not a precise, repeatable signature

– Most common non-explosive source

– Truck mounted vibrator passes low amplitude continuously

varying frequency known as a sweep frequency (up to a few

tens of seconds long, maybe 10-80 Hz).

– Vibrator needs a firm surface (tarmac road).

– To increase the transmitted energy vibroseis trucks are typically

employed in groups that transmit in phase.

– Each recorded seismogram is cross-correlated with the know

sweep signal to produce a correlated seismogram, or

Other Sound Sources

• Weight Drop

– Hammer, weight drop truck

• Shotguns, buffalo guns, and rifles

– Basically firing a gun into the ground.

Basic Theory: Listening

Deploying The Streamer

• Tail Buoy at the end stabilizes and records position

strength member

• Digitizers convert analogue signal to digital

Deploying The Streamer

constant depth

azimuth

Recording The Data

Shots and CMPs

To help you understand next classes field exercise, I will briefly define shot and Common Mid-Point (CMP) gathers Simply put:

•Shot gather: All the data recorded on all the channels by a single shot

•CMP gather: A collection of traces that have been recorded at the same location

Basin, in Proceeding of the Ocean Drilling Program, Scientific Results, Leg

180, edited by P Huchon, B Taylor, and A Klaus, Ocean Drilling Program,

College Station, 2001

3 Mussett, A.E and M.A Khan, Looking into the Earth: An introduction to geological geophysics, 2000

4 http://www.ldeo.columbia.edu/res/fac/oma/sss (airgun schematics)

5 http://www.ldeo.columbia.edu/res/fac/oma/sss/bubble.html (bubble pulse)

6 http://www.ldeo.columbia.edu/res/fac/oma/sss/tuning.html (array tuning)