Download Geophysical Methods: Refraction Seismology Critical and head

Geophysical Methods: Refraction Seismology
Critical and head waves - critical angle ic intersects and travels at 90o. If they are more
oblique they are reflected.
so: sinic/V1 = sin90o/V2
==> sinic = V1/V2(sin90o) = V1/V2 ==> c = sin-1(V1/ V2)
Huygen's principle: each point on a wave front acts as a new source for waves.
Diffraction - bending of waves into shadows. Radial scattering of incident seismic
energy.
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Edges of faulted layers
Small isolated object
e.g., boulder in a homogeneous layer
difficult to identify
Fermat's Principle - a ray that reaches a particular point does so in a minimum time.
Head Waves:
sin ihead = V1/V2 = sin ic
So head waves are the same as the critical refraction. The refracted rays are than the
measurements of the time to receivers for head waves.
Travel time diagram
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head wave
direct wave - travels just below the interface
reflected waves
The direct wave is measured by 1/V1
The refracted wave is measured by 1/V2
The refracted wave starts at the critical distance. This is where the reflection and
refraction coincide.
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Reflected ray: 2hi/V1 - never first arrivals
First arrivals for near surface distances are direct rays whereas beyond a certain distance
(the crossover distance) are the fastest.
Source - shot point
Locating the first arrival is called picking
The calculation between the picks and distance to source will yield velocities
To calculate the depth of an interface can use:
=>
For Multiple Layers:
This continues for each interface.
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Dipping Interfaces:
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There is no way by looking at the ray that it is coming off of a dipping layer. So
velocities calculated from these interfaces are apparent velocities.
Reversed lines: this becomes very important. By reversing the line the travel time
curve shows the dipping interface.
If the interface has a small dip <5o than the approximation is calculated:
1/V2 ~= 1/2(slope2f + slope2r)
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The interfaces are calculated differently as well:
Flat Layers vs. Dipping Layers
1. The slope of the refraction line is less steep up dip, steeper down dip
2. The intercept is less at the up-dip than down-dip end
3. The slopes for direct rays (first sections on the travel time plot) are unchanged
The true dip
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Shooting along strike gives is a parallel view. The structure appears horizontal.
Obliquely the dip will be too low
True dip can be obtained by using two line perpendicular to each other
where  is the angle between the dip direction and the seismic line from 
Seismic Velocity
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Generally velocity increases with depth
The best measurement is in boreholes
Weathering of rocks will lower the velocity of rocks
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Hidden Layers Thin layer of strata
Low Velocity Layer (LVL)
A seismic interface is not necessarily a geologic boundary. Some rocks may be to similar
in type. So velocity would not separate these interfaces out.
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Survey
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Various scales
Need:
o Source
o line of receivers
o Timing
Hammer seismic
o strike a plate on the ground with a hammer, a switch closes, and the
recording unit starts.
o Distance of profile is dependent on:
 the system
 size of the hammer
 rocks
 noise - to reduce noise you can bury the cables and/or lay the cable
flat
o Can stack data by hitting the hammer several times in the same location
o typically goes ~ 100m distance for such profiles
o shallow surveys rule of thumb:
10x the depth to interface in 100 m line = recording depth of 10 m
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Explosion seismic
o want energy to go down not up, so bury the charges
o drill holes are expensive, the best holes are below the water table for better
coupling. can try to take advantage of a water body as well. Shooting in
water is cheaper as well.
o this type of survey is just a hammer survey on steriods
Land surveying in general
Have to think about station spacing. Two far apart and you may miss
varying layers.
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Marine seismic
o use explosives, but most of the time air guns
o receivers are called hydrophones, measure the pressure changes
o shooting and receiving take place concurrently so several lines are
acquired. Boat time is very expensive.
o large explosives may be used for deep surveys
o Can have 2 ships out for reversals
o Sonobouys may be left out to record
o OBS sit at the bottom like land surveying. can get 3-component data this
way.
Undulating interfaces delay times
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Some surfaces may undulate
o topography
o basin/bedrock contacts
o moho
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Delay times:
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The delay time is easiest calculated at the receiver:
can get tf,tr, and ttotal from the travel time plot.
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The depth can be deduced:
Have to find V1 and V2 than you can determine the intercept time.
Plus-Minus Method
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plot 2 additional lines on the travel time curve
tf + tr
tf - tr
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slope is 2/V2 for tf - tr; so can get V2
calculate V1 from the direct arrivals
So ttotal for each receiver is subtracted from tf + tr and then halfed (equation 6.11)
then use eq. 6.12 to get depth of interface
only used for part of the the profiles, has to be long, typically only used for the
first interface, not so good is the dip >10o
Ray tracing and synthetics
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have previously described inverse methods for interpretating refraction data
forward modeling or ray tracing
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Using Snell's law
can vary velocities
point to fit the observed and calculated arrivals
Can calculate amplitude
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comparing synthetic with original data
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Can detect faults if the travel times are offset
Fan shooting - simple tomography
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shooting in a fan of receivers
increases coverage area
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Geophysical Methods: Reflection Seismology
Reflection seismology is similar to echo or depth sounding. Images the layers within the
Earth.
reflectors - seismic energy travels down and is reflected back to the surface after
contacting a rock interface.
The record of such reflectors produces a seismic section.
This is not a true vertical section, because:
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section in time not depth; the time is two-way travel time(TWT).
Rule of thumb for converting to depth is to multiply by ~ 3 for the a
crustal scale survey
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reflections may not come directly below the source
multiples
Normal move out (NMO)
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for reflection we need receivers on either side of the source to a certain offset.
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some rays will travel vertically and then seom will travel at some angle offend.
This creates a hyperbola in the reflections, which is what we are looking for.
approximation of t, which is typical.
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NMO - the later arrival times offset from receivers for a horizontal reflector.
Looking for velocity:
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to, t, and x are measured from a travel time curve.
One of the goals of reflection is to flatten these hyperbolas
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Multiple layers
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for deeper layers where there is interference with refractions => calculate vrms:
n is the one travel time for each layer.
for the second interface:
and so on ...
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the velocity of any particular layer can be calculated using Dix's equation:
tT, tB are TWT to the top and base reflectors.
this velocity is know as the interval velocity. Determining the velocities of
various layers is velocity analysis.
to determine depth:
Stacking
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adding traces together that have a commonality.
the receivers are repeated instead of the shot. so then each trace has to be
corrected for their move out. "flattening the hyperbolas".
typically velocity analysis and stacking are done together.
this is an interactive step and requires educated guesses.
Dipping reflectors and migration
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reflectors will appear shallower with a less steep dip
the hyperbola is displace up-dip by 2hsin,  is the dip of the interface
if the reflector is curved it becomes more complicated (fig 7.9a)
to correct for this we use migration; can be computationally time consuming.
over migrating can be visible in the data by apparent smiles and under migrating
by apparent frowns.
Diffractions - faulted reflectors.
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seen as a step in the reflector called a diffraction hyperbola
migration clears this up so you can locate the fault properly
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Multiples
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tends to be less in move out so it stacks in with higher velocity
several types of multiples, have to know that they may exist.
can also exist in refraction data.
Survey
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Acquisition
o Marine:
 Sources: commonly air guns, sparkers, water gun, gas & O2
mixture
 measures the duration of the pulse
 P-waves are measured by hydrophones
 measuring the changes in pressure, not the movement of the water
 hydrophones are typically mounted on a streamer
o Land:
 Sources: typically explosives, weight source, adapted air gun.
 10x as expensive than marine
Recording:
o the signal is recorded at regular sampling intervals
o reflections are not first arrivals
o reflections come in after the ground-roll (surface waves) and refractions
CDP stacking
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where a ray hits the place - common depth point
the shot is typically advanced
the number of channels are added together, which is called the fold. the greater
the number in the fold the better resolved the data will be.
Statics
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correction for topography or refractions.
shifting the data to a chosen datum
Display
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most common is variable area
use trace equalization to see deeper wiggles better
color is typically used in 3-D reflection
Vibroseis
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generates a series of waves of changing frequency
need to have the sweep to pull out the relevant data
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Reflectors:
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looking for impedance contrasts:
o arefl/aincid = reflection coefficient (RC)
o atrans/aincid = transmission coefficient (TC)
o dependent on v and 
where v is the acoustic impedance
if RC=0 => no reflector!!!
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Bright Spots
o where there is a strong reflector
in oil/gas : location of oil or gas
in environmental : location of water
o these may not be easy to find, because of pull-ups or pull-downs.
o dependent on the thickness of a layer as well as the velocity
Vertical Resolution
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Interference
o constructive : adding
o destructive : subtracting
(figure 7.21a & b)
o happens when interfaces are close to each other
h < /4 ==> can not resolve
use shorter pulse to get better resolution, but give up depth resolution
another problem are gradual changes versus abrupt changes
Synthetics
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need well logs, at least density or sonic; preferably both.
calculate the RC based on this
can then look at teh waveform and use it to march particular interfaces or see if
there are things you are missing
3-D surveying
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in 2-D can get sideswipes, where dipping reflectors are outside the section
can reduce this and other affects by collecting data in 3-D
usually collected in grid all at once or over time
4-D surveys - time lapse; gets a handle on the reservoir evolution
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Hydrocarbons
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To get a reservoir
1. organic matter is buried in a source rock
2. change of the matter @ 100 to 200 oC
3. pool form, oil is less dense than water
4. need a cap rock to prevent leaks. eg., shale
5. reservoir rock needs to be porous and permeable
6. trap
 structural : faulting, folding
 stratigraphic : rock units
 combo : both
Recognition
o bright spots
o changes in velocity with cap rock & reservoir rock
o values of RC
o shape of traces
Sequence Stratigraphy
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strata boundaries often coicide with reflected boundaries
strata seq is defined by a packet of strata bounded by unconformities over a long
period of time
can see things like:
o top lap
o down lap
o on lap
o unconformities - truncations
can record sea level changes
Shallow seismic need to scaled down from the oil or crustal scale.
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