geo 395- case studies mod 8,10 Flashcards
first arrival
muted after NMO correction or deconvolution
Reasons for amplitude correction
- lateral reflector amplitude variations
- spherical divergence
- absorption
reasons for statics in land and marine
land : elevation and velocity variations weathering layer -thickness variations
marine data: source array and depth correction delay due to variations in water depth
consequences of uncorrected static shift
traces are unaligned even after NMO correction appearance of false structure
T D
time correction of datum
E D
elevation of datum
E S
source elevation
D S
depth of shot
E G
elevation of geophone
D SG
depth of shot near geophone
T UGH
uphole time at geophone location
V W
weathering velocity
V
B
velocity of bedrock
Residual statics
static correction with CDP gathers and reflection gathers used for rapid variation in near surface refraction statics and NMO
residual statics- method
use cross correlation for static shift of each trace in CDP gather
shot static
static value applied to all traces of a given shot
receiver static
static value applied to an individual (0, -1,1)
gather types
shot gather common depth pt gather common receiver gather
shot gather
shot and all its traces
common depth point gather
gather and all traces with common reflection pt
common receiver gather
gather of all traces recorded by 1 geophone group
filter
conversiono f an input signmal into an output signmal in such a way that you know the answer
cross correlation
comparison of different traces or wavelets used in residual statics
auto correlation
comparion of identical traces or wavelets output wavelets are either zero phase or symmetrical used in deconvolution
deconvolution
attempts to derive original reflection coeeficient series Rc * w(t) = s(t)
Dereverberation
removal of multiples
Kinds of multiples
- water-bottom multiples
- intra-bed multiples
- inter-bed multiples
- peg-leg multiples
- free surface multipes
water bottom multiples
intra-bed multiples
goes through 2-3 layers
inter-bed multiples
think W
peg-leg multiples
think pirate
free surface
Amplitude spectra
phase spectra
3 values for 3 component traces
CVG
CONSTANT VELOCITY GATHER
single CDP gather is NMO Corrected with a single velocity time pair
CVS
CONSTANT VELOCITY SOURCE
adjacent CDP gather and NMO corrected with a single velocity function derived from CVG
outcomes of NMP correction
3 options
correct velocity
velocity is too slow (overcorrection)
velocity is too fast (undercorrection)
NMO Equation
0
T2 = T02 + X2 / V2
T - non zero offset arrival time
T0 zero offset arrival time
V velocity
X offset
Seismic Data Display Types
wiggle trace
variable area
variable and wiggle trace
variable dencity
Display parameters
Horizontal Scale (Traces per inch or Traces per cm)
Vertical scale (Inches per sec or cm per sec)
Polarity (normal or reverse)
migration
process that repositions to its correct subsurface positions
Migration algorith
Kirchoff
Finite Difference
Bowtie —– migration —> syncline
SONIC LOG
sound generator + 4 receivers
interval velocities are calculated from travel times
used to identify P and S waves
DENSITY LOG
measures gamma ray backscatter from a rock formation
Backscatter is proportional to electron density of rock which is proportional to bulk density
Natural radioactivity has no effect on density logs
GAMMA RAY LOG
Measure of natural radioactivity
used to identify shales
Caliper Log
measurement of borehold diameter
gives info on lithology, porosity and permeability
Well Velocity Survey
identifies relations between surface geology and seismic reflection data
VSP
Vertical Seismic Profile
Downward events (primary and multiples)
increase with time and depth
Upward events (primary and multiples)
decrease with time and depth.
Recipe for constructing synthetic seismogram
- Use sonic and density log to create acoustic impedence AI = p x v
- Create reflection coefficient from AI log Rc=(P2V2 -P1V1 / P2V2+P1V1)
- Convolve RC series with a source wavelet thus creating 1 synthetic trace Rc * w(t) = s(t)
- copy synthetic trate 5-6 times
- create reverse polarity set from the set of synthetic traces due to field acquisition poliarities being opposite
- apply 3-6 bandpass filters to the normal revers synthetic traces
Acquisition reasons for mistie
source type
field filters
sample rate
field polarity
vintage
processing reasons for mistie
NMO
statitcs
deconvolution
migration
velocities
vintage
What should correlations be ?
interval best fit
time variant
filter change down seismic section
time invariant
one filter for the whole section
eg stretching and squeezing
cross correlation
comparison of 2 unequal traces
Cross correlation gives static shift necessary to align data of 2 seismic traces
auto-correlation
comparison of 2 equal traces
output is a zero phase wavelet
good for identifying short multiples
match filter
takes 2 data sets and matches frequency, phase and amplitude.
It’s a way to merge vibe and dynamite data
