Last week’s problems a) Mass excess = 1/2πG × Area under curve 1/2πG = 0.0239 × 10 11 in kgs 2...
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Transcript of Last week’s problems a) Mass excess = 1/2πG × Area under curve 1/2πG = 0.0239 × 10 11 in kgs 2...
Last week’s problems
a) Mass excess = 1/2πG × Area under curve1/2πG = 0.0239 × 1011 in kgs2m-3 Area under curve = -1.8 ×10-6 x 100 m2s-2
So Mass excess (actually deficit) = -4.3 ×105 kg per m
b) The anomaly decreases to zero in 10-15 m. This suggests that z is < 5m.
c) The area of the car park = excess mass per unit length/density contrast, hence the area = 4.3 ×105/1800 = 239 m2
Assuming a rectangular shape, the height is 239/100 = 2.39m.
Problem 4
Problem 5
x1/2 = 2.3 cm = 575 m
For a spherical body zmax = 1.3 x1/2 = 750 m
Today’s lecture Seismic reflection
OverviewData collection - Sources and receiversTheorySeismic reflection processingTwo new developmentsProblems 6 and 7
Marine streamer carries 100s of hydrophones. Detect changes in pressureBirds on streamer keep it levelLand receivers detect movement Reflections occur at boundaries where velocity and/or density change
Horizontal distance ~10 km ~1500 traces
TW
TT
(se
cond
s)
Earth’s surface
~6 kmdepth
Oil explorationcolour = reflection amplitude
0 kmdepth
1 trace
3D image subsurface
Slice at t = 400 ms
Can also track horizons andgenerate fly-bys
Seismic stratigraphy
Unravel depositional changes with time
Normally only interested in p-wave (smallest amplitude/fastest)The higher the frequency the better – reflections appear sharper
Velocity versus density
Seismic boat
Airgun array10-20 guns
Tows 6 streamers at once, each 6 km longEach streamer ~480 channels (group of hydrophones)
5 am in Derbyshire
Dynamite buried ~ 5 m below surface
Packed with filler
Encourage energy to travel down – not up
Ideal source – high frequency
Truck vibrates up and down to produce a continuous seismic signal
Horizontal (S-wave)source
A hammer source
Geophones
Ocean-bottom seismometers Broad band seismometers
i
r
A
AR
i
t
A
AT
1122
1122
vv
vvR
1122
112
vv
vT
For small angle of incidence
Gas beds produce high-amplitude (bright) negative reflections
QA
At ftexp
0
Rays represent the direction of travel of the seismic wave, and are perpendicular to the 3D wavefront
The energy in the wavefront decreases in with increasing distance from the source (geometrical spreading) and is inversely proportional to 4πr2. The amplitude of a seismic wave is proportional to the square root of energy, hence amplitude is proportional to r-1
Absorption and scattering also decrease the energy in the seismic wave as it passes through the Earth
This decrease in energy with increasing travel time (so-called attenuation) can be written as:
Three effects combined
White = primaryRed = source ghostBlack = multiple
Attenuation - decrease in amplitude and frequency content
Multiples and ghosts - wavelet extended
Seismic reflection processing
Raw data contains noise, refractions, S-wave reflections, surface waves, multiples
The aim of processing is to try to remove all of these and leave in only the primary reflections
Filtered trace
Filtered trace
Raw data from a broad-band
seismometer
StaticsIf sources and receivers are not horizontal then reflections from interfaces will not line up correctly in the time section.
After applying statics, horizontal reflectors will appear horizontal
tx
v
z
vor t
x
vtx x
2
2
2
2
22
2
2 0
24 t
z
v0
2
Reflections are hyperbolic
Stacked trace gets plotted at x position that lies at midpoint of traces being stacked
NMO t tx
V tx 0
2
2
02
Test velocity (2-5km/s)
Pow
er o
f st
acke
d tr
ace
at e
ach
velo
city
(A
mpl
itude
2 )
Pick velocity that gives highest amplitude (white dots)
Multiples and S-wave reflectionshave lower velocities thanprimaries
Refractions are not hyperbolic
Semblance plot
Deconvolution
Run a autocorrelation on the seismic tracesto determine the effective wavelet (The wavelet at t = large)
Design an inverse filter (the deconvolution) to convert the wavelet back
As the distance between two reflectors decreases, their reflections start to overlap, and eventually do not appear as two separate reflections.
The minimum vertical resolution (the closest two reflectors can be and still be identified as two separate reflectors) is assumed to be ~ ¼ seismic wavelength
A reflection does not arrive from one single reflection point – but from circular area. Thus small gaps in a reflector will not be obvious in the seismic data
The minimum horizontal resolution (the smallest observable gap) is assumed to be ~ the width of a Fresnel zone
21
2
1
zw
Relatively new developments
4D time lapse
Survey 1985and 1999
Track changes in reservoir
State-of-the-art
Permanent arrayson sea bed
Real-time monitoringof reservoir
e.g. Hydro-fracturing