Metodos de propagacion de ondas
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WS 06-07
Jan van der Kruk
Wave propagation methodsSeismics & GPR
Interpretation, Advanced processing, Related methods, Examples
Interpretation of processed (2D or D3) Seismic or GPR data
• Mapping of geological structures (seismics)• Seismic sequence analysis• Seismic facies-analysis• Hydrocarbon indicators (AVO)• Borehole measurements (seismic and GPR)
– Vertical seismic profiling– Crosshole tomography
• GPR and hydrogeophysics• 3D GPR examples• Comparison between GPR refraction and
reflection seismic.
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Assumptions by interpretation
• Coherent horizons in the processed data arereflections that are emphasized in thesection
• The impedance contrast correspond with thelayering in the subsurface⇒ Reflections reflect this layering
• Seismic details (waveform, amplitudes etc.) have their origin in the lithology
Mapping:
Position of the main horizons
Disturbances
Position and shape of faults
Aim:
geological Profile
Depth charts of Horizons and Disturbances
Analysis of geological Structures
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Horizon 1
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Fault
Fault 2
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Fence Diagram
Problem with „picking“ of Disturbances
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Ínterpretation of seismic data
3D-Seisimik Zurcher Weinland
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Seismic sequence-analysis
The procedure of picking unconformities and correlative conformities on seismic sections so as to separate out the packages involved with different time depositional units
Unconformities
Sequences are terminated by unconformities ora concordant
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Geology versus Seismic
Onlap
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Aim:
Analysis of the character of the reflections(amplitude, continuity, continuity and configuration)
inside a seismic sequenceto
predict the depositional environment
Seismic Facies-Analysis
Reflection patterns on seismic sections
Internal Structures
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Sigmoidal sequence
Hummocky sequence
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Depth sliceDepth slice
Geological interpretationGeological interpretation
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Hydrocarbon indicators
Reflection coefficients
Velocity and density of sedimentary rocks
Porosity and pore-filling fluids
Anomalies that could be associated with hydrocarbon accumulations under some conditions
depend on
depend on
• Bright spot:
Overlying rock has higher velocity than brine filled reservoir rock, lowering the reservoir rock velocity by filling it with hydrocarbon increases the contrast and increases the amplitude
• Dim spot:
Overlying rock has lower velocity than brine filled reservoir rock, filling it with hydrocarbon decreases the contrast and decreases the amplitude
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• Flat spot:
Where a well-defined fluid contact is present (gas-oil or gas-water)
the contrast may be great enough to give a fairly strong reflection
that may stand out because of its flat attitute
• Polarity reversal
Where the overlying rock has a velocity slightly smaller than that
for the reservoir rock, lowering the reservoir rock velocity by
hydrocarbons may invert the sign of the reflection, producing a
polarity reversal
E.R. Tegland, 1973, Dallas Geophysicaland Geological Societies Symposium
Bright spot:
Gas reservoir
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Dim spot
Dim spot associated with gas accumulation in porous carbonatesoverlain by shales.
Sheriff & Geldart
Flat spot
Gas condensate reservoir in the Norwegian North Sea
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Nun-River Field, Nigeria, slice close to fault
Bright spots and flat spots to indicate hydrocarbons trapped against the fault
E.R. Tegland, 1973, Dallas Geophysicaland Geological Societies Symposium
Bright spot:
Gas reservoir
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AVO analysis
Angle-dependent reflection-amplitude
Angle of incidence
Ref
lect
ion-
coef
ficie
nt
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AVO measurements
Synthetic AVO analysis
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• Reservoir properties that change during hydrocarbon extraction:– pore pressure– pore fluids (saturation, viscosity,
compressibility, fluid type)– temperature
• Secondary effects include:– compaction– porosity– density – overburden stress– fracturing– chemical changes
Time-lapse seismic
Delft, University of Technology
• Undesirables that can change with time:– Ambient noise (trucks, vessels, …)– Environmental changes (buildings, rigs, ...)– Near surface velocities and effects
(season, saturation, gas pockets, …)– Recording equipment characteristics– Acquisition parameters
(shot/receiver spacing, fold, offsets, …)– Processing parameters and software
(contractor dependent)
Time-lapse seismic
Delft, University of Technology
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Time-lapse example
Future
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Future
Borehole measurements
• VSP: vertical seismic profilingsurface- borehole measurement
• Crossholeborehole-borehole measurement
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Vertical seismic profiling (VSP)
Dept of receivers is known-> accurate Velocity-depth -Model
Travel times are less:-> Less attenuation-> Improved resolution
Improved distinguishing ofPrimaries and multiples
Direct measurement of the waveform-> Improved deconvolution
Advantages of VSP
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VSP Seismic tools
Synthetic zero-offset VSP recordSource
Receiver
Upgoing waves:Primary refl. from interface 1Primary refl. from interface 2Primary refl. from interface 3
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Downgoing waves removedby kf-filtering
Time shifting of the traces with the upholetime
Stacked seismo-gram from sha-ded corridor
Crosshole tomography
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Crosshole tomography example (GPR)
(Grimsel by H.R. Maurer)
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Observed vs Synthetic Data
100
200
300
400
500
ns
100
200
300
400
500
ns
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GPR and hydrogeophysics
• CMP measurement• Relation between GPR velocity and water
content• Acquifer characterisation: can GPR or other
geophysical techniques bridge the information gap (in terms of resolution and sampled volume) between the more traditional techniques?
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10 sin
90sinsin
vv
cc == θθ
GPR Velocity- or CMP-Measurement
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Velocity- or CMP-Measurement
2,
222 )0()(
irmsii v
xtxt +=
Velocity- or CMP-Measurement
Velocity analysis:
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362422 103.4105.51092.2102.5 rrrv εεεθ −−−− ⋅+⋅−⋅+⋅=
Topps equation: relates water content with GPR velocity
“Hydrogeophysics”
• ~99% of accessible global freshwater reserves correspond to groundwater.
• Water is an increasingly scarce, fragile resource. It is traditionally regarded as a common good and hence hugely undervalued in economic terms: at market price of table water, value of yearly global groundwater production is roughly equal to that of oil!
• Alluvial aquifers play a dominant role due their to high porosities and permeabilities as well as their inherent physical and chemical cleaning potential (filtration, oxidation of pollutants).
• Need for detailed characterization of alluvial aquifers on a local scaleas a prerequisite for protection, remediation and sustainable use.
• No geophysical technique can resolve permeability structure directly. There are, however, high-resolution geophysical techniques that are sensitive to porosity structure, which can then be related to the permeability structure using empirical, site-specific relations.
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What is “aquifer characterization” ?
• Developing a subsurface model of aquifer properties (structuraland parameter information)
• Required resolution depends on the purpose of the study
• For example, for estimating the capacity of an aquifer, averaged properties are often sufficient
• For reliable models of groundwater flow and contaminant transport, a detailed model of the subsurface is needed
A typical unconfined aquifer and its parameters of interest
unsaturated sediments
saturated sediments
bedrock
groundwatertable
bedrockdepth
Aquifer boundaries
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A typical unconfined aquifer and its parameters of interest
Internal geometries
unsaturated sediments
saturated sediments
bedrock
groundwatertable
bedrockdepth
Aquifer boundaries
A typical unconfined aquifer and its parameters of interest
• Internal boundaries
• Distribution of hydraulically relevant parameters
porosity φ
hydraulic conductivity K
unsaturated sediments
saturated sediments
bedrock
groundwatertable
bedrockdepth
• Aquifer boundaries
’
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Conventional field techniquesfor determining aquifer properties
• Core analysis and borehole logs (e.g., flowmeter logging):
⇒ high vertical resolution but low lateral coverage (resolution ∼ 10 -3 – 10 -1 m)
• Pump and injection tests:
⇒ good overall coverage but lack of resolution (resolution ∼ 10 2 – 10 3 m)
Conventional drilling in gravel and sand dominated deposits
1 m
• Difficult to recognize subsurface variations with conventional coring techniques
⇒ usually, cores represent a mix of different lithological units
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Conventional field techniquesfor determining aquifer properties
• Core analysis and borehole logs (e.g., flowmeter logging):
⇒ high vertical resolution but low lateral coverage (resolution ∼ 10 -3 – 10 -1 m)
• Pump and injection tests:
⇒ good overall coverage but lack of resolution (resolution ∼ 10 2 – 10 3 m)
• What is the potential of high-resolution geophysical tools in such studies?
• Can they bridge the information gap (in terms of resolution andsampled volume) between the more traditional techniques?
This is the topic of current research of Jens Tronicke and Klaus Holliger
3D GPR examples
• 3D Pipe / rebar detection• Landmine detection• Fault planes• Fracture detection
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GPR for Humanitarian Demining
GPR for Humanitarian Demining
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Idealized field situation keep away from surface obstacles !
on-screenGPR-data display
in real time
self tracking laser-theodolite:
GPR-coordinates +topo data for DEM
continuous profilingwith GPR antennas
and laser prism
attached to a sled
buriedlaterally displaced
paleochannel
ε1, σ1
ε2, σ2
ε4, σ4
ε3, σ3
ε1≠ε2≠ε3≠ε4
σ1−4 small
vertically and/or laterally displaced strata
San Francisco
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Where is the San Andreas Fault ?
Tasks for GPR: - locate fault zone- structural information- extrapolate localised
information - amount of displacement
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40 m23 m
5 m
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Reconstruction of prefaultinggeometry (GPR)
before faulting...
after faulting
displacement: 7.8 m
restored geometry of buried channel
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3-D migration of fault plane reflection (GPR)
3D interpretationexample
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GPR Advantages
• Simple• Quick • Cheap• High resolution• Significantly more unambiguous than
Potential field or diffusive methods
GPR Disadvantages
• Does not function always • Limited penetration depth (Tradeoff
between resolution and penetration depth)• Reliable interpretations are generally not
without additional information or a prioripossible.
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Reflection seismic Advantages
• High resolution• Functions most of the time at locations
where GPR does not work
Reflection seismic Disadvantages
• Expensive• Needs a lot of specific Knowhow and
specialized Equipment (Hard- und Software)
• Top 5-10 m mostly not interpretable• Reliable Interpretations are in general not
without additional information and/or a priori assumptions possible.
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Refraction seismic Advantages
• Simple• Quick • Cheap• Significantly more unambiguous than
Potential field or diffusive methods
Refraction seismic disadvantages
• Limited on use for simple structures• Considerably less resolution than GPR or
Reflection seismics• Does not work in the presence of velocity
inversions. (needs a higher velocity layer below a low velocity layer).