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).