Kombineret inversion af MEP og seismik data med boringsoplysninger
som á priori
Combined inversion of MEP and seismic data with drilling data as prior information
Roger Wisén1, Anders Vest Christiansen2, Esben Auken2
1. Lunds Tekniska Högskola, Sverige2. GeoFysikSamarbejdet, Aarhus Universitet
Tack till:CityTunnelProjektet ochNordisk Forskerutdanningsakademi
Disposition
• Introduction to the case
• Geology and hydrogeology of the investigated area
• Aim of the investigation
• Part 1 – Combined interpretation of resistivity models from 2D smooth inversion and 2D-LCI
• Part 2 – Combined inversion of seismic and resistivity data
• Summary
Railway trench in Malmö, Sweden
Map of the investigated area
Geology och hydrogeology
•Postglacial sediments
•Clay till
•Intermorainic sediments
•Limestone
•Main aquiferes
Aim of the investigations
Problem:
•Inflow of groundwater from the limestone or intermorainic sediments during construction of the trench
•The effects can be reduced if risk areas are known in advance
Aim:
•Map depth to limestone
•Map extension, thickness and composition of intermorainic sediments
Trench
Multi Electrode Profiling - MEP•Combination of Wenner and Schlumberger measurements
•2 m minimum electrode separation
•About 25-30 m depth penetration
Disposition
• Introduction to the case• Geology and hydrogeology of the investigated area• Aim of the investigation• Part 1 – Combined interpretation of resistivity models
from 2D smooth inversion and 2D-LCI• Part 2 – Combined inversion of seismic and resistivity data• Summary
2D smooth inversion
Pseudo-section
2D LCI
Residuals
Profile along the planned trench
Profile along the planned trench
2D LCI
2D LCI with prior data
Disposition
• Introduction to the case
• Geology and hydrogeology of the investigated area
• Aim of the investigation
• Part 1 – Combined interpretation of resistivity models from 2D smooth inversion and 2D-LCI
• Part 2 – Combined inversion of seismic and resistivity data
• Summary
Multichannel Analysis of Surface Waves - MASW
…………………..
Seismic source Geophones
f (Hz)
VPh (m/s)
FFT
f(Hz)
VPh(m/s)
Dispersion-curve
Depth(m)
VS(m/s)Primary model parameters:
Vs – Shear wave velocity [m/s]
T – Lagertjocklek [m]
Secondary model parameters:
Ny – Poisson’s ratio [m]
Rho – Density [kg/m3]
Invers modelling
Velocity-modell
Combined LCI onresistivity and seismic data
LCI on resistivity dataSeparate LCI on seismic and resistivity data
Combined LCI on seismic and resistivity data
Combined LCI onseismic and resistivity data
Summary – part 1• Smooth 2D inversion provides high lateral resolution• 2D-LCI provides sharp layer boundaries• The combined interpretation of the two resistivity models increase the
possibility to make a precise geological interpretation• The use of a large amount of prior information in 2D-LCI, without increase of
the data residual error, gives great confidence in the resistivity data• The prior information helps solving equivalence problems and decreasing 3D
effects• The geological interpretation points out a few important features
– The limestone level in the eastern part of the profile is very close to the planned bottom of the trench
– Intermorainic sediments with large lateral extension coincide with the planned position of the trench
– In some areas these sediments are high resistive and therefore expected to have high hydraulic permeability
• Further investigations, mainly test pumping and construction of test pits, have already been started
Summary – part 2
• Combined inversion of resistivity and seismic data functions very well in this case
• The seismic model is improved for the combined inversion– Depths to layers in the velocity model gets better determined
when inverted together with resistivity data...
– ... which results in better determined velocities
• With more seismic data available combined inversion could help to solve high resistive equivalence problems
Resultat från resistivitetsmätningarTvärprofil