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