Roger Wisén 1 , Anders Vest Christiansen 2 , Esben Auken 2

1. Kombineret inversion af MEP og seismik data med boringsoplysninger som á prioriCombined inversion of MEP and seismic data with drilling data as prior information Roger Wisén1, Anders Vest Christiansen2, Esben Auken2 • Lunds Tekniska Högskola, Sverige • GeoFysikSamarbejdet, Aarhus Universitet Tack till: CityTunnelProjektet och Nordisk Forskerutdanningsakademi

2. 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 Disposition

3. Railway trench in Malmö, Sweden

4. Map of the investigated area

5. Main aquiferes Geology och hydrogeology • Postglacial sediments • Clay till • Intermorainic sediments • Limestone

6. 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

7. Multi Electrode Profiling - MEP • Combination of Wenner and Schlumberger measurements • 2 m minimum electrode separation • About 25-30 m depth penetration

8. 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 Disposition

9. Profile along the planned trench Pseudo-section 2D smooth inversion 2D LCI Residuals

10. Profile along the planned trench 2D LCI 2D LCI with prior data

11. 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 Disposition

12. ………………….. VPh (m/s) Seismic source Geophones FFT f (Hz) VS(m/s) VPh(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 Dispersion-curve Velocity-modell Depth(m) f(Hz) Multichannel Analysis of Surface Waves - MASW

13. Combined LCI onresistivity and seismic data

14. Combined LCI onseismic and resistivity data LCI on resistivity data Separate LCI on seismic and resistivity data Combined LCI on seismic and resistivity data

15. 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 1

16. 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 Summary – part 2

17. Resultat från resistivitetsmätningar Tvärprofil