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 Wisén 1, Anders Vest Christiansen 2, Esben Auken 2 1.Lunds Tekniska Högskola, Sverige 2.GeoFysikSamarbejdet, Aarhus Universitet Tack till: CityTunnelProjektet och Nordisk 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 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

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) V Ph (m/s) FFT f(Hz) V Ph (m/s) Dispersion- curve Depth(m) V S (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/m 3 ] Invers modelling Velocity- modell

Combined LCI on resistivity and seismic data

LCI on resistivity data Separate LCI on seismic and resistivity data Combined LCI on seismic 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 resistivitetsmätningar Tvärprofil