Seismic and geoelectrical field explorations for parameter estimation in geotechnics

Transcription

1 Bauhaus-University Weimar / Germany Seismic and geoelectrical field explorations for parameter estimation in geotechnics Site Parameter Investigations and Site Monitoring Dipl.-Ing. Frank Wuttke Dr.-Ing. Hans-Gottfried Schmidt Prof. Dr.-Ing. habil. Tom Schanz Bauhaus - University Weimar / Laboratory of Soil Mechanics Content: Site parameters, relations and aim Seismic explorations Geoelectrical explorations Joint inversion of different measurement methods Site investigation Geotechnical parameter estimation Dipl. Phys. U.Lindner Dipl.Geophys. Th. Schicht K-utec Germany

2 Site parameters, relations and aim 1. Description of exist soil stiffness or stiffness change Initial stiffnesses currently only by seismic in situ measurement ascertainable Informations to geometric structures and to characteristic values of undisturbed material 2. Description of exist soil water content / water content change / imaging Informations to geometric structures and to values of specific resistivity of undisturbed material by geoelectrical in situ measurement 3. Aim: Determination of exist porosity and water saturation by specific relations between vs, vp and ρ_specific 4. Different tests are necessary to improve the reliability of final inversion results

3 Seismic Explorations Use of Seismic Waves Advantage of surface waves: Transport of greatest energie content Specific sensitifity for shear stiffness Propagation parallel to surface frequency-dependence storage of layer structure Necessary steps for interpretation of seismic surface wave fields: 1) Experimental wave field analysis 2) Sensitivity analysis and inversion of field data Specific Problem in Geotechnics: distinct impedance differences between layers in flat sediments higher modes become stimulate Surface wave only one part of wave field: weak sensitivity to p-wave velocity, consequently refraction seismic has to used too; the best way is a joint inversion of all information

4 Seismic Explorations Experimental wave field analysis Body wave exploration: (seismic refraction and reflection method) The first arrival of p-wave give important information in seismic refraction method to layer thickness and p-wave velocity Problem: identify the to pick up arrival points and layered softer layer Surface wave exploration: Commonly: dispersion characteristics are used for further analyses 1) Phase difference method mostly used method in geotechnics Basis: phase difference on two points Advantage: - very less geophones necessary (Minimum 2 Geophones), - simple application Disadvantage: - no resolution of higher modes, - often different problems in uniqueness of phase velocity calculation

5 Seismic Explorations Experimental wave field analysis Due to problems in uniqueness and resolution of higher modes, other advanced methods added for phase velocity calculation 2) frequency wave number analysis: math. background: 2D fourier transform Advantage: numerical very robust and fast Disadvantage: weak resolution of higher modes 3) wave field and bessel transformation: math. background: linear radon transform and 1D fourier transform Advantage: high resolution of higher modes Disadvantage: complicated numerical evaluation Disadvantage of the above methods are the required high number of geophones for sufficient resolution and the identify of the extracted modes to a typ of modes at a possible models further informations necessary to find the real solution.

6 Geoelectric Explorations Measurement of the electrolytic conduction of earth materials Different Soil types variable resistivity values distinctive features Advances of geoelectrical methods: straight separation of aquiferous layers extreme sensitive to water saturation in soils sensitive to soil grain size and soil types easy testing performance and analysis applicable on different geometrical structures Essential Aim of Geoelectric Measurement Contour Mapping in Soils Basic Element in Measurements are four electrode arrays (e.g. Schlumberger and Wenner arrays)

7 Used Inversion procedure of seimic or geoelectric field data / Joint inversion of seismic and geoelectric field data geotechnical / geophysical additionally information correction of theoretical model parameter geoelectrical, seismic initial model forward calculation coupled and weigthed data difference vector δd dispersion analysis, p-wave arrival time, apparent resistivity experimental data basis correction of damping terms β error calculation weighting matrix model differenz vector δm no cancel criteria realized? yes resulting profile information to further parameters for geotechnical calculations

8 Site Investigation by Seismic and Geoelectric Methods - Synthetic example synthetic model: Wenner array ( a = 1m) and equidistant seismic array ( s = 2m) Joint inversion of surface wave and geoelectrical data: - improving and stabilize the numerical algorithm - coupling the information of different measurement methods supplementing and increasing of information at site inversion navigate to a more probable result

9 Site Investigation by Seismic and Geoelectric Methods - Site Measurement Seismic Measurements: investigation field flood area in front of a river dyke air sound exist site profile characteristic information in seismic wave field dispersion curve wave field transformation for phase velocity values Using the complete wave field information to avoid exploring problems by using only seismic surface wave method: - ambiguity of solutions - difficulties to assign dispersion curves - small influence by water table - weak p-wave sensitivity shots to different paths and directions refraction seismics for determination of p-wave arrival time

10 Site Investigation by Seismic and Geoelectric Methods - Site Measurement Geoelectric Measurements: 1D Vertical electrical sounding Wenner array apparend resistivity curve at surface by using 4 point array exist site profile exploring problems by using only geoelectric methods: similar numerical problems like surface wave (ambiguity of solutions) more influences due to water table, grain form and ions

11 Site Investigation by Seismic and Geoelectric Methods - Site Measurement Joint Inversion of Seismic and Geoelectric Measurements: 1) 2) 3) Experimental data basis: 1) phase velocity 2) apparent resistivity 3) p-wave arrival time Joint inversion: require congruence between each data basis and inversion result (a) assumption of similar layer boundaries at different physical characteristics by seismic and geoelectric measurements variable like depth is easier informations to p-and s-wave velocity coupled by specific resistivity at the layers (b) (a) Advantage - joint inversion: decrease of number of the date describing models more realistic results possible (b) Problem: instable numerics at different behaviour of physical chacteristics as assumptions

12 Determination of geotechnical parameter in sediment layers 1) Small strain parameter: 2) Porosity of soils: Knowlegde known Determination of porosity from in situ measurents were content of several publications: a) Watkins (1972), empirical relations of basis of experimental p-wave velocities b) Salem (2000); empirical relations of basis of experimental studies (vs and vp wave velocities); (saturated and unsaturated soils) b) Foti et.al. (2002) use Biot s theorie of poroelastic media for saturated soils a) Seismic measurements provide initial values for further stress-strain dependend calculations b) Important parameter in deformation characteristics of soils c) Seismic measurements supply stiffness parameter directly without correlations d) Different compare to other field methods possible and aviable (CPT, SCPT etc.) Results of matrix poisson ratio and porosity (after Foti, in saturated depth) at introduced site; n~0.45 (conventional geotechnical test) Compare of results of the different approaches

13 Determination of geotechnical parameter in sediment layers 3) water saturation in soils: According Archie(1942), Tillmann(2001) and Foti(2000) the porosity affects the mechanical and elektrical behaviour in soils: the common relation between seismic and geoelectrical measured parameter (vs, vp and specific resistivity) are porosity and pore water saturation of soil (a) Dependence of specific resistiviy of porosity (a) and of pore fluid Satuaration (b) after Tillmann. (results of numerous experimental analyses) (b) definition of the macroscopic parameter like bulk modulus, wave velocities and resistivity in dependence of grain size, porosity, pore water sateration a lack on this theories is the neglect of the influence of the pore fluid conductivity information to porosity and saturation obtained after perform the inversion procedure