Geophysical mapping and imaging of soil structures: basic overview

Transcription

1 Geophysical mapping and imaging of soil structures: basic overview Stéphane Garambois Landslide team, LGIT, Université Joseph Fourier 3D imaging of the water table (seismic) Geophysical methods 1

2 Plan 1- Introduction to environmental geophysics 2- Active potential methods 2.1 Electrical 2.2 Electromagnetics 3- Wave methods 3.1 GPR (Ground Penetrating radar) 3.2 Seismic Geophysical methods 2

3 1. Environmental geophysics Relatively new fields compares to oil and mine investigations Fast methods, low cost for preliminary investigations Large scale, 1D, 2D and 3D imaging, non intrusive, monitoring Necessity of independent calibrations (geotechnical, geochemical, ) Questions : Which method to use? Sensitivity : chemico-physical parameter Depth / Resolution Price, time, scale Modern research : number of developments is increasing Geophysical methods 3

4 Geophysical methods 4

5 Sensitivity to geophysical parameters Electrique Geophysical methods 5

6 How do geophysists see soil/rocks? porous media Grain matrix Porosity Fluids (velocity, density, susceptibility, permittivity, conductivity, clays ) + + Nature of the fluid, saturation, viscosity, salinity, presence of pollution,. Chemico-physical parameters of each of the phases Physical parameters of the rocks/soil Geophysical methods 6

7 I. 2 Classification of geophysical methods Profile Observation of a geophysical anomaly: 2D or 3D? Mapping Geophysical methods 7

8 Classification of geophysical methods Images after inversion (tomography) Images in reflexion mode (GPR, seismic) Geophysical methods 8

9 Characteristics of geophysical methods Advantages Light, nondestroying and varied techniques Important Volume of investigation Obtaining images in 2D and 3D Limitations Geophysical methods 9

10 2.1 Electrical methods Geophysical methods 10

11 Sensitivity to resistivity to water (without clays) Geophysical methods 11

12 Resistivity values Geophysical methods 12

13 2.1 Electrical prospecting - Apparent resistivity is measured on the field -It results from the volume integration of all the soil cells crossed by the current lines as a function of the distance from the source - If the soil is homogeneous, apparent resistivity=true resistivity Profondeur (m) To measure the resistivity, we use a quadripole (dipoles of injection of current and of reception of the induced potential difference V Geophysical methods 13

14 Depth penetration as a function of injection electrodes spacing Profondeur (m) Espacement = 10 m Profondeur (m) Espacement = 50 m 50 % of the total injected current flows on a depth lower than electrode spacing. To increase penetration, we increase electrode spacing. Geophysical methods 14

15 Measurement configuration : profiling Geophysical methods 15

16 Measurement method : sounding Geophysical methods 16

17 Measurements method:electrical tomography Geophysical methods 17

18 Application :waste site Geophysical methods 18

19 Agricultural site : nitrates Geophysical methods 19

20 Peat bog of Chirens Geophysical methods 20

21 Water infiltration monitoring Geophysical methods 21

22 Geophysical methods 22

23 2.2 Electromagnetic methods (low frequency) Generalized schematic of the EM surveying method. Both the primary and secondary magnetic fields are shown Geophysical methods 23

24 EM instruments EM31 EM34 EM38 EM61 Geophysical methods 24

25 Geophysical methods 25

26 Everglades National Park Site characterization (Fugro) The main fresh water/ salt water interface is clearly mapped on all the coplanar resistivities as the red-orange to green-blue colour change. Other colour changes represent subtle variations in groundwater salinity, blue as fresh, red-purple as salt. The 56,000 Hz data is mapping the surface conductivity, and so shows lower conductivities as the fresh water rides on top of the salt water Hz 900 Hz Geophysical methods 7200 Hz 26

27 4. Wave propagaating methods for imagery 4.1 GPR (géoradar) : Dielectric permittivity 4.1 Seismics (reflection, refraction) Geophysical methods 27

28 Ground Penetrating Radar (GPR) : high resolution Only in resistive media!!!!!!!!!!!!!!!! Geophysical methods 28

29 Radar equation Maxwell equations can be reduced to (homogeneous, isotropic): 2 r E = µ : magnetic permeability (µ = µ 0 µ r ) σ :electrical conductivity ε : dielectric permittivity (ε= ε 0 ε r ) r δe μσ δt r δ 2 E + με δ 2 t (1) + (2) (1) : diffusion term (2) : propagation term Geophysical methods 29

30 Geophysical methods 30

31 GPR data on a Peat bog 100 MHz Distance (m) Time Depth (ns) (m) CMP1 Pinede CMP Depth (m) Depth (m) P1 Profile 100 MHz Geophysical methods 31

32 CMP acquisition (Annan) Geophysical methods 32

33 Water content derived from GPR CMP data 0 CMP1 Offset (m) CMP2 Offset (m) CMP8 Offset (m) Depth Permit. Water content (Topp et al) CMP1 2.6 m % CMP % CMP8 2.2 m % Geophysical methods 33

34 Studies of the unsaturated zone Geophysical methods 34

35 Geophysical methods 35

36 Ground Penetrating radar: glacier of Sarennes (avril 2006) : Geophysical methods 36

37 Géométrie des rais Angle d incidence Rai incident V 1 ρ 1 A I i 1 A R i 1 Rai réfléchi Loi de Snell-Descartes V 2 ρ 2 i 2 A T Rai Transmis réfracté Si V1 < V2 ρv = Impédance accoustique ρ = Masse volumique V = Vitesse Geophysical methods 37

38 Classification of geophysical methods Seismic data Acquisition of seismic data Modèle de terrain Section sismique

39 Characteristics of P and S velocities Geophysical methods 39

40 Geophysical methods 40

41 Exemple de tomographie sismique Geophysical methods 41

42 2.3 Résonance Magnétique Protonique Geophysical methods 42

43 Geophysical methods 43

44 Geophysical methods 44

45 Geophysical methods 45

46 Geophysical methods 46

47 3. Méthode électrique passive : PS Geophysical methods 47

48 Geophysical methods 48

49 Geophysical methods 49