UBC-GIF: Capabilities for EM Modelling and Inversion of LSBB data

Transcription

1 The University of British Colubia Geophysical Inversion Facility slide 1 UBC-GIF: Capabilities for EM Modelling and Inversion of LSBB data Douglas W. Oldenburg Departent of Earth and Ocean Sciences June 12, 29

2 LSBB: Karst Aquifer Characterization slide 2 Principle Questions: What is the underground atrix Hydrogeologic properties Storage capacity Production capacity Potential for pollution Sustainability What is the role of geophysics?

3 slide 3 Fraework for Applied Geophysics What is the question to be answered? What are the diagnostic physical properties? Density Magnetic susceptibility Electrical conductivity Electrical perittivity Elastic paraeters Choose survey type Collect data Invert data to get physical property odel Interpret odel and synthesize with other data

4 Geophysical Experient slide 4 Physical property: Electrical conductivity Survey: DC Resistivity Collect data DC survey Data: pseudosection Data iages cannot be directly interpreted in ters of geology Data ust be inverted

5 What is Inversion? slide 5 Data Measureents over the Earth are data.? Model Inversion processing Inversion estiates Earth odels based upon data and prior knowledge.

6 The inverse proble slide 6 Geophysical data are: F[] + = d : odel --- unknown F: forward apping operator : errors d: observations (data) Given: data, errors, a forward odelling ethod Find: the odel that generated easureents. Major Difficulty: Nonuniquenes? F -1

7 Inversion as optiization: 3 parts slide 7 Misfit: obs 2 A priori inforation: reference odel, structural detail... Model objective function: ( ) d Inversion as optiization: s S N i1 Fi [ ] d i 2 ( ) ( ) dv x dv... x S i = d +. < < is a constant Choose such that d < Tolerance 2 i : standard deviation s, x constants : reference odel

8 Nuerical solution slide 8 Discretize: Divide the earth into M cells of constant physical property (M>>N). Miniize d W d 2 obs ( F[ ] d ) W ( ) 2 Use the Gauss-Newton ethod for solution. Solving for : - Discrepancy principle. -GCV. - L-Curve. 1 M

9 slide 9 Nuerical solution (Gauss-Newton ethod) Miniize set 2 2 ) ( ] [ W d F obs d ) ( ) ] [ ( ) ( ) ( W W d F J g T obs T j i ij d J J is the sensitivity atrix:

10 slide 1 Nuerical solution (Gauss-Newton ethod) Miniize set 2 2 ) ( ] [ W d F obs d ) ( ) ] [ ( ) ( ) ( W W d F J g T obs T j i ij d J J is the sensitivity atrix: J F F ) ( ] [ ] [ Expand forward operator (dropping higher order ters):

11 slide 11 Nuerical solution (Gauss-Newton ethod) ) ( ) ( ) ( k T k T k g W W J J where k is the odel at the k th iteration. This is an M x M syste of equations. (M = # odel paraeters, or cells) Solve: ) ( ) ] [ ( ) ( ) ( W W d F J g T obs T J F F ) ( ] [ ] [ Expand forward operator (dropping higher order ters): Miniize set 2 2 ) ( ] [ W d F obs d j i ij d J J is the sensitivity atrix:

12 Inversion Capabilities 2 3D UTEM slide 13 Gravity (3D) Magnetics (3D) DC resistivity and IP (2D and 3D) Frequency doain EM (1D and 3D) Tie doain EM (1D and 3D) Software for inversion is distributed world wide through UBC and third party vendors

13 Field Exaple: San Nicolas Deposit slide 14 Location Geologic cross section Elevation () 2 16 Mafic Volcanics Quartz Rhyolit e Keel Massive Sulphide Tertiary Breccia Mafic Volcanics -2 Easting () -11 Unit Physical properties Density Susceptibility Resistivity Chargeability (g/cc) (S.I. x1) 3 ) (oh- (oh) ) (sec) Qal Tv 2.3 (2) Mst./Lst Mafic Vol Mafic/IntVol Sulphide Qtz. Rhyolite Graphitic Mst

14 Gravity 6 Observed Data San Nicolas slide Northing () Easting () -6 6 Predicted Data Northing () Easting () Gravity data collected at the San Nicholas deposit.

15 Gravity Inversion Results slide 16 Cross-section of Density Contrast Model with Geology Density contrast Depth () g/cc 8-22 Easting ()

16 Magnetics -1 Observed Data slide 17-7 Northing () Easting () nt Predicted Data Northing () Easting () Magnetic data collected at the San Nicholas deposit.

17 Magnetic Inversion Results slide 18 Cross-section of Magnetic Susceptibility Model with Geology O 1 Depth () Easting () -8

18 Field Exaple: San Nicolas Deposit slide 19 Geologic cross section Elevation () 2 16 Mafic Volcanics Quartz Rhyolit e Keel Massive Sulphide Tertiary Breccia Mafic Volcanics -2 Easting () -11 Density Magnetic Susceptibility Electrical Conductivity Chargeability Unit Physical properties Density Susceptibility Resistivity Chargeability (g/cc) (S.I. x1) 3 ) (oh- (oh) ) (sec) Qal Tv 2.3 (2) Mst./Lst Mafic Vol Mafic/IntVol Sulphide Qtz. Rhyolite Graphitic Mst

19 Electrical Conductivity: Different Surveys slide 2 Sources: Galvanic (grounded electrodes) Inductive (current loops) Wavefors Sinusoidal (Frequency doain I Wavefor Tie wavefors (Tie doain) I Tie Tie

20 3D EM: Frequency Doain Wavefor slide 21 Source: Loop or grounded electrode I Tie Data (E,H) Borehole Data (E, H) data E iωh Depth H ( σ iω ) E J e

21 CSEM Survey transitter 3.5 k 1.6 k slide Frequencies between.5hz, 8192 Hz 3 lines, 1.6k long, 2 apart 25 eter station spacing single transitter data are scalar ipedances (Ex/Hy) data collected with the goal of MT interpretation receivers

22 3D CSEM Inversion slide 23 Frequencies.5, 8, 64, 256 Hz Iso-surface cutoff 1 Oh-

23 3D TEM Setup Wavefor slide 24 Source (Loop or grounded electrode) I Tie (half sine, step ) Surface Data (E, H, db/dt) data Borehole Data (E, H, db/dt) Depth

24 Introduction to UTEM data at San Nicolas slide 25 3 large loop transitters 2 k by 1.5 k db/dt receivers ainly z coponent I(t) 15 s transitter wavefor 3 Hz sawtooth wave di/dt constant over half cycle di dt db dt

25 San Nicolas UTEM data 175 UTEM channel 4 (1.513s) slide 26 Loop 2 Loop 1 db z /dt northing Loop easting -22 nt/s

26 Fitting the data slide 27 Observed 15 iso-surface 1. Observed 31. View fro SW 1. One decay curve: Observed and predicted Predicted db z /dt nt/s observed predicted log1(t)

27 San Nicolas inversion results: slide 28 Recovered cross section at 45 S easting 5 Resistivity fro drilling at 45 S easting 5

28 Density, Magnetic susceptibility, Conductivity slide 29 Density contrast 3D CSEM Magnetic susceptibility 3D UTEM

29 Field Exaple: San Nicolas Deposit slide 3 Geologic cross section Elevation () 2 16 Mafic Volcanics Quartz Rhyolit e Keel Massive Sulphide Tertiary Breccia Mafic Volcanics -2 Easting () -11 Density Magnetic Susceptibility Electrical Conductivity Chargeability Unit Physical properties Density Susceptibility Resistivity Chargeability (g/cc) (S.I. x1) 3 ) (oh- (oh) ) (sec) Qal Tv 2.3 (2) Mst./Lst Mafic Vol Mafic/IntVol Sulphide Qtz. Rhyolite Graphitic Mst

30 Induced Polarization slide 31 Source Current (Aps) Measured Voltage (Volts) Non-zero area occurs because charges took tie to equilibrate. IP datu: Vs/Vn V V V s (t) Collect IP data along with DC resistivity data

31 DC/IP data at San Nicolas slide 32 Pole-dipole Real Section Pseudo-section 3D Inversion Chargeability

32 Suary of physical property inversion at San Nicolás Density contrast Magnetic susceptibility slide Chargeability D UTEM Sulfide: dense, chargeable, susceptible, conductive.

33 Back to Karst Aquifers: LSBB slide 34 Soil Epikarst Unsaturated Saturate What scale? Which physical properties?

34 Back to Karst Aquifers: LSBB slide 35 Tunnel Scale (eters to k): Conductivity (indicative of water) (DC resistivity, FEM or TEM) IP ight be useful for clay layers if they are chargeable Tie-lapse DC (EM) resistivity can provide inforation about hydraulic conductivity Electrical perittivity (GPR)

35 Geophysics for large scale aquifer slide 36 Conductivity Airborne EM for 2 eters (Epikarst delineation) ZTEM or MT for deeper structure (large voids/conduits, depth of saturated zone)

36 Geophysics for large scale aquifer slide 37 Density: (Change in water volue) Magnetics?? Self-potential (fro fluid otion) Chargeability?? Self-potential, MRI, Seisic Good news fro geophysics side: We can invert ost types of survey data to recover 3D distribution of physical properties.

37 Thank you! slide 38