A scale/resolution problem

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1 Near Surface 2005 September 4-7, 2005, Palermo Italy WORKSHOP on Hydrogeophysics a tool for sustainable use of groundwater resources Integrated Seismic and GPR characterization of fractured rocks Pipan, M., Forte, E.*, Dal Moro, G., Gabrielli P. University of Trieste, Dept. Geological, Environmental and Marine Sciences Exploration Geophysics Group, Via Weiss, 1 Trieste Italy eforte@univ.trieste.it Web site:

$(joints, layers, fractures, aults). illing material estimation.$ iscontinuities network (links, favourite ater/fluid directions, ).

ertical and lateral lithological variations.

2 Statement of the problem A scale/resolution problem icro scale (cm-m): iscontinuities (joints, layers, fractures, aults). illing material estimation. iscontinuities network (links, favourite ater/fluid directions, ). oids (Karstic phenomena, open ractures, ). ertical and lateral lithological variations. Macro scale (m-km): Global rock mass characteristics. Homogeneity zones (from geological, hydrogeological, geomechanical point of view). Main discontinuities (large faults, bedrock). Voids (caverns, tunnels).

3 Objectives Summary Imaging of rock mass characteristics (joints, fractures, voids, internal structures and heterogeneous volumes) 3D patterns reconstruction Definition of rock mass homogeneity zones Methods - SURFACE and BOREHOLE GPR - SEISMICS: Multi channel Analysis of Surface Waves - MASW Results 1) Imaging of rock discontinuities 2) Imaging of lithologic variation 3) Definition of different rock mass parameters 4) Geomechanical hydrogeological problem assessment Conclusions - Remarks

GPR VERTICAL RESOLUTION and PENETRATION Tracce

120ns 200 MHz 160ns 10m 200ns 0m Tracce 51 101

MHz 160ns 10m 200ns Tracce 120ns 50 MHz 10m

4 GPR VERTICAL RESOLUTION and PENETRATION Tracce 0m m 40ns 80ns 120ns 200 MHz 160ns 10m 200ns 0m Tracce m 40ns 80ns 120ns 100 MHz 160ns 10m 200ns Tracce 0m m 40ns 80ns 120ns 50 MHz 10m 160ns 200ns

5 GPR processing: 200 MHz STACK and MIGRATION Trace n ns Stack section 80ns 120ns S A Trace n Pre-stack Kirchhoff depth migrated (time converted) section 40ns 80ns 120ns A S

6 GPR INTERPRETATION: Layering, fractures, faults and cavities

7 N Distanza lungo i profili (in m) Inline distance 5m 0m 0m 3D reconstruction and interpretation 10m 15m 190ns 20m 5m 90ns 130ns 190ns Distanza tra i profili (in m) 50ns 170ns 70ns 90ns 30ns 130ns 150ns 110ns 50ns Crossline distance ns 200ns 190ns 180ns 170ns 160ns 150ns 140ns 130ns 120ns 110ns 100ns 90ns 80ns 70ns 60ns 50ns 40ns 30ns 20ns 10ns Example of ISOPACH SURFACE reconstruction (TWT in ns) Precise geological attitude determination: strike dip direction of dip and local variations

8 GPR interpretation and validation 0m 6.0m 0m Depth [m] 0m 6.0m 3.7m 0m Depth [m] 3.7m Interpreted migrated section 800 MHz

9 3D discontinuities reconstruction

10 Surface GPR Constrains and Limits 1. Maximum reachable penetration depth (high conductivity sediments on surface) 2. Good definition of single rock discontinuities but poor information on global rock mass characteristics 3. Resolution and imaging limits for: - very thin fractures (virtually no electromagnetic contrast) - vertical/sub-vertical discontinuities To overcome some of these limits GPR tomography

11 Borehole GPR Data acquisition methods Reflection T1 R1 T2 T2 T3 R2 T4 T3 R3 T5 Tomography T1 T2 T3 R1 R2 R3 R4 R5 VRP T1 T2 T3 T4 T5 R1 T and R into the same borehole T and R into different boreholes T into a borehole R on the surface

12 Borehole GPR measurements: Tomography T R

13 A Borehole GPR Tomography example Boreholes Location Map BOREHOLE 1 Azimuth=84 Dip=50 Depth= 15.5m BOREHOLE 2 Azimuth=260 Dip=68 Depth= 20m Geological settings: Grey or blackish limestone, with laminithic levels characterised by different organic material content. Presence of fractures locally with karstic phenomena and vertebrate fossils. P. Near Surface 2005

14 A Borehole GPR Tomography example Example of borehole GPR tomography acquisition scheme: Tx increment = 50cm Rx increment = 10cm T1 Rx1 RxN T2 RxN Rx1 T3 Rx1 RxN

15 A Borehole GPR Tomography example First break picking Traveltime inversion velocity Amplitude inversion attenuation

16 6Db/ orehole-1 Borehole-2 A Borehole GPR Tomography example Velocity Field Velocity [mm/ns] 12cm/ns 9cm/ns Attenuation field Attenuation Db/m 20Db Borehole-1 Borehole-2

17 A Borehole GPR Tomography example Ground surface 5.0 Distance 0 NORTH SOUTH Distance Approximate depth (m) Velocity [mm/ns] Approximate depth (m)

18 An integrated approach: seismic MAIN OBJECTIVE: efinition of global rock mass parameters useful for geologica hydrogeological, geomechanical, environmental problems Which method? Refraction Reflection Borehole Tomography 2D 3D P waves S waves or more?

19 Why surface waves? 1. The percentage of energy converted into Rayleigh waves is by far higher (67%) with respect to the energy involved in the P (7%) and S (26%) wave generation 2. Surface wave amplitude depends on r and not on r (body waves)

20 Why surface waves? 3. Differently than the refraction method, surface wave analysis does not suffer for limitations due to the presence of possible velocity inversions 4. Rayleigh wave velocity is mainly influenced by the shear wave velocity fundamental parameter for many geomechanical/geotechnical analyses 5. Applicable also with low impedance contrasts 6. Very simple acquisition and pre-processing required 7. Low overall costs 8. No a-priori constrains

21 MASW: Two steps 1. Dispersion curve picking 2. Dispersion curve inversion

22 A new implemented program Velocity spectrum determination Dispersion Curves inversion

23 Dispersion curves determination and picking 24 traces, bandpass filtered ( Hz) and offset balanced Common-shot gather F-K τ-p Principal Phase Shift mode Higher modes Phase Shift method was selected and applied to data according to an adopted velocit A sum is then performed for each considered frequency (see e. g. Park et al., 1998, SEG, Expanded Abstracts, )

24 Dispersion curves inversion Main problem: it is a multi-modal problem and common linear methods typically strongly depend on the initial model Genetic Algorithms for Surface Wave Inversion It is a global search tool able to explore a wide search space and exploit the btained information starting with very low constrains and required assumptions. The algorithm can identify a mean model (and not only the last model) and calculates the standard deviations for each considered variable. For further details see e.g.: ia J., et al., Utilization of High-Frequency Rayleigh Waves in Near-Surface Geophysics, he Leading Edge, Vol. 23, No. 8, al Moro G., et al., Rayleigh Wave Dispersion Curve Inversion via Genetic Algorithms and arginal Posterior Probability Density Estimation, submitted to the Journal of Applied Geophysics.

Dispersion curves inversion: parameters Fixing boundaries of the search space for all the chosen variables Number of strata All these parameters can be set as

25 Dispersion curves inversion: parameters Fixing boundaries of the search space for all the chosen variables Number of strata All these parameters can be set as wide as possible. The only limitation to reach the final model is the Population number Generation number Crossover rate Mutation rate request CPU-TIME Velocity range

12 Search space Mean (best reliable) velocity model

26 Dispersion curves inversion The inversion procedure is performed on several close seismic shots SHOT 1 SHOT 12 Search space Mean (best reliable) velocity model for Shot-1 Mean (best reliable) velocity model for Shot-12

ASW-reflection seismic comparison and validation WASTE DISPOSALS AND LANDFILL SILTY SAND GRAVEL SILTY CLAY Borehole P1 0m 5m 0 5 Borehole P1 DISTANCE [m] 0 5 10 15 20 25 0 5

27 ASW-reflection seismic comparison and validation WASTE DISPOSALS AND LANDFILL SILTY SAND GRAVEL SILTY CLAY Borehole P1 0m 5m 0 5 Borehole P1 DISTANCE [m] SILTY SAND AND SANDY SILT WITH ORGANIC LEVELS GRAVEL 10m 15m Depth [m] FRACTURED GRAY LIMESTONE 20m Shear-wave depth converted reflection seismic section 30

ASW-reflection seismic comparison and validation WASTE DISPOSALS AND LANDFILL SILTY SAND GRAVEL SILTY CLAY 0m 5m 0 5 DISTANCE [m] 0 5 10 15 20 25 0 5 SILTY SAND AND SANDY SILT WITH ORGANIC LEVELS 10m

28 ASW-reflection seismic comparison and validation WASTE DISPOSALS AND LANDFILL SILTY SAND GRAVEL SILTY CLAY 0m 5m 0 5 DISTANCE [m] SILTY SAND AND SANDY SILT WITH ORGANIC LEVELS 10m GRAVEL FRACTURED GRAY LIMESTONE 15m 20m Depth [m] Depth [m] Velocity [m/s] 100 DISTANCE [m] MASW Velocity profile Depth [m] Depth [m]

29 Conclusions - Remarks Geophysisics can give important information with different scale levels that are very difficult to achieve with direct methods. PR profiles and borehole tomography allow unambiguous high resolution D and 3D imaging of bedding planes, fractures, joints and cavities also n complex environments. his techniques are particularly efficient if performed with antennas irectly coupled with the rock surface and with a required penetration epth not exceeding 20-30m or a similar distance between boreholes. n order to overcome this constrain, and to assess some global rock ass parameters Seismic MASW techniques can be very useful in terms f costs and reliability of results. urther efforts are required to obtain more detailed quantitative nformation.

30 Thank you for the attention! Pipan M., Forte E.*, Dal Moro G., Gabrielli P. University of Trieste - Exploration Geophysics Group Dept. Geological, Environmental and Marine Sciences ViaWeiss, 1 Trieste Italy eforte@univ.trieste.it Web site:

SEISMIC RADAR AND ELECTRICAL TECHNIQUES FOR WASTE DISPOSAL ASSESSMENT. M. Pipan, G. Dal Moro, E. Forte & M. Sugan

SEISMIC RADAR AND ELECTRICAL TECHNIQUES FOR WASTE DISPOSAL ASSESSMENT M. Pipan, G. Dal Moro, E. Forte & M. Sugan Department of Geological, Environmental and Marine Sciences, University of Trieste Via Weiss,