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Sediment Properties Determined through Magnetotellurics

Sediment Properties Determined through Magnetotellurics. By: Andrew Frassetto University of South Carolina July 17, 2002. Outline:. Overview of field area The Magnetotelluric Method Examples of MT Curves and 1-D Inversion Models Description of the Geoelectric profile

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Sediment Properties Determined through Magnetotellurics

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  1. Sediment Properties Determined through Magnetotellurics By: Andrew Frassetto University of South Carolina July 17, 2002

  2. Outline: • Overview of field area • The Magnetotelluric Method • Examples of MT Curves and 1-D Inversion Models • Description of the Geoelectric profile • My focus: Determining sediment properties of shallow, low resistivity layer • Problems in determining sediment properties using Archie’s Law (and Wyllie’s Equation) • Summary of results and interpretations regarding porosity and seismic velocity • Implications of the sediment properties

  3. Field Area: MT/TEM Line N 8 km

  4. The Basics of MT: • Low frequency, passive, deep imaging of the Earth’s lithosphere • Traditionally uses Ex and Ey, along with Hx, Hy, Hz (Titan-24 System did not measure Hz) (Jiracek et al., 1995)

  5. MT at SAGE 2002: • On an MT curve, a positive slope indicates a resistive layer, while a negative slope shows increasing conductivity. The increasing period represents a lowering frequency at depth. (Jiracek et al., 1995) • SAGE 2002’s MT setup consisted of 41 separate • Data collection points spread at 100 m intervals over 4.1 km. The data was collected in 2 days.

  6. Examples: App. Resistivity (ohm-m) Period (sec)

  7. Examples: App. Resistivity (ohm-m) Period (sec)

  8. Examples: App. Resistivity (ohm-m) Period (sec)

  9. Examples: App. Resistivity (ohm-m) Period (sec)

  10. Initial Observations: • From the 1-D Inversion model, 4 basic layers can be seen: • -a thin resistive surface layer • -a 150-650 m thick layer of low resistivity • -a 1000-1500 m thick layer of high conductivity • -the highly resistive basement at 2500-3500 m • The basement layer becomes shallower down the line, with the conductive layer becoming thinner • The subsequent 1-D Inversion stitch illustrates these layers fairly well

  11. Geoelectric Profiles: Area of Focus Depth (m) Precambrian Basement: 2.5-3.5 km depth Distance (km)

  12. Geoelectric Profiles: Possible Power Line Effect Depth (m) Distance (km)

  13. Well Data: Data from a geochemical analysis was used to estimate the resistivity of water in this region using a Salinity-Porosity Nomogram. Thus, porosity can be calculated using Archie’s Law. Flora Barres Well (Longmire, 1985)

  14. Calculations: The well data include temperature (18.1 ˚C) and equivalent salinity (385 ppm). Plotting these on the Nomogram and connecting them with a best fit line yields ρw ≈ 14 ohm-m. (SAGE 2002 Notes)

  15. Calculations - Porosity: Archie’s Law: ρr / ρw = aΦ-m …where a is the coefficient of saturation and m is the cementation factor. ρr was taken from the 1-D inversion model Values range from 8 ohm-m to 34 ohm-m, with most approximately 20 ohm-m. Humble Formula: a = 0.62, m = 2.15 …used in sandstone environments and this study Archie’s Law cannot be applied to clay environments, as clay drastically increases the conductivity and renders porosity estimates useless.

  16. Calculations – Seismic Velocity: Wyllie’s Equation: 1/v = Φ/vf + 1-Φ/vm …where vf is the velocity of the fluid and vm is the velocity of the matrix rock, in this case assumed to be granite. As such, vf = 1510 m/s and vm = 5375 m/s.

  17. Data: Calculated Results: Φ ≈ 29% vp = 3148.28 m/s Several data points were dropped due to power lines in center and clays near the end of MT line. (SAGE 2002 Notes)

  18. Graphs:

  19. Graphs:

  20. Interpretations: Possible clay zone Φ≈ 25-35%: potential aquifer Significant clay & possible increase in salinity?: poor aquifer

  21. Conclusions: • Resistivities are a reasonable method to estimate the porosity of buried sediments or rocks. • Calculated values for porosity and sand maintain fairly consistent across the profile. • The values suggest a large amounts of loosely consolidated, non-lithified sand to a depth of 660 m. • This region of basin has low the potential to be an excellent freshwater aquifer.

  22. References: Jiracek, G.R., Haak, V., Olsen, K.H., 1995, Practical magnetotellurics in a continental rift environment. In: K.H. Olsen (ed.), Continental Rifts: Evolution, Structure, and Tectonics, Developments in Geotectonics Vol. 25, Elsevier, Amsterdam, p. 103-128 Longmire, P., 1985, A Hydrogeochemical Study Along the Valley of the Santa Fe River, Santa Fe and Sandoval Counties, New Mexico. Ground Water and Hazardous Waste Bureau, Santa Fe, p. 01-35 Ward, S.H., 1990, Resistivity and induced polarization methods. In: Ward, S.H. (ed.), Geotechnical and environmental geophysics, Vol. 1, Society of Exploration Geophysicists, p. 147-189 SAGE 2002 Handbook

  23. Acknowledgements: • Quantec and Zonge Engineering for their equipment and expertise • Cochiti Pueblo for allowing us the privilege of working on their land • David and George for always taking the time to answer one of my many questions • Lauren for the TEM data, through which all static shift corrections were possible • The entire SAGE 2002 group for making this such a great experience

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