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SAGE 2002 Field Camp for Geophysicists By: Andrew Frassetto October 21, 2002 What is SAGE?: •Summer of Applied Geophysical Experience •Centered in Santa Fe, NM and run by Los Alamos National Lab and The University of California •Seven days of lectures on geophysical techniques (seismic, gravity, magnetic, electrical) •Seven days of field work (6 in the primary area, 1 at an archaeological site) •Four days of data analysis and interpretation The Advantages: •One of the few opportunities for undergraduates to gain a skill in a wide variety of geophysical techniques •Exposure to numerous career paths via industry lectures (environmental, mining, petroleum) •An opportunity to practice geophysics in the Rio Grande Rift, an area of active extension in the Basin and Range Province The Outcome: •Present a 12 minute talk on your topic: seismic reflection, seismic refraction, gravity, transient electromagnetics, magnetotellurics, the archeological site (GPR, refraction, magnetics, DC resistivity) or structural geology •Integrate results with other team members •Write a four-five page “expanded abstract” on your topic Sediment Properties Determined with Magnetotellurics By: Andrew Frassetto University of South Carolina Presented on July 17, 2002 Outline: •Avoiding an “MT Stare”: An introduction to Magnetotellurics •Overview of the field area •Examples of MT Curves and 1-D Inversion Models •Description of the Geoelectric profile •My study: 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 •Integrated results The Basics of MT: •Low frequency, passive, deep imaging of lithosphere •Uses naturally occurring electric and magnetic fields (influenced by lightning strikes, solar storms, etc.) •Traditionally uses Ex and Ey, along with Hx, Hy, Hz (Titan-24 System did not measure Hz) (Jiracek et al., 1995) 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 two days. MT at SAGE 2002: •MT sounding curves contain TE and TM components: TE assumes that Ex is continuous across a conductive-resistive boundary. •A large separation of TE from TM on a curve represents a drastic change in the apparent resisitivity of layers. (Jiracek et al., 1995) Field Area: MT/TEM Line N 8 km (Terraserver, 2002) Field Area: Power Lines App. Resistivity (ohm-m) Examples: Period (sec) App. Resistivity (ohm-m) Examples: Period (sec) App. Resistivity (ohm-m) Examples: Period (sec) App. Resistivity (ohm-m) Examples: Period (sec) Initial Observations: •From the 1-D Inversion model, four 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 Geoelectric Profiles: Depth (m) Area of Focus Precambrian Basement: ~2.5-3.5 km depth Distance (km) Geoelectric Profiles: Depth (m) Power Line Effect Distance (km) Well Data: Data from a geochemical analysis were 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) Calculations: The well data include temp (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) 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 sand/sandstone environments and applicable to the SAGE 2002 field environment Archie’s Law cannot be applied to clay environments, as clay drastically increases the conductivity and renders porosity estimates useless. 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. (SAGE 2002 Notes) 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) Graphs: Graphs: Interpretations: Φ ≈ 25-35%: potential aquifer Clay & possible increase in salinity?: poor aquifer Near river: more clay? Conclusions: •Resistivities are a reasonable method to estimate the porosity of buried sediments or rocks. •Calculated values for porosity and sand are fairly consistent across the profile. •The values suggest a large amounts of loosely consolidated, non-lithified sediments (sand) to a depth of 660 m. •This region of basin has the potential to be an excellent freshwater aquifer. Integrated Results: •Seismic Refraction – Velocity model for shallow layers, fault location •Seismic Reflection – Velocities, fault location •Gravity – Depth to Basement •TEM – Location of La Bajada fault, imaging of possible shallow resistive layers •MT – Depth to Basement, imaging of deeper conductive layer TEM – Fault Location/Shallow Aquifer: La Bajada? (Ugarte and Larkin, 2002) Seismic - Fault Location: Flag ~222 1800 m/s (Supak, 2002) Flag ~230 (Parkman, 2002) 1900 m/s 2000 m/s 2200 m/s Gravity - Depth to Basement: (Khatun, 2002) •Gravity models show a basement depth that varies between 1.5 and 3.0 km at the basin end of the MT line. •SAGE 2002 gravity group believes the lowest model is correct Depth to Basement Estimates: Gravity Estimates MT Estimate Acknowledgements: •Quantec and Zonge Engineering for their equipment and expertise •Cochiti Pueblo for the privilege of working on their land •David Alumbaugh and George Jiracek for their guidance and expertise 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