# Presentation: 489248

## 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
- 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

## Field Area:

**MT/TEM Line**

** ****N**

**8 km**

## 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)

## 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.

## Examples:

- App. Resistivity (ohm-m)

- Period (sec)

## Examples:

- App. Resistivity (ohm-m)

- Period (sec)

## Examples:

- Period (sec)

- App. Resistivity (ohm-m)

## Examples:

- App. Resistivity (ohm-m)

- Period (sec)

## 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

## Geoelectric Profiles:

**Precambrian Basement: 2.5-3.5 km depth**

**Area of Focus**

**Depth (m)**

**Distance (km)**

## Geoelectric Profiles:

**Possible Power Line Effect**

**Depth (m)**

**Distance (km)**

## Well Data:

**Flora Barres Well**

- 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.

- (Longmire, 1985)

## 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)

## 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.

## 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.

## Data:

- Calculated Results:
-    Φ ≈ 29%
-    vp = 3148.28 m/s

- (SAGE 2002 Notes)

- Several data points were dropped due to power lines in center and clays near the end of MT line.

## Graphs:

## Graphs:

## Interpretations:

**Φ**** ****≈**** 25-35%: potential aquifer**

**Possible clay zone**

**Significant clay & possible increase in salinity?: poor aquifer**

## 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.

## 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

## 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