Title: Sediment Properties Determined through Magnetotellurics
1Sediment Properties Determined through
Magnetotellurics
- By Andrew Frassetto
- University of South Carolina
- July 17, 2002
2Outline
- 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
Archies Law (and Wyllies Equation) - Summary of results and interpretations regarding
porosity and seismic velocity - Implications of the sediment properties
3Field Area
MT/TEM Line
N
8 km
4The Basics of MT
- Low frequency, passive, deep imaging of the
Earths lithosphere - Traditionally uses Ex and Ey, along with Hx, Hy,
Hz (Titan-24 System did not measure Hz)
(Jiracek et al., 1995)
5MT 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 2002s 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.
6Examples
App. Resistivity (ohm-m)
Period (sec)
7Examples
App. Resistivity (ohm-m)
Period (sec)
8Examples
App. Resistivity (ohm-m)
Period (sec)
9Examples
App. Resistivity (ohm-m)
Period (sec)
10Initial 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
11Geoelectric Profiles
Area of Focus
Depth (m)
Precambrian Basement 2.5-3.5 km depth
Distance (km)
12Geoelectric Profiles
Possible Power Line Effect
Depth (m)
Distance (km)
13Well 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 Archies Law.
Flora Barres Well
(Longmire, 1985)
14Calculations
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 ?w 14 ohm-m.
(SAGE 2002 Notes)
15Calculations - Porosity
Archies Law ?r / ?w aF-m where a is the
coefficient of saturation and m is the
cementation factor. ?r 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 Archies Law cannot
be applied to clay environments, as clay
drastically increases the conductivity and
renders porosity estimates useless.
16Calculations Seismic Velocity
Wyllies Equation 1/v F/vf 1-F/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.
17Data
Calculated Results F 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)
18Graphs
19Graphs
20Interpretations
Possible clay zone
F 25-35 potential aquifer
Significant clay possible increase in
salinity? poor aquifer
21Conclusions
- 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.
22References
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
23Acknowledgements
- 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