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Sediment Properties Determined through Magnetotellurics

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Summary of results and interpretations regarding porosity and seismic velocity ... geophysics, Vol. 1, Society of Exploration Geophysicists, p. 147-189 ... – PowerPoint PPT presentation

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Title: Sediment Properties Determined through Magnetotellurics


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
    Archies Law (and Wyllies 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
    Earths 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 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.

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 Archies 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 ?w 14 ohm-m.
(SAGE 2002 Notes)
15
Calculations - 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.
16
Calculations 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.
17
Data
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)
18
Graphs
19
Graphs
20
Interpretations
Possible clay zone
F 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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