Magentotelluric Estimates of Sediment Porosity and Seismic Velocity

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
•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
Sediment Properties Determined
with Magnetotellurics
By: Andrew Frassetto
University of South Carolina
Presented on July 17, 2002
•Avoiding an “MT Stare”: An introduction to
•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:
8 km
(Terraserver, 2002)
Field Area:
Power Lines
App. Resistivity (ohm-m)
Period (sec)
App. Resistivity (ohm-m)
Period (sec)
App. Resistivity (ohm-m)
Period (sec)
App. Resistivity (ohm-m)
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)
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)
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)
Φ ≈ 25-35%: potential aquifer
Clay & possible increase in
salinity?: poor aquifer
Near river:
more clay?
•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
•SAGE 2002 gravity group believes the lowest
model is correct
Depth to Basement Estimates:
Gravity Estimates
MT Estimate
•Quantec and Zonge Engineering for their
equipment and expertise
•Cochiti Pueblo for the privilege of working on their
•David Alumbaugh and George Jiracek for their
guidance and expertise
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.
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