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Introduction
This article provides an explanation of the
 MT technique (what is it?) and its application, together with some case
histories.
Dissected, the word “magnetotellurics”
has two parts – “magneto” for magnetic and “telluric” for earth
currents. MT is a geophysical method that measures magnetic and electric
fields that are found in the earth. Basically, MT is a geophysical method
that measures naturally occurring, time-varying magnetic and electric
fields. From these measurements we can derive resistivity estimates of the
subsurface, from the very near surface to tens of thousands of feet.
Figure
Captions
 Figures 1-2: Interaction of solar particles
with earth’s magnetic field creates high-energy EM energy, which travel
around the earth via thunderstorms.
 Figure 3: This area is a typical candidate
for MT application.
 Figure 4: Resistivity values vary with
lithology.
 Figure 5: A typical MT station layout. The
stations can be anywhere from a quarter-mile to tens of miles apart,
depending on the type of survey.
 Figure 6: Actual MT time series recording.
The amplitudes of electric and magnetic waves are measured as a function
of time. Channels 4,3,2,1, respectively, correspond to Ex, Ey, Hx, Hy.
 Figure 7 – An example of MT apparent
resistivity curves (in ohm-meters) vs. frequency (in Hertz).
 Figure 8
– An MT resistivity section in the Columbia Plateau.
 Figure 9 – An MT interpretation across an
anticline in the fold belt in Papua New Guinea.
 Figures 10 – Interpretation of MT data (a)
and seismic section (b) from Turkey.
 Figure 11 – An inversion of 15 MT stations
acquired along a north-trending profile in southern Wyoming. Precambrian
granite was thrust from north to south in this MT resistivity section.
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What
is the Source of the MT Signal?
The MT signal is caused by two things:
1. In the lower frequencies (generally less
than 1 Hertz, or 1 cycle per second), the source of the signal is
interaction of the solar wind with the earth’s magnetic field. As solar
storms emit streams of ions, this energy disturbs the earth’s magnetic
field and causes low-frequency energy to penetrate the earth’s surface (Figure
1).
2. The higher frequency signal (greater than
1 Hertz) is created by world-wide thunderstorms, usually near the equator.
The energy created by these electrical storms travels around the earth (in
a wave guide between the earth’s surface and the ionosphere), with some
of the energy penetrating into the earth (Figure
2).
Both of these sources of signal create
time-varying electromagnetic waves.
Although these electric and magnetic fields
are small, they are measurable. That’s the good news. The bad news is
that these signals vary in strength over hours, days, weeks and even over
the sunspot cycle (which is about 11 years and creates an increase in the
number of solar storms). So, geophysicists measuring MT have to measure
for hours at each station in order to get enough signal to ensure
high-quality data. This is especially true when measuring them at the
lowest frequencies (about 0.001 Hertz, or 1 cycle per 1000 seconds). At
these low frequencies, we need to record for 16 minutes to get one sample
of data! That means we really need to record for several hours just to get
a decent statistical average of good data.
What
is MT used for?
The MT method itself has only been in
existence since the 1960s. Practical systems came into use in the 1970s,
with large improvements made in the 1980s. The last two or three years
have seen the advent of smaller systems, taking advantage of GPS and
faster computers, as well as 24 bit A to D conversion (with further
discussion below).
At first, MT was used mostly for
reconnaissance mapping of basins and geothermal prospects. In the 1980s,
MT came into use for petroleum exploration, mainly in frontier areas. This
is because MT is very portable (a station can be placed almost anywhere
with access by horse, helicopter, snowmobile, etc.) and because MT works
best where seismic has problems; i.e., areas of high-velocity cover such
as volcanic provinces, overthrusts, carbonate cover, or salt. Figure 3
shows an area that is a typical candidate for application of MT.
These days, MT can be used in frontier areas
where seismic acquisition is difficult or prohibitive (due to cost or
environmental factors) to map prospects (usually structural). The data are
normally integrated with whatever other information is available (gravity,
magnetics, geology, and borehole).
MT can be used in lieu of seismic in areas
where seismic acquisition is not possible. MT can be applied in lead of
seismic (to help determine the best placement of seismic lines). MT can
also be acquired in conjunction with seismic, in order to have a
“second” opinion of subsurface structure or stratigraphy.
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What
Can MT Tell Us About the Subsurface?
The main parameter that is derived from MT is
resistivity. The main factor affecting resistivity is lithology; however,
other parameters can come into play as well (such as pore fluid, pressure,
and temperature).
Figure 4 shows how resistivity can vary with
lithology. Resistivity is given in ohm-meters. Note that the main
contrasts are between the volcanic/igneous/carbonate groups with higher
resistivities and the clastics, but resistivity can be used to map sands
versus shales – it all depends on the actual resistivity contrast
between the units and the thickness of the units.
The deeper the unit is, the thicker it has to
be in order to be mappable by MT. The MT data can be interpreted to give
an estimate of resistivity variations with depth. This is normally
displayed as a cross-section, where formations or units of differing
resistivity are mapped in the subsurface.
Because MT needs a resistivity contrast to be
present in order to map a boundary, and because these units need to be
fairly thick to be mapped, the sections will not have the resolution of
seismic sections.
How
Are MT Data Acquired?
An MT crew will normally acquire between two
and six stations at the same time. Each station is independent of the
others. One MT station consists of a set-up as shown in Figure
5. The
stations can be anywhere from 1/4 mile to tens of miles apart, depending
on the type of survey – reconnaissance or detail (prospect
high-grade).
At each MT station, five measurements
(channels) are recorded. They are the magnetic field in two horizontal
directions and in the vertical direction, and the electric field in two
horizontal directions.
The horizontal measurements are at 90-degree
angles to each other (e.g., north and east) and as close to level as
possible. The vertical electric field is not measured because it is
assumed to be zero. The directions are labeled as x, y and z, with z being
the vertical direction. The electric field is abbreviated “E” and the
magnetic field is abbreviated “H.” Hence, we measure Ex, Ey, Hx, Hy
and Hz.
Ten to thirty channels are recorded at one
time. More channels could be recorded, but this is usually limited by
logistics.
The magnetic fields are measured with a type
of magnetometer, basically an iron-cored coil with thousands of turns of
wire. These coils are encased in waterproof containers, like PVC, and have
a cable extending from one end. The coils are extremely sensitive to noise
from wind, walking or trucks, and are buried in soil or under rocks to
prevent movement.
The electric fields are measured with long
“antennae,” or dipoles – usually wires about 300-500 feet long. The
ends of the wires are connected to “pots” – sealed containers a few
inches in diameter and about six inches high. The pots have a porous
ceramic base and are filled with an electrolyte solution (like
silver/silver-chloride). These pots are buried a few inches in the ground
and measure the voltage drop along the dipole length.
Because the wires are susceptible to wind
noise, they are usually placed directly on the ground. The coils and
electric-field dipoles are all connected to “sensor” boxes where
filtering and amplification of the signals take place. Remember, these are
very small signals we are measuring.
The data then are sent to a laptop computer
where they are digitized and recorded on digital media. This is where the
new 24-bit A to D systems come in. These new systems allow for a much
larger amount of data (in amplitude) to be transferred from analog to
digital (A to D) signal, meaning that we can get more information out of
the data and have more to work with when processing. The older systems
were 16-bit A to D – now we can record 156 times more information than a
few years ago.
The electric and magnetic fields are measured
as a function of time. An example of a time-series record is shown in
Figure 6. Notice how the signals coincide with each other. The four
channels, from top, are Ex, Ey, Hx, Hy. Remembering basic physics, the
electric field in the x direction (Ex) should correlate with the magnetic
field in the y direction (Hy), and similarly Ey correlates with Hx. Hz
(not shown) is recorded only to give us some information about the
geologic strike. We will use Ex, Hy, Ey and Hx to tell us about subsurface
resistivity.
The data are synchronized with GPS signals.
This is important because, as we record two or more stations at one time,
these data are compared with each other for noise. This method, known as
“remote referencing,” allows the data at one station to be compared to
data at another station, recorded at exactly the same time, and compared
for coherency. Any non-coherent data are thrown out and considered as
noise. This greatly improves data quality. Recording at each station takes
6-18 hours, depending on signal strength and survey parameters.
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Processing
and Displaying the Data
After magnetotelluric
(MT) data are acquired, they are run through several processing steps. In
part, noise is removed from the data. Examples of noise are thunderstorms,
power lines, pipelines, and trains.
Part of the processing involves comparing the
data at one station to another station that was recorded at exactly the
same time (remote-referencing). As noted above, any non-coherent signal
between the two stations is considered noise and discarded from the time
series.
MT measures changes in the electric and
magnetic fields with time. The data are transformed from the time-domain
to the frequency domain. At each station, about 40 points in the
subsurface are derived, as a measurement of apparent resistivity (and
phase) vs. frequency.
Some sample MT data are displayed in Figure
7, which shows apparent resistivity (in ohm-meters) vs. frequency (in
Hertz). The data are plotted on a log-log curve, so “2” means 102
or 100, and 0 means 100 or 1. It should be remembered that the
lower the frequency, the deeper the information.
Why are there two curves?
1. One curve shows the apparent resistivity (rho)
determined from the electric field in, for example, the north direction
(Ex) and the magnetic field, in the east direction (Hy).
2. The other curve plots the data for the
other two orthogonal horizontal fields, Ey and Hx.
Hence, at every MT station we get two curves.
These data are processed so that they align with approximate geologic dip
and strike, regardless of the layout in the field.
The processing takes several hours per
station.
Interpretation
The MT method assumes that the earth
structure is two-dimensional; i.e., that there is a dip and strike.
Therefore, most MT stations are acquired along profiles (2-D) or on a grid
(3-D) from which profiles can be extracted.
Almost all MT interpretation is done in 2-D,
usually dip lines. There are 3-D codes available, but they still require
large amounts of computing power and are not normally practical for
prospect-level exploration problems.
The MT interpreter takes the processed data
and interprets it to a representation of true resistivity versus depth.
This can be done using forward or inverse modeling.
With forward modeling, the interpreter
creates a cross-section, computes the MT response and compares it with the
acquired data; for inverse modeling, the interpreter allows the computer
to create a cross-section from the acquired data.
Both types of modeling result in
cross-sections or maps of the subsurface where the resistivity of the
subsurface is interpreted to represent certain geologic formations or
units (See Figures 8 and 9).
MT interpretation is not easy, and a good
interpreter must look at the data (not rely on inversion only) and must
integrate geology. There are two commercial MT workstations running on
PCs. They allow the interpreter to process, review, edit, interpret, plot,
and map data. They also allow for the integration of other types of
geophysical and geological data (e.g., structure, well logs, surface
dips).
Often MT interpretation can be done rapidly
enough in the field to allow for changes or additions to field programs
during acquisition.
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Current
Application of MT
There are hundreds of MT systems in use
throughout the world for petroleum exploration, most being run by national
oil companies (such as China, Japan, and India) and a handful of
contractors.
Because MT works best in areas of seismically
high-velocity cover, many of these areas are frontier provinces. In recent
years, MT data for petroleum exploration have been acquired in Italy,
northern Africa, China, Japan, the western United States, Colombia,
Turkey, Greece, Albania, Jordan, Greenland, Pakistan, the Arabian
Peninsula, Papua New Guinea, and the Gulf of Mexico (marine MT).
Given below are a few case histories
involving the use of MT.
1.
Columbia Plateau, Washington state
Thousands of MT stations were recorded in the
Columbia Plateau during the 1980s in an effort to map the basin beneath
the thick cover of flood basalts. In places the basalt thickness exceeds
20,000 feet.
Shown in Figure 8 is a 2-D MT model
cross-section, from west to east, extending from central Washington to
near the Idaho border. Station locations are shown across the top of the
section. The section shows the Miocene flood basalts (light blue), the
Oligocene/Eocene clastics (including volcanoclastics) in yellow, and
basement (in dark blue). The section is vertically exaggerated about 5:1.
The resistivity of these units (as modeled) is shown on the right
scale.
The MT model shows the basalts and clastics
thinning dramatically from west to east, with the clastic section absent
at the east end. In this area, the basalts were probably deposited
directly on basement rocks. Seismic data are almost impossible to acquire
because of the thickness of the basalt cover. Several wells were drilled
on the Plateau that had good ties to the MT data.
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2.
Papua New Guinea
The Papuan Fold Belt, lengthwise trends
northwest along the island of New Guinea. Here, Tertiary carbonates have
been thrust and folded into structures trapping large quantities of oil
and gas. Several large fields have been discovered here in the past
decade.
The thickness of the carbonates in the fold
belt is about 3,000 feet, and in places it doubles or triples in thickness
due to thrusting. Seismic acquisition is expensive (more than $100,000 per
mile for 2-D data), and the data are usually poor in quality.
MT data have been acquired over many
structures to map the base of the surface carbonate unit and the thickness
of subthrust carbonates (if present). The target reservoirs are Cretaceous
sands, sealed by younger shale, and trapped in folds created by the
thrusting.
Figure 9 shows an MT model through an
anticline in the fold belt. Only 11 MT stations were acquired along a dip
profile. From the interpretation of the MT data, the resulting 2-D model
shows the Tertiary Darai limestone (in blue) and the older clastics in
orange.
The limestones are very resistive compared to
the clastics (a contrast of almost 500:1). The primary thrust is shown,
emplacing limestone and clastics in the hanging wall, with limestone also
present in the footwall. The target is the folded clastics in the hanging
wall. There are also possible footwall plays.
MT interpretations on some structures in
Papua New Guinea have estimated the base of limestone (pre-drill) to
within 2 percent to 7 percent of drilled depth.
3. Turkey
Much MT data have been acquired in Turkey
owing to the outcrop of carbonates, volcanics, and other high-velocity
rocks. Figure 10 shows an interpreted MT profile and the corresponding
seismic data. The red areas indicate more resistive units, and the blue
areas show the more conductive units. The section is plotted with north on
the left.
The Kocali (an ophiolitic melange) is thrust
over clastics and carbonates, all Mesozoic in age. The target is the
Mardin carbonates. The seismic data are of poor quality. Nevertheless, the
principal reflectors were converted to depth and plotted on the MT section
(red lines). The MT data show a more resistive section at depth
corresponding to the Mardin. The results show good correlation to well
data.
4.
Granite Overthrust, southern Wyoming
Figure 11 shows an inversion of 15 MT
stations acquired along a north-trending profile in southern Wyoming.
Precambrian granite was thrust from north to south. The section is true
scale, with no exaggeration. The granite is high in resistivity (500 + ohm
- m). The subthrust Cretaceous/Jurassic rocks are 10-50 ohm-m.
A thin Tertiary section is present on the
Precambrian at the surface. A possible secondary thrust fault is seen
deeper in the section. Possible normal faults cut the thrust plate. The
structure has not been drilled. This survey was conducted to investigate
the subthrust structure before acquiring seismic data.
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