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Magnetotellurics (MT): Technique, Interpretation, and Application
Karen Rae Christopherson*
Search and Discovery Article #40025 (2001)
*Chinook Geoconsulting Inc., Evergreen, CO ([email protected])
Adapted for online presentation from two articles by same author, entitled “MT Gauges Earth’s Electric Fields” in Geophysical Corner, AAPG Explorer, December, 1998, and “MT
Data
Throws Curves to Viewers” in
Geophysical Corner, AAPG Explorer,
January, 1999. Appreciation is expressed to the author and to M. Ray Thomasson,
former Chairman of the AAPG Geophysical Integration Committee, and Larry Nation,
AAPG Communications Director, for their support of this online version.|
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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.
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 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 These days, MT can be used in frontier areas
where MT can be used in lieu of 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 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
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 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
After magnetotelluric
(MT) Part of the MT measures changes in the electric and
magnetic fields with time. The Some sample MT 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 Hence, at every MT station we get two curves.
These The 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
( Almost all MT interpretation is done in 2-D,
usually dip lines. There are The MT interpreter takes the processed With forward modeling, the interpreter
creates a cross-section, computes the MT response and compares it with the
acquired 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 Often MT interpretation can be done rapidly enough in the field to allow for changes or additions to field programs during acquisition. 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 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. 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. MT 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 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. Much MT The Kocali (an ophiolitic melange) is thrust
over clastics and carbonates, all Mesozoic in age. The target is the
Mardin carbonates. The 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 acquiringseismic data .
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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
Figure 9 – An MT interpretation across an
anticline in the fold belt in Papua New Guinea. 
Figures 10 – Interpretation of MT 
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.