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). 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.
The lower the frequency, the deeper the information.
Some sample MT data are displayed in Figure 1, 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 10 or 1.
Why are there two curves?
- One curve shows the apparent resistivity (rho)
determined from the electric field in say, the north direction (Ex) and
the magnetic field, in the east direction (Hy).
- 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.
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
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 2 and 3).
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 in
the field, allowing for changes or additions to field programs during acquisition.
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.
Since 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).
Listed here are a few case histories involving
the use of MT.
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
2 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 far
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.
Papua New Guinea.
The Papuan Fold Belt extends lengthwise trending
northwest along the island of New Guinea. Here, Tertiary carbonates have
been thrust and folded into structures trapping large quantities of oil
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 ñ and in places it doubles or triples in thickness
due to thrusting. Seismic acquisition is expensive (more than $100K 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 subtrust
carbonates (if present). The target reservoirs are Cretaceous sands, sealed
by younger shale, and trapped in folds created by the thrusting.
Figure 3 shows
an MT model through an anticline in the fold belt. Only 11 MT stations
were acquired, along a dip profile.
The MT data were interpreted and this resulting
2-D model shows the Tertiary Darai limestone (in blue) and the older clastics
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 base limestone (pre-drill) to within 2 percent
to 7 percent of drilled depth.
Much MT data have been acquired in Turkey owing
to the outcrop of carbonates, volcanics, and other high-velocity rocks.
shows an interpreted MT profile and the corresponding seismic data. The
red areas indicate more resistive units, and blues shows the more conductive
units. The section is plotted with north on the left.
The Kocali (an ophiolitic melange) was thrust over
clastics and carbonates. All are Mesozoic in age. The target is the Mardin
carbonates. The seismic data are of poor quality. However, 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.
Granite Overthrust, southern Wyoming.
Figure 5 shows
an inversion of 15 MT stations acquired along a north-trending profile
in southern Wyoming. Precambrian granites were thrust from north to south.
The section is true scale, with no exaggeration.
The Precambrian granites are 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 done to investigate the subtrust
structure before acquiring seismic data.