existed for many years, but it is only in the last decade
it has been widely used offshore. While CSEM is used
primarily for detection of relatively shallow and thin
horizontal resistors such as hydrocarbon reservoirs,
MMT can only image larger-scale structures. It can
improve the interpretation of deep structures, and due
to the very low frequency content the energy can
penetrate several tens of kilometres into the subsurface. More importantly it can image conductive strata
below thick resistors such as salt or basalt. Because of
this, MMT application has also created interest in the
hydrocarbon exploration industry. Presence of salt or
basalt in a basin often complicates seismic imaging,
and MMT’s ability to see through such layers has
shown to be very valuable.
The naturally varying electromagnetic field is set
up by two external sources. The low frequency content
(<1 Hz) is caused by solar winds (stream of protons
and electrons) interacting with the Earth’s magnetosphere, while the high frequency content (>1 Hz) is
worldwide lightning, mostly around the equator,
which charges the Earth’s ionosphere. These complex
processes create a plane electromagnetic field
travelling vertically downward through the atmosphere. Most of the energy is reflected back again
when it hits the Earth surface, but part of the signal
penetrates into the subsurface. This field induces
currents in the Earth called telluric currents which
again set up a secondary magnetic field. The strength
of the secondary field is dependent on the electrical
properties in the Earth. Measuring this signal will
therefore provide information about subsurface resistivity. In marine magnetotelluric it is the low frequency signal set up by the interaction between the
solar wind and the magnetosphere that will dominate.
The conductive seawater will filter out all frequencies
above approximately 1 Hz. The signal strength also
varies with latitude. Areas close to the poles have
stronger source signals than areas close to the equator.
20.3.2 Acquisition of MMT Data
An MMT survey is acquired by placing receivers on
the seabed. The MMT signal is extremely weak, so
high sensitivity receivers, recording both the electric
and magnetic fields, are needed. Receivers can be
placed as a single line, or in a receiver grid with typical
receiver spacing of 1–5 km. Often a MMT survey is
done in combination with CSEM acquisition. However, due to the low frequency signal it is important
that enough listening time, undisturbed by the controlled source, is allowed when planning a survey.
Typically it is necessary to listen for several days in
order to get enough samples to describ the low frequency signal (Fig. 20.13). Required listening time
will also vary with latitude. Close to the equator the
signal is weaker, and longer listening time is needed to
maximise the number of periods describing the low
frequency signal.
MMT surveys in areas with large water depth will
only obtain the lower frequency signals. This filtering
of the high frequencies has implications for the imaging capabilities and will result in reduced resolution in
the shallowest part of the section.
20.3.3 Basic Interpretation of MMT Data
The Earth resistivity can be described by impedance
tensors. These tensors can be found by analysing the
relationship between the recorded E and H fields (electric and magnetic). The processed receiver data results
in apparent resistivity and impedance phase as a function of the signal period (T). In Fig. 20.14 the TE- and
TM modes are plotted as a function of period. The
transverse electric mode (TE) is the electrical
polarised mode where the electric field is in the geological strike direction. The transverse magnetic mode
(TM) is the magnetic polarised mode where the magnetic field is in the geological strike direction.
The apparent resistivity plot can be directly
interpreted as it gives the resistivity average of the
subsurface. The time period can roughly be transferred
to minimum approximate burial depth by using the
skin depth δ $ 500
ffiffiffiffiffi ffi
ρT
p
ð
Þ . For example a signal period
(T) of 10 s with apparent resistivity (ρ) of 1.5 Ωm in
Fig. 20.14 (upper) corresponds to a minimum burial
depth of 2.0 km. The two different modes start to
separate at a signal period of 100 s (Fig. 20.14,
lower), indicating that there is a deeply buried three
dimensional structure at minimum 7 km depth.
Displaying the apparent resistivity or impedance
phase for a complete receiver line with the signal
period along the y-axis, and the different receiver
responses along the x-axis can therefore provide a
528
S.E. Johansen and P.T. Gabrielsen
it has been widely used offshore. While CSEM is used
primarily for detection of relatively shallow and thin
horizontal resistors such as hydrocarbon reservoirs,
MMT can only image larger-scale structures. It can
improve the interpretation of deep structures, and due
to the very low frequency content the energy can
penetrate several tens of kilometres into the subsurface. More importantly it can image conductive strata
below thick resistors such as salt or basalt. Because of
this, MMT application has also created interest in the
hydrocarbon exploration industry. Presence of salt or
basalt in a basin often complicates seismic imaging,
and MMT’s ability to see through such layers has
shown to be very valuable.
The naturally varying electromagnetic field is set
up by two external sources. The low frequency content
(<1 Hz) is caused by solar winds (stream of protons
and electrons) interacting with the Earth’s magnetosphere, while the high frequency content (>1 Hz) is
worldwide lightning, mostly around the equator,
which charges the Earth’s ionosphere. These complex
processes create a plane electromagnetic field
travelling vertically downward through the atmosphere. Most of the energy is reflected back again
when it hits the Earth surface, but part of the signal
penetrates into the subsurface. This field induces
currents in the Earth called telluric currents which
again set up a secondary magnetic field. The strength
of the secondary field is dependent on the electrical
properties in the Earth. Measuring this signal will
therefore provide information about subsurface resistivity. In marine magnetotelluric it is the low frequency signal set up by the interaction between the
solar wind and the magnetosphere that will dominate.
The conductive seawater will filter out all frequencies
above approximately 1 Hz. The signal strength also
varies with latitude. Areas close to the poles have
stronger source signals than areas close to the equator.
20.3.2 Acquisition of MMT Data
An MMT survey is acquired by placing receivers on
the seabed. The MMT signal is extremely weak, so
high sensitivity receivers, recording both the electric
and magnetic fields, are needed. Receivers can be
placed as a single line, or in a receiver grid with typical
receiver spacing of 1–5 km. Often a MMT survey is
done in combination with CSEM acquisition. However, due to the low frequency signal it is important
that enough listening time, undisturbed by the controlled source, is allowed when planning a survey.
Typically it is necessary to listen for several days in
order to get enough samples to describ the low frequency signal (Fig. 20.13). Required listening time
will also vary with latitude. Close to the equator the
signal is weaker, and longer listening time is needed to
maximise the number of periods describing the low
frequency signal.
MMT surveys in areas with large water depth will
only obtain the lower frequency signals. This filtering
of the high frequencies has implications for the imaging capabilities and will result in reduced resolution in
the shallowest part of the section.
20.3.3 Basic Interpretation of MMT Data
The Earth resistivity can be described by impedance
tensors. These tensors can be found by analysing the
relationship between the recorded E and H fields (electric and magnetic). The processed receiver data results
in apparent resistivity and impedance phase as a function of the signal period (T). In Fig. 20.14 the TE- and
TM modes are plotted as a function of period. The
transverse electric mode (TE) is the electrical
polarised mode where the electric field is in the geological strike direction. The transverse magnetic mode
(TM) is the magnetic polarised mode where the magnetic field is in the geological strike direction.
The apparent resistivity plot can be directly
interpreted as it gives the resistivity average of the
subsurface. The time period can roughly be transferred
to minimum approximate burial depth by using the
skin depth δ $ 500
ffiffiffiffiffi ffi
ρT
p
ð
Þ . For example a signal period
(T) of 10 s with apparent resistivity (ρ) of 1.5 Ωm in
Fig. 20.14 (upper) corresponds to a minimum burial
depth of 2.0 km. The two different modes start to
separate at a signal period of 100 s (Fig. 20.14,
lower), indicating that there is a deeply buried three
dimensional structure at minimum 7 km depth.
Displaying the apparent resistivity or impedance
phase for a complete receiver line with the signal
period along the y-axis, and the different receiver
responses along the x-axis can therefore provide a
528
S.E. Johansen and P.T. Gabrielsen