IN WESTERN AUSTRALIA - Department of Mines and Petroleum

24
PWA April Edition - Magnetotelluric Surveys
Peter Kirk, Petroleum Geophysicist
Peter Kirk Geophysical Consultancy Pty Ltd
Of all the geophysical techniques used in
petroleum exploration in Australia over the last 50
years or more, the magnetotelluric or MT method
has been used the least frequently and is probably
the least familiar method to explorationists. There
have, in fact, been five MT surveys conducted in
WA, one conventional MT survey and four audio
frequency MT (AMT) surveys, all of which have
been conducted onshore.
Although to date the technique has not resulted in
any major discoveries, it has produced some
interesting results and in the case of one survey it
accurately predicted the results of two
unsuccessful wells. It deserves to be used more
frequently, particularly in areas where the seismic
technique has significant problems (mainly due to
shallow limestone) or where seismic acquisition is
restricted due to environmental problems.
The Magnetotelluric Method
Magnetotellurics (MT) is a division of geophysics
which studies the earth’s naturally occurring
electromagnetic field and the telluric (from the
Greek Tellus meaning earth) currents caused by
fluctuations therein. Many researchers say that this
source is “many orders of magnitude greater than
the strengths of fields that can be generated with
man-made sources on the surface of the earth”.
The primary energy source is naturally occurring
electromagnetic waves that are confined to the
space between the ionosphere and the earth’s
surface, these circumnavigate the globe and are
consequently known as ‘spherics’. The frequencies
of these waves cover a spectrum from 10 -3 Hz to
10 4 Hz (about 22 octaves). The low to mid
frequencies are caused by the interaction of the
natural solar wind with the earth’s magnetic field,
whilst the higher frequencies are generally
Magnetotelluric Surveys for Petroleum
Exploration in Western Australia
attributed to distant lightning strikes. Nearby
lightning strikes, powerful man-made transmitters
and highly irregular solar activity due to solar flares
all produce levels of unwanted noise that prevent the
recording of useful signal and also introduce
spurious ‘static’ delays or depth shifts. Constant
monitoring of sunspot activity or ‘space weather’ is
necessary but this is greatly facilitated by the
availability of reliable online data compiled by solar
observatories, including the one at Learmonth in WA.
Since the bulk of the useful energy is generated by
the interaction with the solar wind, it is normally only
possible to record during daylight hours.
The primary energy source induces telluric currents
just below the earth’s surface in large sheets,
preferentially through conducting layers such as
brine filled sedimentary rocks and certain mineral
deposits. These currents flow more slowly through
resistive layers such as dense limestone, volcanics
such as basalt, tight non-porous rocks and
evaporites including salt (although salt in solution is
highly conductive, solid salt is highly resistive). The
currents can be readily measured as a result of the
horizontal potential gradients and the horizontal and
vertical magnetic gradients that they produce at the
surface. A modern recording instrument normally
incorporates two electrical antennae horizontally at
right angles and three magnetic coiled antennae
horizontally and vertically at right angles. These
signals may be recorded separately or the magnetic
and electrical signal may be combined and recorded
in stereo on digital magnetic tape. A high sampling
rate is required for the higher frequency
components and this is provided by modern DAT
recorders, principally used by the music industry.
The depth of penetration of the induced currents
within the earth depends upon the frequency of the
primary source with lower frequencies necessary to
induce currents at greater depths. In order to
measure currents kilometres below the surface, we
need frequencies with periods of several minutes.
Typically the length of each individual recording is
20 minutes for petroleum exploration. The digitally
recorded signals, which contain information for all
depths, are demodulated by analogue or digital
computer to final form for analysis. The recorded
signal contains frequency-phase vs. amplitude
information relating to the incoming field at the
surface, the decaying earth carrier field, and the
modulation resulting from the earth’s resistivity
reflection coefficients. The former two fields are
extracted using least square methods. Only the
earth’s resistivity profile remains as a function of
frequencies. The depth of investigation is a result of
the frequency of the data and the resistivity, and this
is approximately described by the well known ‘skin
depth equation’ - skin depth (m) = 500 p/f.
Simplification of the “skin depth” equation is used to
convert the profiles to depth. The result of this
process is a series of electric and magnetic
reflection coefficients. These are then combined to
form the apparent resistivity series defined by
Z = E/H as a function of depth. In addition phase
values with depth are also obtained. The depths
derived from the skin depth equation can then be
corrected at a calibration well within the survey
area. However, changes in the overburden
composition, irregular variations in the earth’s
magnetic field and accelerated solar activity can
affect the depth accuracy. This can occur from dayto-day
or from one survey area to another. These
inaccuracies can be reduced by recording at a
known calibration point at least every day and
sometimes continuously throughout the day.
Sophisticated inversion algorithms can also be used
to produce 2D and even 3D plots of apparent
resistivity using all 5 recorded signals at each