All,
There has also been some discussion about depth of EM sounding here lately. As the Mad one has stated the only thing that will bring positive proof is the flow of liquid gold! I posted this at #119:
cass.jsc.nasa.gov
This info was removed on or about 14 March but I kept a copy of the important stuff, after my comments, as I thought it related to this discussion:
Read the part that says:"Electromagnetic
sounding can determine the presence (or absence) of sharp variations in the
resistivity to four to five hundreds kilometers depth and thus provide
information on the thermodynamic conditions within this zone." and "The
effectiveness of the technique can be strengthened considerably by the
addition of a magnetometer in orbit to assist in the source determination."
and it talks about: "extremely long booms are impractical"
For me if NASA thinks this works to hundreds kilometers, more effective if
airborne, and a long boom is required (sounds like that boom mounted on a
helicopter?) why couldn't IMMM's?
To convert from km to m multiply by: 1000
MAGNETISM
Magnetic measurements yield information both on the electromagnetic
material properties (primarily resistivity) of the interior of a planet and
on processes of internal field generation (such as a dynamo). The primary
magnetic measurements of a planet are better suited to orbital techniques.
However, surface measurements can play an important role in investigating
the interior through electromagnetic sounding.
The transient variations of the magnetic field at the surface of a planet
have a primary external source, the interaction between the environment of
the planet and solar radiation, and a secondary source, the electric
currents induced in the conductive planet. Continuous recording of the
transient variations of the magnetic field at the surface can therefore
provide information on its internal structure. The depth of penetration of
an electromagnetic wave in a conductive medium depends on both the period
of the wave and the electrical resistivity of the medium. The larger the
period and the resistivity, the greater the depth of penetration.
Electromagnetic sounding can determine the presence (or absence) of sharp
variations in the resistivity to four to five hundreds kilometers depth and
thus provide information on the thermodynamic conditions within this zone.
Electromagnetic sounding techniques are based on the analysis of the
electromagnetic field observed at the surface of the planet. The
resistivity distribution within the Earth is usually determined by
measuring the magnetotelluric tensor which relates, in the frequency
domain, the horizontal components of the electric field to those of the
magnetic field simultaneously recorded at a station. When (i) the
resistivity varies with depth only, and (ii) the externally originating
variations are, as a first approximation, homogeneous at the scale of the
studied area, the magnetotelluric tensor is antisymmetric; the antidiagonal
terms are equal to plus or minus the transfer function between the magnetic
and electric fields respectively. This transfer function is called the
impedance of the conductive medium.
Within the same approximation, the impedance may also be deduced from the
ratio, in the frequency domain, between the vertical and horizontal
components of the magnetic field at a given station, provided the geometry
of the source (which is provided by the interaction of the interplanetary
field with the planet and/or its ionized environment) is known. In this
case, the relative error in the impedance determination is on the order of
the relative error of the source wavelength determination. The
electromagnetic sounding technique can therefore allow the determination of
the resistivity distribution even in the case of one magnetic station
operating at the surface of the planet. The effectiveness of the technique
can be strengthened considerably by the addition of a magnetometer in orbit
to assist in the source determination.
The impedance is directly related to the variations of the resistivity with
depth. Information about these variations can then be deduced from the
observed frequency dependence of the impedance. The resistivity is
dependent on the petrological nature of the materials and their
thermodynamic conditions. Laboratory results on terrestrial materials show
that the electrical resistivity varies greatly with respect to the
thermodynamic conditions such as the temperature and the percentage of
conductive fluids within the solid matrix (e.g., partial melting,
water-rich fluids). For non-hydrated rocks, the resistivity remains very
high for temperatures up to 1200§ C or even 1800§C in some cases. Molten
rocks have low resistivities (1 - 0.1 W-m) and in the presence of partial
melting, the effective resistivity falls sharply by several orders of
magnitude at constant temperature. In the presence of even a small fraction
of a conductive liquid phase the resistivity sharply drops.
Extremely light, sensitive magnetometers have been developed over the years
for many deep-space and planetary missions. These instruments can be easily
adapted for use on planetary surfaces. The primary difficulties encountered
on the surface are deployment away from magnetically-noisy landers
(extremely long booms are impractical) and the extremes of temperature
often encountered.
I'm also in Harvard. Check out VVUS's thread. Sounds like early Zitel thread! Good luck also.
Jay |
