Diapositiva 1

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FIRST GEOMAGNETIC DEEP SOUNDING IN THE WESTERN
SECTOR OF THE ANTI ATLAS, SOUTHERN MOROCCO
Domenico Di Mauro1, Faiçal Ramdani2, Massimiliano
Fois1

• ISTITUTO NAZIONALE DI GEOFISICA E VULCANOLOGIA,
  ROME - ITALY
• INSTITUTE SCIENTIFIQUE, RABAT - MOROCCO

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GENERAL INTRODUCTION
Seismology tell us how the inner parts of the
Earth are composed using the mechanical waves
propagating from the source to the rest of the
planet (for strong earthquakes).
Its important to note that seismology revealed
that a liquid inner core was present, since
seismograms were available.
Another (independent) way to probe our planet is
offered by the time changes of the Earths
magnetic field (from below and from above).
Geomagnetism gives us inexpensive methods for
probing the planet at different depth, from
mantle up to shallow crust, depending on the time
windows we are looking at.
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WHAT IS A GEOMAGNETIC (DEEP) SOUNDING?
Short variations of the field are associated to
EM waves of external origin, generated by changes
in the electrical currents in the ionosphere and
magnetosphere, and penetrating into the Earth.
Here in turn the electrically conducting layers
of the Earth produce their own surface magnetic
fields.
The depth of penetration is controlled by the
skin depth relationship d, expressed as follows
given in km when r, the electrical resistivity is
given in W.m and the period T, in seconds.
(From the Danish National Space Center website)
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CONCEPT OF SKIN DEPTH
Skin depth for a uniform half-space in dependence
on resistivity r and period T. (From
Encyclopedia of Geomagnetism and Paleomagnetism,
Springer 2007)
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WHAT IS A GEOMAGNETIC (DEEP) SOUNDING? (3)
In geomagnetic deep sounding (GDS) the deduction
of the internal electrical conductivity
distribution is based only on three time-varying
geomagnetic field components X, Y and Z (or H, D,
and Z).
Basically, the relationship between external and
internal magnetic fields, at a range of
frequencies, holds information on the radial
conductivity distribution. However, the presence
of lateral electrical conductivity differences at
depth perturbs the flow of induced currents and
produces frequency-dependent anomalies in the X,
Y and Z components. The detection of such
anomalies can be facilitated by correlating data
from closely spaced sites, recorded preferably by
an array of simultaneously operating magnetometers
. The detection and interpretation of these
frequency-dependent anomalies, related to lateral
electrical inhomogeneities, give the basis and
objectives of the GDS technique. In an
alternative sense, the GDS combines the principle
of both, soundings (depth) and profiling
(lateral), to image lateral conductivity
inhomogeneities at varying depths.
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Recipe for making a Geomagnetic Deep Sounding

• Collecting data (of good quality ) in the
  planned area for as long as possible
• Overlapping (possibly) the periods of the data
  collection for the different stations (for making
  possible a inter-station reference)
• Selection of data segments (for ex. during
  night-time) to be used for the analysis
• Calculation of the TRANSFER FUCTIONS A(f) and
  B(f) between the vertical and horizontal
  components from the linear formula
• Computation of the INDUCTION ARROWS (elsewhere
  known as Parkinsons vectors or Wiese vectors) as
  follows
• Vr,i Ar,i(f)2 Br,i(f)21/2 and
  Phaser,i arctg Br,i(f)/Ar,i(f)180
• Modeling the area of interest (for a reliable
  estimation of the electrical conductivities and
  depths of the stratified Earth)

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OPERATIONS ON THE FIELD
EQUIPMENT LEMI-018 (manufactured by Lviv Centre
of Institute of Space Research Resolution 0.1
nT Noise level (0.01-1)Hz drift 8
WHERE IN MOROCCO?
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WHY JUST THERE?
The project was funded by the Italian Ministry of
Foreign Affair in the frame of a special law to
sustain the development policy of North African
countries.
The equipment (2 vectorial magnetometer, 1 scalar
magnetometer PCs other) was left to the
Moroccan institution as a donation from the
Italian government
A secondary aim of the project was to improve the
AVERROE geomagnetic observatory capability in
data collecting, storage and dissemination
(hopefully via Intermagnet network)
IAGA Code AVE Latitude 331753 NLongitude
72448 W Altitude 230m
Averroe obs. is used as reference station for
this GDS application
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TECTONIC SECTING of the investigated area
(from Burkhard, M., S. Caritg, U. Helg, C.
Robert-Charrue and A. Soulaimani. 2006. Tectonics
of the Anti-Atlas of Morocco, C.R. Geoscience, v.
338,1-2, .pp11-24)
The Anti-Atlas Mountains are a 700 km long Pan
African fold, formed as a result of the collision
of the African and Eurasian plates about 80
million years ago. It exposes the oldest rocks
of the Atlas domain.
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EXAMPLES OF INDUCTION ARROWS (from a selection of
night-time dataset)
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EXAMPLES OF INDUCTION ARROWS (from a selection of
night-time dataset)
Station Vrerr ReAzerr Vierr ReAzerr ----------
--------------------------------------------------
------------------------ Stat1 0.43.02 16215.5 0
.18.02 11210.0 Stat2 0.45.05 15112.3 0.08.02
16715.3 Stat3 0.46.02 15810.4 0.09.01 -17912.
0 Stat4 0.57.02 17715.7 0.27.02 12210.0
Stat5 0.55.05 16215.8 0.17.02 13513.3
Stat6 0.57.08 -17010.4 0.35.10 11912.5
Stat7 0.43.02 15112.6 0.09.05 14114.6
Stat8 0.38.05 15315.7 0.08.05 16810.2
Stat9 0.57.08 14612.9 0.28.10 15610.8
Stat10 0.62.02 15410.5 0.19.02 13912.0
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• Conclusion
• Geomagnetism can be used as a probing tool for
  local, regional and global scale surveys
• In particular, results from GDS can give a
  valuable contribution to establish the tectonic
  frame of a studied area
• In the studied area (West Sector of the Anti
  ATLAS chain) from Induction Arrows we can
  preliminary say that an evident and persistent
  indication of an electrical conductivity contrast
  is pointing the magnetic anomalies of the
  Atlantic rifting system (West of the studied
  area)
• Further deep investigations and appropriate
  modelings are necessary to better define the
  frame
• THANK YOU