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UPSL has recently aquired a state
of the art Phynix V5-2000 24-bit MT system. Our main focus is the interpretation of
Magnetotelluric data
and field QC of data
acquisition. We also work with other techniques such as AMT, CSAMT, and EM.
UPSL's personnel have supervised field acquisition in various
countries for petroleum, mineral, and geothermal resources.
Recent applications of Magnetotellurics have produced results
superior to those of earlier years by many improvements in the technology, the
application and the interpretation of the method. MT continues to be useful in
those areas where seismic acquisition is either difficult or expensive, where
the surface inhibits effective seismic penetration, or where the use of MT is
cost-effective as a predecessor to seismic.
Improvements made to MT
within the last few years include: multi-site acquisition with varied station
spacings and omni-directional, low frequency antennae; smaller, higher storage
field computers, sensor-box modifications for filtering unwanted signal (such as
lightning); post-acquisition re-processing of the data via robust schemes; and
faster, larger 2-D and 3-D modeling capabilities.
Within the last two
years, MT has been used world-wide for a variety of exploration targets. These
include the continued use of MT for mapping sub-carbonate plays in Papua New
Guinea with on-going drilling. Here, use of seismic is precluded due to thick,
high-velocity surface limestone. MT has been used as a precursor to seismic in
overthrust regimes in Nevada and Colorado where MT is used to highgrade areas as
prospects before seismic programs are acquired. In Greenland, MT has been used
for reconnaissance mapping in large basin environments
sub-basalt.
On-going system modifications are being made to provide
larger channel systems and future marine acquisition of MT.
Exploration programs are being driven into areas of
increasing seismic difficulty with associated escalation in cost. Depth imaging,
potential field and electrical methods geophysics are playing an increasingly
important role to help the explorationist reduce risk in these areas. In so far
as electrical methods are concerned, exploration environments can be broadly
cast into the following categories: limestone, volcanic, basement thrusts or
other dense layer at the surface or in the geological section, severe
topography, and environmentally sensitive areas. Now, thanks to new technology,
electrical methods, in particular Magnetotelluric (MT), are making a
strong comeback as a cost-effective aid to exploration showing an increasingly
valuable role augmenting seismic where seismic data quality is poor and
difficult to interpret. Hundreds of MT systems are currently in use throughout
the world (Christopherson, 1997). However the results of several surveys
conducted in highly noisy environments, as in Italy. have shown the severe
limitation of the MT method when applied to these extreme. Good news! This paper
focuses on the practical application of the current state of the art in
acquisition and processing capable of producing reliable and interpretable MT
data in these challenging environments.
The problem
The
noise sources in difficult MT areas are mainly of two kinds. The first kind is
due to the presence of power plant equipment, power lines and other local
cultural disturbances. All these local disturbances produce an incoherent noise
mainly affecting higher frequencies - usually above 1 Hz, - which can be removed
using a remote-reference site only few kilometers away. The second kind of noise
is due to the presence of very strong sources of coherent electromagnetic
signals such as direct current electric trains and active cathodic protection of
pipelines that can render MT data useless at frequencbelow 1 Hz. Although the
data may appear very coherent, the apparent resistivity and phase derived from
these data look very much like the response due to a local dipole source instead
of the plane wave MT source. Once the reference processing has removed the bias
noise, the data scatter and poorer point-to-point continuity indicate the poor
signal to noise ratio of what remains. The effects of the electromagetic field
produced by electric trains and cathodic protection systems are sources of
serious artificial disturbance for natural magnetic and electric fields
measurements even if the observation sites are situated some ten kilometers away
from the noise source. Standard remote reference processing coupled with various
sort of robust data adaptive weighting scheme are ineffective when confronted
with these sorts of difficulties, in particular, if there are outliers or strong
correlated noise signals in most data sections.
The
solution
I It is our view, that in order to fully optimize the
potential of MT applied to highly noisy environments; a comprehensive overall
approach is required. To this end, critical parts of the acquisition and
processing flows have been reworked. New acquisition and processing technologies
have been introduced specifically tuned to high coherent noise constraints. The
following elements have formed the overall frame of the comprehensive
approach:
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The V5 System 2000 uses a new
approach to EM data acquisition based on multiple, rugged, lightweight,
independent data acquisition units (MTUs), which are synchronized by time
signals broadcast by the GPS (Global Positioning Satellite) system. Use of
24-bit analog-to-digital converters provides wide dynamic range,
simplified circuitry, and lower cost.
The V5 System 2000 is offered
for the MT (Magnetotelluric, AMT (Audiofrequency MT) techniques. Future
enhancements will provide data acquisition capability for other commonly
used EM techniques, including Long Period Transient EM and Induced
Polarization (IP).
The field configuration and
spacing of the MTUs can be varied with complete flexibility, according to
the requirements of the survey. No cable links are required between the
boxes - an important advantage in areas with rugged topography, lakes,
swamps and other access difficulties.
Where access is easier, or if
required, the MTUs can be deployed at very close spacing or for continuous
profiling. |
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The MTU is provided in 3
configurations. The MTU-2E measures 2 channels of electric field data; the
MTU-3H measures 3 channels of magnetic field data; the MTU-5 combines
both.
When the V5 System 2000 is
used for MT, the electric field is preferably measured at a closer spacing
than the magnetic field. Thus, most of the deployed units are usually
MTU-2Es, with a smaller number of MTU-3Hs or MTU-5s. The MTU-2E with
electric field sensors costs significantly less than the MTU-3H / MTU-5
with magnetic sensors, so that the required sampling density can be
achieved at an acceptable cost.
Each MTU is configured with
ample solid state memory, onboard processor and diagnostics, onboard GPS
antenna and auxiliary precision timing. The solid aluminum enclosures are
equipped with rugged, military style connectors.
The lightweight MTUs can be
easily carried to remote locations. In the distributed (stand alone)
configuration, data is retrieved by ruggedized PCs for transfer to base
camp.
An important benefit of the
innovative architecture of the V5 System 2000 is the light weight and
extreme portability of the MTUs. All the equipment for a single 2-E
measurement is easily backpacked by two field workers, reducing labor
costs and permitting economical MT and other EM surveys even in remote,
rugged, off-road areas.
The MTUs can be deployed
without any cable links between the boxes, providing great logistic
flexibility.
In reconnaissance of large
areas, a multi-pass data acquisition strategy is used. The MTUs are
deployed at relatively wide spacing in a first "pass" of data acquisition.
If areas of interest are discovered in the first pass the MTUs can be
immediately redeployed in those areas at closer spacing in the second pass
to provide a higher resolution image. In this way the spatial sampling
density is always optimized and cost is always minimized.
New data acquisition
techniques provide far more data than previous generation equipment. New
multi-site processing techniques provide better interpretation, even in
noisy areas. |
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