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Advances in Applications of Magnetotellurics
to Hydrocarbon Exploration in Papua New Guinea
By Karen Rae Christopherson
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This paper was given at the AAPG meeting
in Reno in July 1995.
More than 800 stations have been acquired in PNG since 1988. There are
several unique acquisition and data problems found in applying the MT
method in PNG. Over the past seven years, many improvements and changes
have been made to acquisition and interpretation in order to provide
solutions to these data problems.
Acquisition and Data Problems:
More difficult than most areas of the
world owing to:
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Low signal strength (as PNG is close
to the equator) |
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Almost continuos lightning noise
(which can travel long distances in the resistive limestone outcrop
and can appear as coherent noise) |
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Rugged terrain, which requires
helicopter transport, difficult layout, and prohibits hard-wire
telemetry in most cases |
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Complex geology which creates static
shifts, 2D to 3D data, and noise |
Solutions
Several changes, improvements, and
modifications have been made to the acquisition equipment and logistics,
processing software, interpretation methodology and interpretation
software in order to improve the results from MT surveys in PNG. These
have all been done in the last two to three years and are on-going.
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a) Radio Telemetry: The current
Zephyr system allows for the recording of up to 5 stations
simultaneously. This speeds up acquisition and reduces overall
costs. Current antennae are omni-direction and low-frequency
allowing for transmission over hills and karsts. |
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b) Sensor Box Modifications: The
"SP" boxes (where data inputs are filtered and amplified
before transmission) have been re-designed to limit lightning
effects by the use of low pass filters, different corner
frequencies, and specialized diodes. These changes have greatly
limited the effect of lightning on the times series data. |
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c) Robust processing:
Post-acquisition re-processing with a robust scheme has been vary
effective in removing unwanted noise (lightning) from the time
series data. Although data are edited in the field, in severe cases,
they can be re-edited where lightning effects are strongly biasing
the data. |
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d) CSAMT and other near-surface
techniques: The use of CSAMT, or AMT, has been attempted in order to
acquire higher frequency data than MT. These data can be used to
help in statics corrections and to confirm the MT interpretation as
deep as base HW Darai. |
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e) Interpretation: All MT
interpretation is currently done on a Geotools workstation using a
PC, Sparc2 or equivalent. This has meant a faster turn-around on
interpretation (in as short as two days), larger models for
the 2-D and 3-D codes, and better integration of other information.
Advances in the understanding of PNG geology has helped also. |
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