Download Geotech VTEM-ZTEM : Airborne Acquisition Systems and

Geotech VTEM-ZTEM :
Airborne Acquisition Systems
and Data Formats
Prepared by
Geoffrey Plastow ([email protected])
AEM Data Processing Manager
Presented by
Carlos Izarra, AEM Interpreter
Collaborators:
Paolo Berardelli ([email protected])
General Director of Sales & Business Development
Fabiana Domingos ([email protected])
Manager of Operations in Brasil
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VTEM/ZTEM Survey Design
Defining
the survey area and airborne geophysical
survey.
 Ideally contain minimal culture – power lines, houses, pipelines,
other current or ongoing geophysical surveys ground or airborne.
Absent of major bodies of water (salt or fresh water).
 Safety concerns flying over large bodies of water.
Salt Water
Masks EM.
Line direction – Typically flown perpendicular to Geological strike
to ensure maximum coverage of Geological contacts which in mining
exploration are sometimes associated with mineral occurrences.
 Can be determined from local / regional geological or geophysical
maps.
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VTEM System
Results of a TEM sounding: Voltage
decay (blue) and apparent specific
resistivity (red) as a function of time after
switchoff of the primary field Source:
BGR
TEM sounding and interpretation.
Source: BGR
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VTEM System
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VTEM Survey Design
The
footprint – or the surface area of the earth that will be
energized will be approximately be 200-250 meters.
Line
Spacing – Exceeding 250 meters may potentially leave
gaps or generate issues with the visualization of the survey data.
Traditionally flown with 100-200 meter line space to ensure
complete coverage for modeling and interpretation.
Survey altitude – As the system is an active TEM system, we
must induce a primary EM field into the earth. The higher from
ground level the system is the less energy penetrates into ground.
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VTEM Survey Design
•
Severe or rugged topography may pose a challenge for any
airborne survey.
•
•
Topography may impact the survey:
–
Sensor altitude (above ground level, a.g.l). Inconsistent line-linetie intersections resulting in inconsistent earth response or signal
levels.
–
Personnel safety above 10,000 ft asl. Oxygen deprivation.
–
Decreased survey speeds, decreased production
Terrain analysis required (strongly adviced) before survey
design.
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VTEM Survey Design

Consider pre-planned drape surface to obtain safe
consistent speed and sensor height.


Can be used to predict potential sensor height agl in any
topography.
Important to use high resolution topographic data to
generate drape surface.


Aim for 30 m sensor agl in flat, 45 m in hills and 65 in
rugged terrain.
Airspeed should be around 80 km/h. Depending on
topography +/- 20km/h
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Loose-Drape
Tight-Drape
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VTEM Instrument Calibration

High Altitude Calibration – 2,000-2,500 ft system
altitude a.g.l. No earth response from system, measure only
system response and or primary field. Also used to measure ideal
system waveform – current waveform.


The shape of the current waveform should be precisely known and
measured continuously.
EM receiver calibration – Coil measures an induced current /
voltage so it is important to ensure the system electronics are not
acting as a resistor to the measured earth response.


The EM receiver should be calibrated to measure and remove the
system response.
EM Parallax or Lag – A conductive object – man made or
geologic to correct for offset in observed data and true geographical
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position.
Ancillary Instrument Calibration
• A heading test should be performed to
remove any magnetic heading error
associated with the direction of flight. The
heading error should be less than 1.5 nT.
• A radar altimeter test should be completed
prior to low level survey flights. The test has
two purposes: (1) To ensure the radar altimeter
measures the correct altitude and increases linearly
with GPS altimeter. (2) To determine the offset in
altitude measuring equipment. Radar altimeter &
GPS altimeter separation.
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Ancillary Instrument Calibration

Airborne GPS Static Test – Recorded on the ground for 2 hours to
watch instrument drift. Displayed in a scatter plot. Mean separation
of points 1 m.
All positional information should be post-processed using a local
GPS base station.

Daily forecasts of DGPS availability should be consulted.

Magnetic Base station
Should be within approx. 100 km of survey area and relocated
during the survey to maintain this minimum distance.
Sample magnetic base station data should be acquired prior to the
survey for inspection and assurance no cultural influence is present
in the dataset.
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Magnetic Data Processing
• Synchronized with airborne data via GPS.
• Lag and heading error correction.
• Removal of magnetic diurnal variations.
• Control line or tie line leveling.
• Micro-leveling.
• Visualization – 1VD, 2VD analysis.
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EM Data Processing
Time Series Analysis and Field Processing
 Sferic Rejection
 EM System Response Correction
 EM Transient Stacking
 EM Time Channel Generation
TDEM Processing in the Office
(using Geosoft)
 EM Data Compensation (Background Removal)
 EM Filtering
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EM Data Processing
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Data Analysis :
EM Products - Basic
•
Time Channel Amplitude Plots – Early, Mid, Late
•
Decay Constant Analysis
•
B Field Analysis – Suppress Overburden response.
•
X Coil – Fraser Filter.
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EM Products – Advanced

Anomaly Picking

Target Grade – Conductance Calculations

Maxwell Modeling

Resistivity Depth Imaging Or Conductivity Depth Imaging

Depth Sections

Depth Slices

1D, 2D, 3D Inversions

Pattern Recognition Analysis with Magnetics
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EM Products – Advanced
South Nuqrah VTEM dBz/dt (ch30 / 880ms)
with Maxwell 2.5D Plates and CVG Contours
L10070 VTEM dBz/dt (0.880-7.04ms)
and Maxwell 2.5D Modelling + RDI Resistivity
30siemens
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VTEM: DELIVERABLES and FORMATS
Two copies of the data and maps on
DVD were prepared to accompany the
report. Each DVD contains a digital file
of the line data in GDB Geosoft Montaj
format as well as the maps in Geosoft
Montaj Map and PDF format.
The survey report describes the
procedures
for
data
acquisition,
processing, final image presentation and
the specifications for the digital data set.
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VTEM: Geosoft GDB Data Format
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ZTEM System
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ZTEM
System
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ZTEM System
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ZTEM Survey Design
• Larger footprint – typically survey lines are spaced 200500m. We lose spatial resolution as we increase survey
line spacing.
• Less sensistive to Terrain Clearance – Due to slow fall
of natural fields the ZTEM system can be flown at higher
sensor altitudes relative to active EM systems. Terrain
clearance is still important and should be measured and
controlled.
 EM sensor altitude should be 80 m to 150m agl (or
less).
• Airspeed should be around 80 km/hr
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ZTEM Survey Design
• Offer a range of frequencies from 25 Hz to 720 Hz.
• A minimum of the 5 lowest frequencies should be
available for surveying: 25 Hz – 360 Hz.
• ZTEM base station should be within 50 km of the
survey area. This ensures the a strong correlation
between airborne components and ground
components for tipper evaluation.
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ZTEM Survey Design
• Following compensation, small non-linear and low pass filters are applied to
the data.
Raw + Corrected In-line (Tzx) xIP
Corrected + Final Smoothed In-line (Tzx) xIP
Raw + Corrected Cross-line (Tzy) yIP
Corrected + Final Smoothed Cross-line (Tzy) yIP
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ZTEM: Geosoft GDB Data Format
Polarity Convention
DT Maps
TPR Maps
PR = Phase Rotation
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ZTEM Survey Design
A) ZTEM 90Hz In-Phase Total Divergence
B) Total Magnetic Intensity
2D
0
5km
n
io
n
io
ct
ct
Se
5km
Se
2D
0
C) ZTEM 2D INVERSION ACROSS PALEOZOIC SEDIMENTARY BASIN
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ZTEM Products – Advanced
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VTEM-ZTEM
Pre-Survey HSE
• Safe location to assemble equipment and land
aircraft.
Cooperation with local community.
• Emergency response plan.
• Helicopter Safety and Awareness training for on-site
staff.
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Obrigado pela sua atenção
À ANP, muito obrigado pelo
seu convite .
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