University of SaskatchewanSeminar 8 October 2004 Markku Peltoniemi Helsinki University of Technology (TKK)

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University of Saskatchewan Seminar 8 October
2004Markku Peltoniemi Helsinki University of
Technology (TKK)

• Current Trends in Airborne Geophysics

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Contents

• Quick Overview
• High-Accuracy GPS
• Aeromagnetic Method
• Gradient measurements
• Airborne Electromagnetics
• Depth of penetration
• Airborne Gamma-Ray Spectrometry
• Spectral component analysis
• Airborne Gravity
• Wavelet Analysis

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Quick Overview How and Why Airborne Geophysics
Aircraft Sensors
(Geological Survey of Finland GTK Cessna Caravan)
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(GTK 2003) Twin Otter
Cessna Caravan
Instrumentation Data Acquisition
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Digital Data Quality Control
(GTK 2003)
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Digital Images Interpretation
http//www.gsf.fi/midnord/
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GPS (Global Positioning System)
The travel time of the signal from a satellite to
a receiver is measured and converted into a
precise distance
24 satellites z 20.200 km t 11 h 58 min
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GPS Pseudo-Range ri c Dt
DGPS using two receivers, one stationary, one
mobile
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GPS Signal Structure
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New DGPS Developments - VRS

• VRS Virtual Reference Station
• carrier wave phase measurements closely-spaced
  stationary receivers
• cm-range accuracy

(Landau et al. 2001)
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VRS Service in Finland
opened in Dec. 2003, full coverage in 2005
(Hakli 2004)
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VRS Accuracy
(Hakli 2004)
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AeromagneticMethod

• Rock magnetism
• Gradient measurements
• Levelling of aeromagnetic data

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Rock Magnetism Crustal Field

• Mi kH (induced magnetisation)
• Mtot Mi Mr (total induced remanent
  magnetisation)
• Measured field
• Bt moH moMi moMr
• mo(1k)H moMr
• demagnetisation term
• high-magnetisation effects

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Aeromagnetic Gradient Measurements

• Advantages
• temporal field variations are cancelled
• near-surface source effects increase
• contact effects increase
• gridding accuracy is improved
• Difficulties
• increased accuracy in data acquisition is needed
• increased accuracy in navigation and positioning
  is needed

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HM5 Gradiometer / Fugro AS350 helicopter
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Magnetic Gradiometer (Sander Cessna Caravan)
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AM Horizontal Gradient Gridding
Bps Blinja Dy ()dypsDy measured
dBt/dyBlinja (BleftBright)/2
Bps
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AM Horizontal Gradient
(GTK 2003)
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TRIAX / Fugro
source between flight lines
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Factors Affecting AM Accuracy
Table summarizes the sources of, and typical
ranges of, expected noise for various types of
aeromagnetic surveys. SURVEY ACCURACY (in nT)
Survey sensor Alkali Vapor Proton Fluxgate Res
olution .01 -.25 .1 - 1 .1 - 2 Instrumental
error .01 - .5 .1 - 1. .5 - 1. Diurnal etc. .5
- 2. .5 - 2 .5 - 2 Positioning Errors .25 -
5 .25 - 5 .25 - 5 Total .77 - 4.75 .95 -
9 1.35 - 10 As is evident in this table, the
major noise sources are the temporal changes and
positioning errors.
(http//www.geoexplo.com/airborne_survey_workshop.
htmlsurvey_costs)
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Levelling Datasetsfrom Different Surveys
60 surveys in Alaska unlevelled local
levelling corrections final levelling
corrections, including long-wavelength
corrections (IGRF)
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Airborne Electromagnetic Method
GTK Cessna Caravan AEM - wingtip intallation 2
frequencies, rigid-coil system (HMD)
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Helicopter AEM Frequency Domain Fugro RESOLVE
rigid-coil system
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Time Domain Towed Bird AEM
Fugro Airborne Surveys
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Helicopter AEM in Time Domain
Aeroquest AEROTEM
Geotech VTEM
Rationale Improved Spatial Resolution
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Skin Depth vs Depth of Penetration
Skin depth di is defined as the depth at which
the intensity of EM field in a conductive medium
is deceased into 1/e (37 ) from the original
value at the surface. Skin depth for a plane wave
excitation is defined easily as di ????????
503v(r/f ) (m) 1D Skin depth is a measure
for electrical attenuation of EM field in a
conductive (lossy) medium. Geometrical
attenuation is determined by the geometry of the
source, and in case of dipolar AEM sources is
governed by the basic formula
3D
Both types of attenuation affect AEM measurements
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3D Skin Depth Footprint
VMD
(Beamish 2004)
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AEM TD dB/dt vs BMegatem (Fugro)
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Response Function in Time Domain
traditional (dB/dt signal) new (B
signal)
TD Quadrature systems
(Smith Annan 1998)
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TD In-phase Component(On-time new)
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Broadband AEM (new)

• Frequency range                        300 Hz to
  48 kHz
• Number of frequencies              Programmable
  (typically 5)
• GEM-2A is a programmable, broadband EM sensor.
  The system utilizes a single set of
  transmitter-receiver coils for all frequencies

GEM-2A (Geophex)
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Time DomainS/N-Ratio andPenetrationFugro
GEOTEM
GEOTEM noise levels in
(Smith Annan 1997)
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State-of-the-Art AEM Inversion Display
US DoE / Fugro
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State-of-the-Art AEM Forward Modelling
coaxial HEM Inphase the barren in red the
target in blue
(Raiche et al., 2003)
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Airborne Gamma-Ray Spectrometry
KUTh map on topographic relief
(AGSO Journal 1997 Vol 17 No 2)
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Cumulative Gamam-Ray Spectrum
(GTK 2003)
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Noise in Airborne g-Ray Measurements

• Statistical noise
• Radioactivity is a statistical phenomenon
  (Poisson process). If N counts per unit time are
  measured, the standard deviation is s ÖN
• Relative noise De due to low count rate N is
• De ÖN / N 100
• Athmospheric radon
• Meteorological factors - Rn222 half-life is 3.82
  days

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Noise Filtering
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Noise Filtering Principal Component Analysis
principal components 1 - 4
(Grasty Hovgaard 1997)
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Noise Filtering Principal Component Analysis
principal components 5 - 8
(Grasty Hovgaard 1997)
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Uraniumresults before and after processing
(Grasty Hovgaard 1997)
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U Map, Incomplete Rn Correction
(AGSO Journal 1997 Vol 17 No 2)
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U Map after Background Rn Correction
(AGSO Journal 1997 Vol 17 No 2)
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K,U,Th (IHS Color Scheme) on 3D Relief
(IAEA Tech Doc 1363, 2003)
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BHP Billiton FALCONTM airborne gravity meter
Airborne Gravity
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Gravity Gradient Sensors
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Partial Gravity Tensor Measurement
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Eotvos Correction Two parts 1) Aircraft
acceleration gd 2) Coriolis force (deviation of
aircracft acceleration from that of normal
centrifugal acceleration of the Earth) -gt gm -gt
Eotvos correction Eotvos unit 1 Eo 10-4
mGal / m , or 10-9 1/s2
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Example of Eotvos Correction
(Peltonemi Pirttivaara 2002)
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FALCONTM vs GROUND GRAVITY
GRADIENT
GRAVITY
Cross Section
Ground colour FalconTM contours
(ASEG 2001 Melbourne)
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Full Tensor Gradiometer

• introduced by Bell Geospace in 2004
• three perpendicular, rotating inertial platforms
• two pairs of accelerometers on every rotating
  platform

(ASEG Preview August 2003 p. 22)
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Fourier vs Wavelet Analysis

• The Fourier power spectrum will tell you that
  there are two assemblages of sources, shallow and
  deep - this is useful
• The problem is that the Fourier analysis does not
  tell you, where these sources are located (x,y)

(G.Cooper 2002)
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Problems with Fourier Transform

• Instability Stationarity requirement
• Trend in data

(G.Cooper 2002)
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Problems with the Fourier Power Spectrum
(Phase spectra will tell that the two series are
different, but not how in space domain)
(G.Cooper 2002)
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The Wavelet Transform

• Capable of analysing non-stationary signals!

(energy of the wavelength)
time (or position)
(G.Cooper 2002)