inversion of z-axis tipper electromagnetic (z-tem) data the ubc geophysical inversion facility...
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Inversion of Z-Axis Tipper Electromagnetic (Z-TEM) Data
The UBCThe UBCGeophysical Inversion FacilityGeophysical Inversion Facility
Elliot Holtham and Douglas Oldenburg
Outline
Introduction
Transfer functions and Z-TEM data
Conducting block example
Synthetic inversion
Field data inversion
Conclusions
Questions / Discussion
Z-TEM Technique
Z-TEM technique uses natural source fields similar to the magnetotelluric (MT) technique
Vertical magnetic fields due to natural sources are recorded over the survey area
Z-TEM Technique
Data relates the vertical magnetic field to the horizontal field at some fixed grounded reference station
Reference station compensates for unknown source field amplitude
Large areas can be surveyed quickly and economically
Promising technique to find large scale structures at depth
Transfer Functions
Transfer functions relate the vertical magnetic field measured above the earth to the horizontal magnetic field at some fixed reference station
Two unknowns and only one equation
Source polarization is assumed to be random
Use measurements from independent source polarizations
Computation of Transfer Functions for a Conducting Block
Source electric field into the page
Resulting charge buildup leads to a secondary current
Conducting block
Transfer Function Computation for Synthetic Example
Secondary electric fieldResulting transfer functions
Inversion of Z-TEM Data
Inversion algorithm has been implemented
2)( re fm mmW
)(][,)][(2
Q umdmW d fFF o b sd where
: Regularization parameterQ: Projection matrix u: Fields : Observed data : Model and Reference model
o b sd
re fmm ,
Wd, W : Data error, model weighting
Minimize = d + m
Inversion of Z-TEM Data
Minimize = d + m
)()( mgmWWJJ δTT Gauss-Newton method
)(]][[)( re fTo b sT
dT F mmWWdmWJmg
m
Synthetic Inversion Example
•Data computed at 1, 3.2, 5.6, 10, 18, 32 Hz
•Reference Station: (-3000, -3000, 0)m
•Data collected at a constant height of 100m
•Data collected over an area of 2500 x 2500m
•10m data spacing and 50m line spacing
Bingham Canyon Field Data Inversion
Bingham Canyon site is 50 km west of Salt Lake city. Z-TEM data acquired in the winter of 2008
Rugged topography
30, 45, 90, 180, 360 Hz, real and imaginary components
1000 m flight line spacing. 497.5 line-km of data
96 x 92 x 95 cell mesh
Error Assignment
The range of the data is determined by sorting each data (ie. real Tzx , Imag Tzx, Real Tzy, Imag Tzy)
The range is set to be the difference between the 90th and 10th percentile data
The assigned standard deviation is then a small fraction of this range. Initially C=0.125 for all frequencies.
Error Assignment
Invert each frequency of the data with the assigned standard deviations.
Adjust the constant C, until the final misfit achieves the target misfit
Misfit
Error Assignment
Inverting and adjusting the errors on each frequency separately gives the correct weighting of each frequency
Ensures that no frequency dominates inversion
45 Hz misfit – scale factor 1.50 180 Hz misfit – scale factor 1.90
Comparison of Inverted Model with Geology(Surface resistors and conductors)
Inverted Model Geologic Model
Comparison of Inverted Model with Geology(Surface resistors and conductors)
Inverted Model Geologic Model
Comparison of Inverted Model with Geology(Surface resistors and conductors)
Inverted Model Geologic Model
Comparison of Inverted Model with Geology(resistors and conductors below 1600m)
Inverted Model Geologic Model
Conclusions
Z-TEM data can be forward modeled
Inversion algorithm exists for inverting Z-TEM data
Inversion yields encouraging results on a synthetic model
Z-TEM technique has been applied to a field dataset and yields good results
Shows promise to find large scale structures at depth
Acknowledgements
Michael Zang and Exploration Syndicate, Inc.
Thank You