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Polynomial Equivalent Layer. Vanderlei C. Oliveira Jr. Observatório Nacional. Valéria C. F. Barbosa*. Observatório Nacional. Contents. Classical equivalent-layer technique. The main obstacle. Polynomial Equivalent Layer (PEL). Synthetic Data Application. Real Data Application. - PowerPoint PPT Presentation
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Polynomial Equivalent Layer
Valéria C. F. Barbosa*
Vanderlei C. Oliveira JrObservatório Nacional
Observatório Nacional
Contents
• Conclusions
• Classical equivalent-layer technique
• Polynomial Equivalent Layer (PEL)
• Real Data Application
• Synthetic Data Application
• The main obstacle
yxN E
zDep
th
3D sources
Potential-field observations produced by a 3D physical-property distribution
Potential-field observations
Equivalent-layer principle
can be exactly reproduced by a continuous and infinite 2D physical-property distribution
yxN E
Dep
th
Potential-field observations
can be exactly reproduced by a continuous and infinite 2D physical-property distribution
Potential-field observations produced by a 3D physical-property distribution
Equivalent-layer principle
z
2D physical-property
distribution
This 2D physical-property distribution is approximated by a finite set of equivalent sources arrayed in a layer with finite horizontal dimensions and located below the
observation surface
yxN E
zDep
thD
epthLayer of equivalent
sources
Potential-field observations
Equivalent sources may be
magnetic dipoles, doublets,
point masses.Equivalent Layer
(Dampney, 1969).
Equivalent-layer principle
Equivalent sources
• Interpolation
To perform any linear transformation of the potential-field data
such as:
• Upward (or downward) continuation
• Reduction to the pole of magnetic data
(e.g., Silva 1986; Leão and Silva, 1989; Guspí and Novara, 2009).
(e.g., Emilia, 1973; Hansen and Miyazaki, 1984; Li and Oldenburg, 2010)
(e.g., Cordell, 1992; Mendonça and Silva, 1994)
• Noise-reduced estimates
(e.g., Barnes and Lumley, 2011)
Equivalent-layer principle
How ?
Classical
equivalent-layer technique
Classical equivalent-layer technique
yxN E
De
pth
Potential-field observations
d NR
We assume that the M equivalent sources are distributed in a regular grid with a constant
depth zo forming an equivalent layer
zo
Equivalent sources
Equivalent Layer
Classical equivalent-layer technique
yx
N E
y
E
x
N
Ph
ysic
al-p
rop
ert
y
dis
trib
uti
on
Estimated physical-property
distribution
Equivalent Layer D
epth
Transformed potential-field data
p*
t T p*=
How does the equivalent-layer technique work?
?
Potential-field observations
Step 1: Step 2:
Why is it an obstacle to estimate the physical property
distribution by using the classical equivalent-layer technique?
Classical equivalent-layer technique
A stable estimate of the physical properties p* is obtained
by using:
Parameter-space formulationp* = (GT G + I ) -1 GT d,
p* = GT(G GT + I ) -1 d Data-space formulation
or
The biggest obstacle
(M x M)(N x N)
A large-scale inversion is expected.
Objective
We present a new fast method for performing any linear
transformation of large potential-field data sets
Polynomial Equivalent Layer
(PEL)
Polynomial Equivalent Layer
kth equivalent-source window
with Ms equivalent sources
The equivalent layer is divided into a regular grid of Q equivalent-source windows
Ms <<< M
Inside each window, the physical-property distribution is described by a
bivariate polynomial of degree .
12
Q
dipoles (in the case of magnetic data)
Equivalent sources
point masses (in the case of gravity data).
Phy
sica
l-pro
pert
y di
strib
utio
n
The physical-property distribution within the equivalent layer is
Polynomial Equivalent Layer
Equivalent-source window
Polynomial function
assumed to be a piecewise polynomial function
defined on a set of Q equivalent-source windows.
Phy
sica
l-pro
pert
y di
strib
utio
n
Equivalent-source window
Polynomial Equivalent Layer
How can we estimate the physical-property distribution within the entire equivalent layer ?
Phy
sica
l-pro
pert
y di
strib
utio
nkth equivalent-source window
Polynomial Equivalent Layer
Physical-property distribution pk
Relationship between the physical-property distribution pk within the kth
equivalent-source window and the polynomial coefficients ck of the th-order polynomial function
Polynomial coefficients ck
kckB
kp
Phy
sica
l-pro
pert
y di
strib
utio
n
Polynomial Equivalent Layer
Physical-property distribution p
How can we estimate the physical-property distribution p within the entire equivalent layer ?
All polynomial coefficients cEntire equivalent layer
B c(H x 1)
p(M x 1) (M x H)
QB00
0B0
00B
Β
2
1
Q equivalent-source windows
Estimated polynomial
coefficients
How does the Polynomial Equivalent Layer work? Polynomial Equivalent Layer
Step 1:
N E
Potential-field observationsD
epth
Equivalent layer with Q equivalent-source
windows
c*
Phy
sica
l-pro
pert
y di
strib
utio
n
Computed physical-property
distribution p*
EN
Transformed potential-field data
t T p*=
c*Bp*
Step 3:
Step 2:
?
How does the Polynomial Equivalent Layer estimate c*?
H is the number of all polynomial coefficients describing all polynomial functions
H <<<< M H <<<< N
Polynomial Equivalent Layer
(H x H)
A system of H linear equations in H unknowns
Polynomial Equivalent Layer requires much less computational effort
c dGB TT
R BRBIG BGB TTTT ] ) ( [ 10
-1
A stable estimate of the polynomial coefficients c* is obtained by
Polynomial Equivalent Layer
the smaller the size of the equivalent-source window
THE CHOICES:
The shorter the wavelength components of the anomaly
the lower the degree of the polynomial should be.
A simple criterion is the following:
and
• Size of the equivalent-source window
• Degree of the polynomial
Gravity data set Magnetic data set
Polynomial Equivalent Layer
Large-equivalent source window andHigh degree of the polynomial
Small-equivalent source window and Low degree of the polynomial
EXAMPLES
Ph
ysic
al-p
rop
erty
di
strib
utio
n
How can we check if the choices of the size of the equivalent-source window and the degree of the polynomial
were correctly done?
Acceptable data fit.
Polynomial Equivalent Layer
A smaller size of the equivalent-
source window and (or) another
degree of the polynomial
must be tried.
Unacceptable data fit.
Estimated physical-property
distribution via PEL yields
Application of
Polynomial Equivalent Layer (PEL)
to synthetic magnetic data
Reduction to the pole
Simulated noise-corrupted total-field anomaly
computed at 150 m height
Polynomial Equivalent Layer
A
B
C
The number of observations is about 70,000
The geomagnetic field has inclination of -3o and declination of 45o.
The magnetization vector has inclination of -2o and declination of -10o.
Polynomial Equivalent LayerTwo applications of Polynomial Equivalent Layer (PEL)
Large-equivalent-source window Small-equivalent-source window
First-order polynomials
First Application of Polynomial Equivalent Layer
Large window
Large-equivalent-source windows and First-order polynomials
M ~75,000 equivalent sources
H ~ 500 unknown polynomial coefficients
The classical equivalent layer
technique should solve
75,000 × 75,000 system
The PEL solves a 500 × 500 system
Computed magnetization-intensity distribution obtained by PEL
with first-order polynomials and large equivalent-source windows
A/m
First Application of Polynomial Equivalent Layer
Differences (color-scale map) between the simulated (black contour lines)
and fitted (not show) total-field anomalies at z = -150 m.
Large windownT
Poor data fit
First Application of Polynomial Equivalent Layer
Small-equivalent-source windows and First-order polynomials
Small window
M ~ 75,000
equivalent sources
H ~ 1,900 unknown polynomial coefficients
Second Application of Polynomial Equivalent Layer
The PEL solves a 1,900 × 1,900
system
The classical equivalent layer
technique should solve
75,000 × 75,000 system
Computed magnetization-intensity distribution obtained by PEL
with first-order polynomials and small equivalent-source windows
A/m
Second Application of Polynomial Equivalent Layer
Differences (color-scale map) between the simulated (black contour lines)
and fitted (not show) total-field anomalies at z = -150 m.
Small window
nT
Acceptable data
fit.
Second Application of Polynomial Equivalent Layer
Polynomial Equivalent LayerTrue total-field anomaly at the pole
(True transformed data)
Polynomial Equivalent LayerReduced-to-the-pole anomaly (dashed white lines) using the
Polynomial Equivalent Layer (PEL)
Application of
Polynomial Equivalent Layer
to real magnetic data
Upward continuation and
Reduction to the pole
São PauloRio de Janeiro
Aeromagnetic data set over the
Goiás Magmatic
Arc, Brazil.
Brazil
Real Test
Aeromagnetic data set over the Goiás
Magmatic Arc in central Brazil.
The geomagnetic field has inclination of -21.5o and declination of -19o.
The magnetization vector has inclination of -40o and declination of -19o.
N
M ~ 81,000 equivalent sources
H ~ 2,500 unknown polynomial coefficients
N ~ 78,000 observations
Small-equivalent-source windows and First-order polynomials
Small-equivalent source window
The classical equivalent layer
technique should solve
78,000 × 78,000 system
The PEL solves a 2,500 × 2,500
system
Real Test
Computed magnetization-intensity distribution obtained by
Polynomial Equivalent Layer (PEL)
N
Real Test
N
Observed (black lines and grayscale map) and
predicted (dashed white lines) total-field anomalies.
Acceptable data
fit.
Real Test
N
Transformed data produced by applying the upward continuation and the
reduction to the pole using the Polynomial Equivalent Layer (PEL)
Conclusions
Conclusions
We have presented a new fast method (Polynomial Equivalent Layer- PEL)
for processing large sets of potential-field data using the equivalent-layer principle.
The PEL divides the equivalent layer into a regular grid of equivalent-source
windows, whose physical-property distributions are described by polynomials.
The PEL solves a linear system of equations with dimensions
based on the total number H of polynomial coefficients within all
equivalent-source windows, which is smaller than the number N
of data and the number M of equivalent sources
The estimated polynomial-coefficients via PEL are transformed into the physical-
property distribution and thus any transformation of the data can be performed.
Polynomial Equivalent Layer
H <<<<< N H <<<<< M
Thank youfor your attention
Published in GEOPHYSICS, VOL. 78, NO. 1 (JANUARY-FEBRUARY 2013)
10.1190/GEO2012-0196.1