![Page 1: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/1.jpg)
Lecture 14
Nonlinear ProblemsGrid Search and Monte Carlo Methods
![Page 2: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/2.jpg)
SyllabusLecture 01 Describing Inverse ProblemsLecture 02 Probability and Measurement Error, Part 1Lecture 03 Probability and Measurement Error, Part 2 Lecture 04 The L2 Norm and Simple Least SquaresLecture 05 A Priori Information and Weighted Least SquaredLecture 06 Resolution and Generalized InversesLecture 07 Backus-Gilbert Inverse and the Trade Off of Resolution and VarianceLecture 08 The Principle of Maximum LikelihoodLecture 09 Inexact TheoriesLecture 10 Nonuniqueness and Localized AveragesLecture 11 Vector Spaces and Singular Value DecompositionLecture 12 Equality and Inequality ConstraintsLecture 13 L1 , L∞ Norm Problems and Linear ProgrammingLecture 14 Nonlinear Problems: Grid and Monte Carlo Searches Lecture 15 Nonlinear Problems: Newton’s Method Lecture 16 Nonlinear Problems: Simulated Annealing and Bootstrap Confidence Intervals Lecture 17 Factor AnalysisLecture 18 Varimax Factors, Empircal Orthogonal FunctionsLecture 19 Backus-Gilbert Theory for Continuous Problems; Radon’s ProblemLecture 20 Linear Operators and Their AdjointsLecture 21 Fréchet DerivativesLecture 22 Exemplary Inverse Problems, incl. Filter DesignLecture 23 Exemplary Inverse Problems, incl. Earthquake LocationLecture 24 Exemplary Inverse Problems, incl. Vibrational Problems
![Page 3: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/3.jpg)
Purpose of the Lecture
Discuss two important issues related to probability
Introduce linearizing transformations
Introduce the Grid Search Method
Introduce the Monte Carlo Method
![Page 4: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/4.jpg)
Part 1
two issue related to probability
not limited to nonlinear problemsbut
they tend to arise there a lot
![Page 5: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/5.jpg)
issue #1
distribution of the data matters
![Page 6: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/6.jpg)
d(z) vs. z(d)0 2 4 6
0
1
2
3
4
5
6
z
d
0 2 4 60
1
2
3
4
5
6
d
z
0 2 4 60
1
2
3
4
5
6
z
d
not quite the sameintercept -0.500000 slope 1.300000intercept -0.615385 slope 1.346154
d(z) z(d) d(z)
![Page 7: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/7.jpg)
d(z)d are Gaussian distributedz are error free
z(d)z are Gaussian distributedd are error free
![Page 8: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/8.jpg)
d(z)d are Gaussian distributedz are error free
z(d)z are Gaussian distributedd are error free
not the same
![Page 9: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/9.jpg)
lesson learned
you must properly account for how the noise is distributed
![Page 10: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/10.jpg)
issue #2
mean and maximum likelihood point can change under reparameterization
![Page 11: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/11.jpg)
consider the non-linear transformation
m’=m2
with
p(m) uniform on (0,1)
![Page 12: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/12.jpg)
0 0.5 10
0.5
1
1.5
2
m
p(m
)
0 0.5 10
0.5
1
1.5
2
m
p(m
)
p(m’)= (m’)½ -½
m m’
p(m) p(m’)
p(m)=1
![Page 13: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/13.jpg)
Calculation of Expectations
![Page 14: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/14.jpg)
although
m’=m2
<m’> ≠ <m>2
![Page 15: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/15.jpg)
right way
wrong way
![Page 16: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/16.jpg)
Part 2
linearizing transformations
![Page 17: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/17.jpg)
Non-Linear Inverse Problemd = g(m)transformationd→d’m→m’
d’ = Gm’Linear Inverse Problem
solve with least-squares
![Page 18: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/18.jpg)
Non-Linear Inverse Problemd = g(m)transformationd→d’m→m’
rarely possible, of course
d’ = Gm’Linear Inverse Problem
solve with least-squares
![Page 19: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/19.jpg)
an example
log(di) = log(m1) + m2 zi
d’i=log(di)m’1=log(m1)m’2=m2
di = m1 exp ( m2 zi )
di’ =m’1+ m’2 zi
![Page 20: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/20.jpg)
0 0.5 10
0.2
0.4
0.6
0.8
1
z
d
0 0.5 1-1
-0.8
-0.6
-0.4
-0.2
0
z
log1
0(d)
z z
d
log10(d)
(A) (B)
trueminimize E=||dobs – dpre||2 minimize E’=||d’obs – d’pre||2
![Page 21: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/21.jpg)
againmeasurement error is being treated
inconsistently
if d is Gaussian-distributed
then d’ is not
so why are we using least-squares?
![Page 22: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/22.jpg)
we should really use a technique appropriate for the new error ...
... but then a linearizing transformation is not really much
of a simplification
![Page 23: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/23.jpg)
non-uniqueness
![Page 24: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/24.jpg)
considerdi = m12 + m1m2 zi
![Page 25: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/25.jpg)
linearizing transformationm’1= m12 and m’2=m1m2 di = m’1 + m’2 zi
considerdi = m12 + m1m2 zi
![Page 26: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/26.jpg)
linearizing transformationm’1= m12 and m’2=m1m2 di = m’1 + m’2 zi
considerdi = m12 + m1m2 zi
but actually the problem is nonuniqueif m is a solution, so is –m
a fact that can easily be overlooked when focusing onthe transformed problem
![Page 27: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/27.jpg)
linear Gaussian problems have well-understood non-uniqueness
The error E(m) is always a multi-dimensioanl quadratic
But E(m) can be constant in some directions in model space (the null space). Then the problem is non-unique.
If non-unique, there are an infinite number of solutions, each with a different combination of null vectors.
![Page 28: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/28.jpg)
0 1 2 3 4 50
100
200
300
400
500
600
m
E
m
E(m)
mest
![Page 29: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/29.jpg)
a nonlinear Gaussian problems can be non-unique in a variety of ways
![Page 30: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/30.jpg)
0 1 2 3 4 50
200
400
600
m
E
0 1 2 3 4 50
100
200
300
400
500
mE
0 1 2 3 4 50
20
40
60
m
E
0 1 2 3 4 50
5
10
15
20
25
m
E
0 0.2 0.4 0.6 0.8 1
0
0.2
0.4
0.6
0.8
1
x2
x1
0 0.2 0.4 0.6 0.8 1
0
0.2
0.4
0.6
0.8
1
m2
m1
m m
m m
E(m)E(m)
E(m)E(m)
m2
m2
m1
m1
(B)(A)
(C) (D)
(E)
(F)
![Page 31: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/31.jpg)
Part 3
the grid search method
![Page 32: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/32.jpg)
sample inverse problemdi(xi) = sin(ω0m1xi) + m1m2with ω0=20true solutionm1= 1.21, m2 =1.54N=40 noisy data
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
2
4
x
d
![Page 33: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/33.jpg)
strategy
compute the error on a multi-dimensional grid in model space
choose the grid point with the smallest error as the estimate of the solution
![Page 34: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/34.jpg)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
2
4
x
d
0 0.5 1 1.5 2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
m2
m1
20
40
60
80
100
120
140
160
180
200
220
(A)
(B)
![Page 35: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/35.jpg)
to be effective
The total number of model parameters are small, say M<7. The grid is M-dimensional, so the number of trial solution is proportional to LM, where L is the number of trial solutions along each dimension of the grid.
The solution is known to lie within a specific range of values, which can be used to define the limits of the grid.
The forward problem d=g(m) can be computed rapidly enough that the time needed to compute LM of them is not prohibitive.
The error function E(m) is smooth over the scale of the grid spacing, Δm, so that the minimum is not missed through the grid spacing being too coarse.
![Page 36: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/36.jpg)
MatLab
% 2D grid of m’sL = 101;Dm = 0.02;m1min=0;m2min=0;m1a = m1min+Dm*[0:L-1]';m2a = m2min+Dm*[0:L-1]';m1max = m1a(L);m2max = m2a(L);
![Page 37: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/37.jpg)
% grid search, compute error, EE = zeros(L,L);for j = [1:L]for k = [1:L] dpre=sin(w0*m1a(j)*x)+m1a(j)*m2a(k); E(j,k) = (dobs-dpre)'*(dobs-dpre);endend
![Page 38: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/38.jpg)
% find the minimum value of E[Erowmins, rowindices] = min(E);[Emin, colindex] = min(Erowmins);rowindex = rowindices(colindex);m1est = m1min+Dm*(rowindex-1);m2est = m2min+Dm*(colindex-1);
![Page 39: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/39.jpg)
Definition of Errorfor non-Gaussian statistcis
Gaussian p.d.f.: E=σd-2||e||22 but sincep(d) ∝ exp(- E)½ and L=log(p(d))=c- E½ E = 2(c – L) → -2L since constant does not affect location of minimum
in non-Gaussian cases:define the error in terms of the likelihood L E =– 2L
![Page 40: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/40.jpg)
Part 4
the Monte Carlo method
![Page 41: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/41.jpg)
strategy
compute the error at randomly generated points in model space
choose the point with the smallest error as the estimate of the solution
![Page 42: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/42.jpg)
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 10
2
4
x
d
0 0.5 1 1.5 2
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
m2
m1
20
40
60
80
100
120
140
160
180
200
220
0 500 1000 1500 20000
50
100
iteration
E
0 500 1000 1500 20000.5
1
1.5
iteration
m1
0 500 1000 1500 20001
1.5
2
iteration
m2
(A)
(B)
(C)
B)
![Page 43: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/43.jpg)
advantages over a grid search
doesn’t require a specific decision about grid
model space interrogated uniformly soprocess can be stopped when acceptable
error is encountered process is open ended, can be continued as long as desired
![Page 44: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/44.jpg)
disadvantages
might require more time to generate a point in model space
results different every time; subject to “bad luck”
![Page 45: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/45.jpg)
MatLab
% initial guess and corresponding errormg=[1,1]';dg = sin(w0*mg(1)*x) + mg(1)*mg(2);Eg = (dobs-dg)'*(dobs-dg);
![Page 46: Lecture 14 Nonlinear Problems Grid Search and Monte Carlo Methods](https://reader035.vdocuments.us/reader035/viewer/2022062501/568166e9550346895ddb2abd/html5/thumbnails/46.jpg)
ma = zeros(2,1);for k = [1:Niter] % randomly generate a solution ma(1) = random('unif',m1min,m1max); ma(2) = random('unif',m2min,m2max); % compute its error da = sin(w0*ma(1)*x) + ma(1)*ma(2); Ea = (dobs-da)'*(dobs-da); % adopt it if its better if( Ea < Eg ) mg=ma; Eg=Ea; endend