Note
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Sobol’ sensitivity indices from chaos¶
In this example we are going to compute global sensitivity indices from a functional chaos decomposition.
We study the Borehole function that models water flow through a borehole:
With parameters:
: radius of borehole (m)
: radius of influence (m)
: transmissivity of upper aquifer ()
: potentiometric head of upper aquifer (m)
: transmissivity of lower aquifer ()
: potentiometric head of lower aquifer (m)
: length of borehole (m)
: hydraulic conductivity of borehole ()
import openturns as ot
from operator import itemgetter
import openturns.viewer as viewer
from matplotlib import pylab as plt
ot.Log.Show(ot.Log.NONE)
borehole model
dimension = 8
input_names = ["rw", "r", "Tu", "Hu", "Tl", "Hl", "L", "Kw"]
model = ot.SymbolicFunction(
input_names, ["(2*pi_*Tu*(Hu-Hl))/(ln(r/rw)*(1+(2*L*Tu)/(ln(r/rw)*rw^2*Kw)+Tu/Tl))"]
)
coll = [
ot.Normal(0.1, 0.0161812),
ot.LogNormal(7.71, 1.0056),
ot.Uniform(63070.0, 115600.0),
ot.Uniform(990.0, 1110.0),
ot.Uniform(63.1, 116.0),
ot.Uniform(700.0, 820.0),
ot.Uniform(1120.0, 1680.0),
ot.Uniform(9855.0, 12045.0),
]
distribution = ot.JointDistribution(coll)
distribution.setDescription(input_names)
Freeze r, Tu, Tl from model to go faster
selection = [1, 2, 4]
complement = ot.Indices(selection).complement(dimension)
distribution = distribution.getMarginal(complement)
model = ot.ParametricFunction(
model, selection, distribution.getMarginal(selection).getMean()
)
input_names_copy = list(input_names)
input_names = itemgetter(*complement)(input_names)
dimension = len(complement)
design of experiment
size = 1000
X = distribution.getSample(size)
Y = model(X)
create a functional chaos model
algo = ot.FunctionalChaosAlgorithm(X, Y)
algo.run()
result = algo.getResult()
print(result.getResiduals())
print(result.getRelativeErrors())
[0.00175769]
[3.70801e-06]
Quick summary of sensitivity analysis
sensitivityAnalysis = ot.FunctionalChaosSobolIndices(result)
sensitivityAnalysis
draw Sobol’ indices
first_order = [sensitivityAnalysis.getSobolIndex(i) for i in range(dimension)]
total_order = [sensitivityAnalysis.getSobolTotalIndex(i) for i in range(dimension)]
graph = ot.SobolIndicesAlgorithm.DrawSobolIndices(input_names, first_order, total_order)
view = viewer.View(graph)
We saw that total order indices are close to first order, so the higher order indices must be all quite close to 0
for i in range(dimension):
for j in range(i):
sij = sensitivityAnalysis.getSobolIndex([i, j])
print(f"{input_names[i]} & {input_names[j]}: {sij:.4f}")
plt.show()
Hu & rw: 0.0088
Hl & rw: 0.0088
Hl & Hu: 0.0000
L & rw: 0.0083
L & Hu: 0.0012
L & Hl: 0.0011
Kw & rw: 0.0020
Kw & Hu: 0.0003
Kw & Hl: 0.0003
Kw & L: 0.0003