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Inverse Problems in Geophysics

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Inverse Problems in Geophysics ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) InverseInverse ProblemsProblems inin GeophysicsGeophysics What is an inverse problem? - Illustrative Example - Exact inverse problems - Linear(ized) inverse problems - Nonlinear inverse problems Examples in Geophysics - Traveltime inverse problems - Seismic Tomography - Location of Earthquakes - Reflection Seismology Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) WhatWhat isis anan inverseinverse problem?problem? Forward Problem Model m Data d Inverse Problem Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt X X X X Gravimeter ? Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt –– FForwardorward ProblemProblem We have observed some values: µ X X X 10, 23, 35, 45, 56 gals X X How can we relate the observed gravity Gravimeter ? values to the subsurface properties? We know how to do the forward problem: Gρ(r') Φ(r) = dV ' ∫ r − r' This equation relates the (observed) gravitational potential to the subsurface density. -> given a density model we can predict the gravity field at the surface! Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt –– TTrialrial andand ErrorError What else do we know? 3 X X X Density sand: 2,2 g/cm X X Density gold: 19,3 g/cm3 Gravimeter ? Do we know these values exactly? How can we find out? Where is the box with gold? One approach: Use the forward solution to calculate many models for a rectangular box situated somewhere in the ground and compare the theoretical (synthetic) data to the observations. ->Trial and error method Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt –– MModelodel SpaceSpace But ... X X X ... we have to define plausible models X X for the beach. We have to somehow describe the model geometrically. Gravimeter ? -> Let us - divide the subsurface into rectangles with variable density - Let us assume a flat surface surface xxx x x sand gold Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt –– NNon-uniquenesson-uniqueness Could we go through all possible models and compare the synthetic data with the X X X observations? X X - at every rectangle two possibilities Gravimeter (sand or gold) -250 ~ 1015 possible models - Too many models! -We have 1015 possible models but only 5 observations! - It is likely that two or more models will fit the data (possibly perfectly well) -> Nonuniqueness of the problem! Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt –– AA prioripriori informationinformation Is there anything we know about the treasure? X X X X X - How large is the box? - Is it still intact? Gravimeter - Has it possibly disintegrated? - What was the shape of the box? - Has someone already found it? This is independent information that we may have which is as important and relevant as the observed data. This is called a priori (or prior) information. It will allow us to define plausible, possible, and unlikely models: plausible possible unlikely Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt –– UUncertaintiesncertainties (Errors)(Errors) Do we have errors in the data? X X X - Did the instruments work correctly? X X - Do have to correct for anything? (e.g. topography, tides, ...) Gravimeter Are we using the right theory? - Do we have to use 3-D models? - Do we need to include the topography? - Are there other materials in the ground apart from gold and sand? - Are there adjacent masses which could influence the observations? How (on Earth) can we quantify these problems? Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt -- EExamplexample Models with less than 2% error. X X X X X Gravimeter Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt -- EExamplexample Models with less than 1% error. X X X X X Gravimeter Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TreasureTreasure HuntHunt –– EExercisexercise Exercise: X X Now let us assume that we know the X X box has not disintegrated into less X than two pieces. Change the calculations of the synthetic data Gravimeter and try to find the box, does it make a difference? Parametrization of the box with two pieces Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) InverseInverse ProblemsProblems -- SSummaryummary Inverse problems – inference about physical systems from data X X X X X Gravimeter - Data usually contain errors (data uncertainties) - Physical theories are continuous - infinitely many models will fit the data (non-uniqueness) - Our physical theory may be inaccurate (theoretical uncertainties) - Our forward problem may be highly nonlinear - We always have a finite amount of data The fundamental questions are: How accurate are our data? How well can we solve the forward problem? What independent information do we have on the model space (a priori information)? Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) CorrectedCorrected schemescheme forfor thethe realreal worldworld Forward Problem True Model m Appraisal Data d Problem Inverse Problem Estimated Model m~ Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) ExactExact InverseInverse ProblemsProblems Examples for exact inverse problems: 1. Mass density of a string, when all eigenfrequencies are known 2. Construction of spherically symmetric quantum mechanical potentials (no local minima) 3. Abel problem: find the shape of a hill from the time it takes for a ball to go up and down a hill for a given initial velocity. 4. Seismic velocity determination of layered media given ray traveltime information (no low-velocity layers). Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) Abel’sAbel’s ProblemProblem (1826)(1826) z P(x,z) dz’ ds x Find the shape of the hill ! For a given initial velocity and measured time of the ball to come back to the origin. Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TheThe ProblemProblem z P(x,z) d 1 2 z’ ds gz = v0 At any point: 2 x 1 −−mg('z z )=m(ds /dt)2 At z-z’: 2 After z ds /'dz tz()= ∫ dz' integration: 0 2 gz('− z) Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TheThe solutionsolution ofof thethe InverseInverse ProblemProblem z P(x,z) z ds /'dz dz’ tz()= ∫ dz' ds 0 2 gz('− z) x After change of variable and integration, and... 1 d a t ( z ) dz f ( z ' ) = − ∫ π dz ' z ' z − z ' Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) TheThe seimologicalseimological equivalentequivalent Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) Wiechert-HerglotzWiechert-Herglotz MethodMethod Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) DistanceDistance andand TravelTravel TimesTimes Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) SolutionSolution toto thethe InverseInverse ProblemProblem Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) Wiechert-HerglotzWiechert-Herglotz InversionInversion The solution to the inverse problem can be obtained after some manipulation of the integral : r1 r 2 / c2 (z) − p2 ⎛ r ⎞ 1 ∆1 ⎛ p ⎞ T = p∆ + 2 dr ⇔ ln⎜ 0 ⎟ = cosh −1⎜ ⎟d∆ ∫ r 2 ⎜ r ⎟ π ∫ ⎜ξ ⎟ r0 ⎝ 1 ⎠ 0 ⎝ 1 ⎠ forward problem inverse problem The integral of the inverse problem contains only terms which can be obtained from observed T(∆) plots. The quantity ξ1=p1=(dT/d∆)1 is the slope of T(∆) at distance ∆1. The integral is numerically evaluated with discrete values of p(∆) for all ∆ from 0 to ∆1. We obtain a value for r1 and the corresponding velocity at depth r1 is obtained through ξ1=r1/v1. Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) ConditionsConditions forfor VelocityVelocity ModelModel Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) Linear(ized)Linear(ized) InverseInverse ProblemsProblems Let us try and formulate the inverse problem mathematically: Our goal is to determine the parameters of a (discrete) model mi, i=1,...,m from a set of observed data dj j=1,...,n. Model and data are functionally related (physical theory) such that d1 = A1(m1,...,mm ) d = A (m ,...,m ) This is the nonlinear 2 2 1 m formulation. # dn = An (m1,...,mm ) Note that mi need not be model parameters at particular points in space but they could also be expansion coefficients of orthogonal functions (e.g. Fourier coefficients, Chebyshev coefficients etc.). Nonlinear Inverse Problems Introduction ρ(d,m)θ (d,m) σ (d,m) = k µ(d,m) Linear(ized)Linear(ized) InverseInverse ProblemsProblems If the functions Ai(mj) between model and data are linear we obtain di = Aij m j or d = Am in matrix form. If the functions Ai(mj) between model and data are mildly non-linear we can consider the behavior of the system around 0 some known (e.g.
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