# Forward Problem¶

## Problem Class¶

The problem is a partial differential equation of the form:

$c(m, u) = 0$

Here, $$m$$ is the model and u is the field (or fields). Given the model, $$m$$, we can calculate the fields $$u(m)$$, however, the data we collect is a subset of the fields, and can be defined by a linear projection, $$P$$.

$d_\text{pred} = P u(m)$

For the inverse problem, we are interested in how changing the model transforms the data, as such we can take write the Taylor expansion:

$Pu(m + hv) = Pu(m) + hP\frac{\partial u(m)}{\partial m} v + \mathcal{O}(h^2 \left\| v \right\| )$

We can linearize and define the sensitivity matrix as:

$J = P\frac{\partial u}{\partial m}$

The sensitivity matrix, and it’s transpose will be used in the inverse problem to (locally) find how model parameters change the data, and optimize!

Working with the general PDE, $$c(m, u) = 0$$, where m is the model and u is the field, the sensitivity is defined as:

$J = P\frac{\partial u}{\partial m}$

We can take the derivative of the PDE:

$\nabla_m c(m, u) \partial m + \nabla_u c(m, u) \partial u = 0$

If the forward problem is invertible, then we can rearrange for $$\frac{\partial u}{\partial m}$$:

$J = - P \left( \nabla_u c(m, u) \right)^{-1} \nabla_m c(m, u)$

This can often be computed given a vector (i.e. $$J(v)$$) rather than stored, as $$J$$ is a large dense matrix.

## The API¶

### Problem¶

class SimPEG.Problem.BaseProblem(mesh, **kwargs)[source]
Problem is the base class for all geophysical forward problems

in SimPEG.

Optional Properties:

• model (Model): Inversion model., a numpy array of <class ‘float’>, <class ‘int’> with shape (*) or (*, *)

counter = None

A SimPEG.Utils.Counter object

surveyPair
mapPair
class Solver(A, **kwargs)
clean()
solverOpts = {}

Solver options as a kwarg dict

mesh = None

A discretize instance.

property mapping

Setting an unnamed mapping has been depreciated in v0.4.0. Please see the release notes for more details.

property curModel

Setting the curModel is depreciated.

property survey

The survey object for this problem.

pair(d)[source]

Bind a survey to this problem instance using pointers.

unpair()[source]

Unbind a survey from this problem instance.

deleteTheseOnModelUpdate = []

List of strings, e.g. [‘_MeSigma’, ‘_MeSigmaI’]

clean_on_model_update = []

List of matrix names to have their factors cleared on a model update

property ispaired

True if the problem is paired to a survey.

Jvec(m, v, f=None)[source]

Effect of J(m) on a vector v.

Parameters
Return type

numpy.ndarray

Returns

Jv

Jtvec(m, v, f=None)[source]

Effect of transpose of J(m) on a vector v.

Parameters
Return type

numpy.ndarray

Returns

JTv

Jvec_approx(m, v, f=None)[source]

Approximate effect of J(m) on a vector v

Parameters
Return type

numpy.ndarray

Returns

approxJv

Jtvec_approx(m, v, f=None)[source]

Approximate effect of transpose of J(m) on a vector v.

Parameters
Return type

numpy.ndarray

Returns

JTv

fields(m)[source]

The field given the model.

Parameters

m (numpy.ndarray) – model

Return type

numpy.ndarray

Returns

u, the fields

class SimPEG.Problem.BaseTimeProblem(mesh, **kwargs)[source]

Sets up that basic needs of a time domain problem.

Optional Properties:

• model (Model): Inversion model., a numpy array of <class ‘float’>, <class ‘int’> with shape (*) or (*, *)

property timeSteps

Sets/gets the timeSteps for the time domain problem.

You can set as an array of dt’s or as a list of tuples/floats. Tuples must be length two with […, (dt, repeat), …]

For example, the following setters are the same:

prob.timeSteps = [(1e-6, 3), 1e-5, (1e-4, 2)]
prob.timeSteps = np.r_[1e-6,1e-6,1e-6,1e-5,1e-4,1e-4]

property nT

Number of time steps.

property t0
property times

Modeling times

property timeMesh

### Fields¶

class SimPEG.Fields.Fields(mesh, survey, **kwargs)[source]

Fancy Field Storage

u[:,’phi’] = phi print(u[src0,’phi’])

knownFields = None

Known fields, a dict with locations, e.g. {"e": "E", "phi": "CC"}

dtype

alias of builtins.float

aliasFields = None

Aliased fields, a dict with [alias, location, function], e.g. {"b": ["e", "F", lambda(F,e,ind)]}

startup()[source]
property approxSize

The approximate cost to storing all of the known fields.

class SimPEG.Fields.TimeFields(mesh, survey, **kwargs)[source]

Fancy Field Storage for time domain problems

u[:,’phi’, timeInd] = phi print(u[src0,’phi’])

### Survey¶

class SimPEG.Survey.BaseSurvey(**kwargs)[source]

Survey holds the observed data, and the standard deviations.

std = None

Estimated Standard Deviations

eps = None

Estimated Noise Floor

dobs = None

Observed data

dtrue = None

True data, if data is synthetic

mtrue = None

True model, if data is synthetic

counter = None

A SimPEG.Utils.Counter object

srcPair

Source Pair

alias of BaseSrc

property srcList

Source List

getSourceIndex(sources)[source]
property prob

The geophysical problem that explains this survey, use:

survey.pair(prob)

property mesh

Mesh of the paired problem.

pair(p)[source]

Bind a problem to this survey instance using pointers

unpair()[source]

Unbind a problem from this survey instance

property ispaired
property nD

Number of data

property vnD

Vector number of data

property nSrc

Number of Sources

dpred(m, f=None)[source]

Create the projected data from a model. The fields, f, (if provided) will be used for the predicted data instead of recalculating the fields (which may be expensive!).

$d_\text{pred} = P(f(m))$

Where P is a projection of the fields onto the data space.

Note

To use survey.dpred(), SimPEG requires that a problem be bound to the survey. If a problem has not been bound, an Exception will be raised. To bind a problem to the Data object:

survey.pair(myProblem)

eval(f)[source]

This function projects the fields onto the data space.

$d_\text{pred} = \mathbf{P} f(m)$
evalDeriv(f)[source]

This function s the derivative of projects the fields onto the data space.

$\frac{\partial d_\text{pred}}{\partial u} = \mathbf{P}$
residual(m, f=None)[source]
Parameters
Return type

numpy.ndarray

Returns

data residual

The data residual:

$\mu_\text{data} = \mathbf{d}_\text{pred} - \mathbf{d}_\text{obs}$
property isSynthetic

Check if the data is synthetic.

makeSyntheticData(m, std=None, f=None, force=False)[source]

Make synthetic data given a model, and a standard deviation.

Parameters
class SimPEG.Survey.BaseSrc(rxList, **kwargs)[source]

SimPEG Source Object

loc = None

Location [x,y,z]

rxPair

alias of BaseRx

rxList = None

property nD

Number of data

property vnD

Vector number of data

class SimPEG.Survey.BaseRx(locs, rxType, **kwargs)[source]

knownRxTypes = None

Set this to a list of strings to ensure that srcType is known

projGLoc = 'CC'

Projection grid location, default is CC

storeProjections = True

Store calls to getP (organized by mesh)

locs = None

Locations (nRx x nDim)

property rxType

property nD

Number of data in the receiver.

getP(mesh, projGLoc=None)[source]

Returns the projection matrices as a list for all components collected by the receivers.

Note

Projection matrices are stored as a dictionary listed by meshes.

class SimPEG.Survey.BaseTimeRx(locs, times, rxType, **kwargs)[source]

projTLoc = 'N'
times = None

Times when the receivers were active.

property nD

Number of data in the receiver.

getSpatialP(mesh)[source]

Returns the spatial projection matrix.

Note

This is not stored in memory, but is created on demand.

getTimeP(timeMesh)[source]

Returns the time projection matrix.

Note

This is not stored in memory, but is created on demand.

getP(mesh, timeMesh)[source]

Returns the projection matrices as a list for all components collected by the receivers.

Note

Projection matrices are stored as a dictionary (mesh, timeMesh) if storeProjections is True

class SimPEG.Survey.Data(survey, dobs=None, standard_deviation=None, floor=None)[source]

Storage of data, standard_deviation and floor storage with fancy [Src,Rx] indexing.

Requried :param Survey survey: The survey descriping the layout of the data

Optional :param ndarray dobs: The data vector matching the src and rx in survey :param ndarray standard_deviation: The standard deviation vector matching the src and rx in survey :param ndarray floor: The floor vector for the data matching the src and rx in survey

calculate_uncertainty()[source]

Return the uncertainty base on standard_devation * np.abs(data) + floor