Our iterative plane-search adds additional flexibility if the user so desires. The conventional conjugate-direction descent stepper (Claerbout, 2008) lacks this feature since it only searches the plane of the gradient and the previous step once. To utilize our plane-search the user may specify another value for this parameter, allowing the solver to scan the local neighborhood around the internally-computed residual values. When the plane-search iteration is greater than one, the solver will estimate that many model updates between each full function evaluation. This is particularly useful if the forward-operator is computationally expensive.
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