ModEM: A modular system for inversion of elecgtromagnetic geophysical data

We have developed a modular system of computer codes for inversion of electromagnetic (EM) geophysical data (ModEM). ModEM allows for rapid adaptation of inversion algorithms developed for one purpose (e.g., three-dimensional magnetotellurics (MT)) to other EM problems (e.g., controlled source EM). The modular approach can also simplify maintenance of the inversion code, as well as development of new capabilities -- e.g., allowing for new data types such as the inter-site transfer functions in MT, or modifying model regularization. Basic data objects (model parameters, solution vectors, data vectors) are treated as abstract data types, with a standard set of methods developed for each class, including creation and destruction, and, as appropriate, linear algebra or other vector space methods. Operators required for gradient computations are developed as mappings between these basic object classes. Only these abstract data objects and mappings are manipulated by higher level Jacobian and inversion routines, with no reference to the problem specific details required for a specific EM method, or for the numerical implementation of the forward solver. Required problem-specific components are instantiated at the lowest levels of the system, with details hidden from generic top-level routines by an interface layer. Parallelization using MPI has been implemented at the top level, and is thus applicable to any problem embedded in ModEM. To illustrate the flexibility of the system, we consider applications to two- and three-dimensional MT inversion, as well as simple controlled source EM problems.