An open source FORTRAN subroutine for calculation of TEM responses and derivatives from 1D models

In this paper, an open source FORTRAN subroutine for the calculation of transient electromagnetic responses of one-dimensional earth models in the quasi-static approximation is presented. The code accommodates the most common TEM instruments configurations used today and includes the modelling of the effect of the system response, i.e. the influence of instrument properties on the responses. It also provides an option for calculating the derivatives of the response with regard to the model parameters of a one-dimensional earth model. Furthermore, induced polarisation effects can be included in the forward responses. The paper presents the considerations behind its creation and outlines the details of the computational elements used in its realisation. The subroutine is intended as a building block that can be included in other programs, specifically as an external computational resource for speeding up calculations in higher order language modelling and inversion codes.