calculation of one-dimensional forward modelling of helicopter-borne electromagnetic data and a sensitivity matrix using fast hankel transforms

the helicopter-borne electromagnetic (hem) frequency-domain exploration method is an airborne electromagnetic (aem) technique that is widely used for vast and rough areas for resistivity imaging. the vast amount of digitized data flowing from the hem method requires an efficient and accurate inversion algorithm. generally, the inverse modelling of hem data in the first step requires a precise and efficient technique provided by a forward modelling algorithm. the exact calculation of the sensitivity matrix or jacobian is also of the utmost importance. as such, the main objective of this study is to design an efficient algorithm for the forward modelling of hem frequency-domain data for the configuration of horizontal coplanar (hcp) coils using fast hankel transforms (fhts). an attempt is also made to use an analytical approach to derive the required equations for the jacobian matrix. to achieve these goals, an elaborated algorithm for the simultaneous calculation of the forward computation and sensitivity matrix is provided. finally, using two synthetic models, the accuracy of the calculations of the proposed algorithm is verified. a comparison indicates that the obtained results of forward modelling are highly consistent with those reported in simon et al. (2009) for a four-layer model. furthermore, the comparison of the results for the sensitivity matrix for a two-layer model with those obtained from software is being used by the bgr centre in germany, showing that the proposed algorithm enjoys a high degree of accuracy in calculating this matrix.