Journal of the Korean Society of Mineral and Energy Resources Engineers ISSN:2288-0291(Print) 2288-2790(Online) 한국자원공학회지

Resistivity logging is believed to be the most accurate method to obtain the resistivity profile with depth. The measured data, however, are merely apparent resistivity, which are not true resistivity of the earth. In particular, for a very thin layer, the measured data may not reflect the true resistivity of this layer. In this study, we have developed an efficient automatic inverse code to obtain true resistivity profile of stratified earth from normal resistivity logs. To overcome the difficulty in selecting a proper initial model and to facilitate the inversion, we divide the earth into many thin layers and the resistivity and thickness of each layer are selected as inverse variables. Moreover, theoretical formula to calculate partial derivatives with respect to these variables is derived and the Active Constraint Balancing method is adopted to enhance the resolving power of the inversion. The efficiency of our inverse code is tested with a layered model with thin resistive layer, which is hard to resolve in resistivity logs, and with randomly generated stratified earth model. Inversion results of field data set show that the developed algorithm is efficient to reveal real resistivity changes with depth.