Using induction coil sensor optimization techniques for designing compact geophysical transmitters

Abstract

We have developed and tested code to optimise electromagnetic (EM) sensors to improve performance of the ARMIT B field induction coil sensor at desired frequencies. We aim to use the optimised parameters to develop a compact air core transmitter, which will form the basis for developing a compact ferromagnetic core transmitter. Techniques for optimising induction coil sensors are well established in literature and use analytical equations for the objective and constraint functions. Alternatives for EM sensor design are also well documented. In contrast, the design of compact transmitter systems needed for portability or in boreholes have limited discussion in the literature and have many more design constraints than sensors. Our ultimate intention is to use established sensor optimisation techniques to build a compact transmitter with sufficient magnetic dipole moment. To optimise an ARMIT induction current sensor we develop the algebraic expression for the total internal sensor noise to use as a constraint function. The objective function is the weight of the sensor. We aim to achieve noise goals of v / and v / frequencies of 1Hz and 2 kHz, respectively. 1 Hz was chosen because that is a common base-frequency for conductive sulphide exploration and 2 kHz was chosen as being appropriate for nuclear magnetic resonance investigations. We use numerical non-linear constraint optimization techniques to predict a target noise level of 1 pT at 1 Hz. , At this stage we predict the best 2 kHz sensor to have 4 fT noise at 2 kHz. This was based on existing dimensional and weight constraints on the induction coil sensor. We introduce an analogous method of transmitter optimisation using transmitter dipole moment as the objective function.