Exploration of RF antenna design and optimization for downhole NMR probe

Detection depth, signal strength and the scope of sensitive area are the main evaluation parameters of NMR logging tool. All these three parameters are determined by the properties of magnet and the radio frequency antenna in the probe ^[1] . By simulating the downhole NMR logging tool such as MRIL-P, this article is focused on exploring methods for NMR logging probe design and optimization.
Firstly, the material, property and structure of magnet is important for numerical simulation of magnet. The distribution of static magnetic field (B₀), the relationship between intensity and depth of B₀, and the intensity of B₀ in every direction is showed as Fig.1 (a) (b) and (c) ^[2] . Secondly, a variety of RF antennas were designed, and those were simulated after adjusting the parameters. Fig.2 (a) shows the relationship between intensity and depth of different RF magnetic fields (B₁) radiated by different antennas, and the intensities of different B₁ in every directions were showed as Fig.2 (b). Then, the influence caused by changes of different parameter of different antennas on their radiation characteristics was summarized. Thirdly, based on the simulation results, the edges of different areas that B₀ and different B₁ orthogonal matched were analyzed and draw out as Fig.2 (c). This result could reflect the detection characteristics of probe preliminarily. Based on it, the antenna could be adjusted pointedly. It is the basic procedures of solving the RF antenna from part of the known data.

Fig.1: (a) The magnetic field distribution of B0. (b) The relationship between intensity and depth of B0. (c) The intensity of B0 in every direction.

Fig.2: (a) The relationship between intensity and depth of different B1. (b) The intensities of different B1 in every direction. (c) The edges of different areas that B0 and different B1 orthogonal matched.