Desenvolvimento de um magnetometro a precessão nuclear

Abstract

The objetive of this thesis was to develop a proton precession magnetometer for geophysical prospecting and base stations. The proton precession magnetometer measures the total magnetic field intensity. It operates on the basis of nuclear magnetic resonance by determining the precession frequency of protons of a non vicous liquid in the terrestrial .magnetic field. The magnetometer consists of two parts: the sensor and the measuring instrument. The sensor is a solenoildal coil with the liquid as the core. Three different liquids were tested: water, n-propanol and a laboratory grade kerosene. The last one was selected because it offers more high precession signal amplitude and, consequently higer signal/noise ratio. The measuring system contains the tunning and amplifier circuits and the logical circuits for programming the measuring cycle and count on the precession frequency. Each cycle has a duration of 3 seconds; 2.3 seconds for the polarization and 0.7 seconds for the reception of the signal. The operation can be carried out manually, automatically or by remote control. The precession signal is amplified selectively in one of 14 different syntonized band-passes for measurements between 22000 and 95000 gammas. The precession frequency is multiplied by 64 and counted for a time of 0.36699 seconds, selected in consideration to the gyromagnetic ratio of the proton. The number of counted pulses is numerically equal to the value of the magnetic field in gammas. The reading is displaed digitally. For remote operation a BCD output is availabe. The precision of the measurements is 1 gamma. The instrument was tested in field to evaluate signal/noise ratio, supportable gradient and battery consuption. Application teste was carried out to take diurnal variation data and, reconnaissance and detail surveys data on an archaeological site in the Marajó Island-Pa. The test results were confronted with two commercial magnetometers-GP-70, McPhar e G-816, Geometrics - and, with data from Observatório Magnetico Ilha de Tatuoca as well. For all cases, the data comparison showed a good performance of the magnetometer tested.