IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 5, MAY 2014
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A Low-Noise Fundamental-Mode Orthogonal Fluxgate Magnetometer Robert Bazinet1 , Alfredo Jacas2, Giovanni A. Badini Confalonieri2 , and Manuel Vazquez2 1 Phoenix
2 Instituto
Geophysics Ltd., Toronto, ON M1W 3K5, Canada de Ciencia de Materiales de Madrid, Consejo Superior de Investigaciones Científicas, Madrid 28049, Spain
We introduce a low-noise fundamental-mode orthogonal fluxgate magnetometer for use in magnetotelluric surveys. The fluxgate makes use of rapidly quenched amorphous wire having vanishing value of saturation magnetostriction constant (λ ≈ 10−7 ) and displaying ultrasoft magnetic behavior. The design of the fluxgate consists of multiple U-shaped sensor heads where pairs of wire pieces are inserted as core material. The novelty of this system resides in the use of a quadruple sensor head. Digital signal processing with effective electronic noise suppression allows this magnetometer to achieve a noise floor of 0.8 pT/Hz1/2 for frequencies above 10 Hz. The possibilities of in situ application are discussed and guidelines on noise suppression strategies are given. Index Terms— Amorphous microwire, low noise, magnetometer, orthogonal fluxgate.
I. I NTRODUCTION
M
AGNETOTELLURIC surveys require very sensitive ac magnetometers. Systems based on induction coils are the common sensor of choice for such surveys as they provide appropriately low-noise level, typically 0.1 pT/Hz1/2 at 1 Hz (Phoenix Geophysics Ltd MTC50 sensor, for example) [1]. This comes at a high logistic price, as these sensors are large and heavy (1.5 m long, 10 kg). Fluxgate magnetometers are small but unfortunately do not provide the performance level required. The best commercially available unit is specified at 6 pT/Hz1/2 [2], which value is also typical of various results reported in the literature. More recently, giant magnetoimpedance (GMI) magnetometers have been shown to be competitive substitutes for fluxgate sensors, generally showing higher bandpass and better high-frequency performance. However, they present higher 1/f noise in the low-frequency region, with relatively strong perming effect and low-frequency temperature drifts [3]–[5]. Alternatively, superconducting quantum interference device magnetometers do provide the required performance but their use in field conditions is mostly impractical. Fundamental-mode orthogonal fluxgate (FMOF), making use of magnetic microwires and originally described in [6], offers a series of advantages over more conventional second harmonic fluxgate magnetometers. The main difference between FMOF and the conventional second harmonic counterpart resides in the physical principles involved in the excitation of the magnetic core. In FMOF, this is achieved by passing throughout the wire a dc current to which an ac sine wave, of lower amplitude, is superimposed. As a consequence, the magnetization vector of the core, instead of reversing, rotates from its radial position toward the axis of the wire. The amplitude of this rotation is detected by a sensing Manuscript received May 27, 2013; revised September 6, 2013; accepted November 16, 2013. Date of publication November 26, 2013; date of current version May 1, 2014. Corresponding author: G. A. Badini Confalonieri (e-mail:
[email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2013.2292834
coil in the form of induced voltage, which is sinusoidal at a fundamental frequency [6]. The use of magnetic microwires as core material, where the circumferential close magnetic flux of the wire can be excited by a circumferential field obtained by passing a current along the wire axis, allows for simpler sensor design together with the easiness to miniaturize it. Traditionally, the functionality of FMOF was limited by the relatively high output noise [7], understood to arise mostly from Barkhausen noise. Recently, however, Butta and Sasada [8] made an important contribution toward understanding the source of noise in FMOF, and reducing it down to the competitive value of 1.8 pT/Hz1/2, by careful control over the amorphous microwire magnetic anisotropy. Other aspects to consider are the contribution to the noise arising from the magnetically harder ends of the wire [9] and the excitation parameters [10]. With noise values of a few pT/Hz1/2 , further thermal and electronic contributions become relevant, alongside Barkhausen noise, to the system noise level. In the following section, we present the development of an FMOF sensor with a noise level of better than 1 pT/Hz1/2 , which, while not being as good as the best large induction coils, it is nevertheless good enough for a wide range of geophysical applications as detailed in the patents applied on several aspects of this magnetometer [11]. II. E XPERIMENTAL D ETAIL In this section, we describe the elements involved in the fluxgate magnetometer. A. Sensing Element The core material is an ultrasoft amorphous cobalt alloy wire obtained with a rotating-water-bath-casting unit, similar to the one described in [12], having composition (Co0.94 Fe0.06 )72.5 Si12.5 B15 and diameter 120 μm, custom developed by ICMM for Quantec Geoscience. CoFe-based amorphous alloys are among the softest magnetic materials and, in wire form, are of particular technological interest for their magnetic and GMI properties [13]. At the origin of the
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Fig. 1.
IEEE TRANSACTIONS ON MAGNETICS, VOL. 50, NO. 5, MAY 2014
Schematic view of the sensor head construction.
magnetic properties is the unique intrinsic magnetic anisotropy of this class of materials, which is the result the balanced contributions from the magnetoelastic anisotropy, the stresses quenched in during the fabrication process, and the shape anisotropy from the cylindrical geometry of the wire. Wires exhibiting vanishing values of saturation magnetostriction, λs , can be obtained with a Co:Fe ratio of approximately 94:6. At this proportion, the magnetization reversal mechanism along the magnetic easy axis, parallel to the wire axis, occurs by the rapid displacement of magnetic domain walls at external field values below 10 A/m [13]. B. Sensor Head The basic design of our sensor head is very similar to the one presented in [14], using a U-shaped magnetic core, but the core is inserted in a single 1 mm diameter and 25 mm long sensing coil, as shown in Fig. 1. An additional solenoid, over the basic sensor assembly, nulls the earth magnetic field. C. Circuit Electronics The circuit electronics operates as digital, as much as possible. Nevertheless, some very good low noise analog circuits are needed for proper operation. The system is actually built to support three magnetic sensor heads for triaxial operation. Fig. 2 shows a block diagram of the circuitry for a single channel. Other channels are identical. The components are either repeated or shared, as appropriate, for the two other channels. The amorphous wire is driven, as any other fundamentalmode orthogonal sensor, by a sinusoidal ac waveform superposed on a dc bias current. We use a 96 kHz drive frequency with a peak amplitude of 40 mA over a 50 mA dc bias. Both the ac waveform and the dc bias are synthesized by programmable logic driving a fast 12 bit digital-to-analog converter. The sensing coil is parallel tuned at 96 kHz and followed by a very low noise preamplifier and a 96 kHz low-pass filter, which then feeds a 24 bit analog-to-digital converter sampling at 192 kHz and synchronized to the same clock as the driver digital-to-analog converter. A floating point digital signal processor chip takes the data from the analog-to-digital converter, synchronously
Fig. 2.
Block diagram of the magnetometer electronics.
demodulates it, applies a low-pass filter and decimates to a 1 kHz output sample rate, with 400 Hz effective bandwidth. A second high-accuracy digital-to-analog converter drives the earth field canceling coil. A very well-designed output filter was necessary to reduce the output noise from this converter to under the sensor intrinsic noise. Data from the DSP are recorded on a Compact Flash card; 1 GB provides for over 12 h of recording. The system is completed by a serial link to a laptop computer used as the operator’s console and by a GPS receiver to which the internal clocks are synchronized. This allows real-time comparison of data from several independent units. III. R ESULTS AND D ISCUSSION A. Performance Noise measurements were performed in a shielded can having three layers of μ metal. All the measurements were performed in a closed loop and the calibration line and a known amplitude of 50 Hz field was applied by a solenoid positioned around the sensor. Acquired data were processed using Quantec proprietary magnetotelluric processing tools. For the purpose of these experiments, we were only extracting the power spectrum of the digitized signal. As currently configured, the sensitivity of the magnetometer is approximately 6900 units/nT. With digital processing, this can be changed at will. The dynamic range, without nulling is approximately 1000 nT. It is limited by the saturation of the analog electronics. The noise floor is more significant. Individual sensor heads consistently provide better than 2 pT/Hz1/2 noise performance and typically 1.5 pT/Hz1/2 at 1 Hz and higher frequencies. B. Multiple Sensor Heads Individual sensor heads may be stacked to improve the noise. Assuming that the noise from each head is not coherent, the noise from multiple units is the square root of the sum of the noise from each individual unit, while the signal adds linearly. This provides a signal-to-noise improvement of 21/2 for each doubling of the numbers of units. A quadruple unit, summing the output of four individual sensors, each with its own preamplifier works quite well.
BAZINET et al.: LOW-NOISE FMOF MAGNETOMETER
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converter was found to be the source of the problem. Both a lower output noise operational amplifier and an output filter were necessary to bring down that noise. As the signal levels are extremely low, power supply coupling may also be an issue. Massive supply filtering was needed to prevent 1 Hz pulses from the GPS to corrupt the signal. IV. C ONCLUSION
Fig. 3.
Noise spectra of the quadruple sensor head.
With careful design, it was found possible to improve the noise of a FMOF gate magnetometer to values below 1 pT/Hz1/2. It looks like the noise is mainly limited by the eddy-current losses in the amorphous core material. This raises the possibility that the noise can be reduced further by the use of a smaller diameter amorphous wire active element. ACKNOWLEDGMENT This work was supported by Quantec Geoscience Ltd. The team at the Instituto de Ciencia de Materiales de Madrid, under contract to Quantec, developed and produced the amorphous wire material. R EFERENCES
Fig. 4.
Noise spectra of the single sensor head.
Signals from the four preamplifiers are summed before the low-pass filter shown in Fig. 2. Fig. 3 shows the noise spectra of the quadruple unit. The spike at 50 Hz is our 1570 pT calibration line. The high-frequency noise floor is approximately 0.8 pT/Hz1/2. C. Noise Origin Fig. 4 shows that the noise floor does not change when the excitation current through the amorphous wire is shut down. This shows that the observed noise is mainly the thermal noise of the sensing coil—amorphous wire core assembly, with most of it contributed by the eddy currents in the amorphous wires. The FMOF mechanism is apparently noise free, at least at this level. Electronic noise can be kept lower than the contribution of the sensor itself but this needs very careful design of the analog parts of the system. The preamplifier input noise was found to be of one order of magnitude less than the sensor noise, but we did have problems, in particular, with noise injection from the earth field nulling circuit. The output noise of the operational amplifier used to buffer out the digital-to-analog
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