Geomatrix Earth Science Ltd
The following test was conducted by Geometrics which proves the excellent bandwidth and low noise characteristics of Geometrics Caesium Vapour (CV) Sensors, these results are totally at odds with claims by rival manufacturers about the poor performance of CV sensors particularly in sensitivity.
The test procedure:
Two CV sensors were set up approximately 4.2 m apart outside Geometrics' building. Two sensors were used so that diurnal and regional magnetic field variations can be removed. however, since this is a gradiometer, the plots shown below are the RMS sum of the "base" sensors and the "test" sensor, this increased the plotted noise level by a factor of the square root of 2 times the individual sensor noise.
Figure 1. A 1/4-20 by 13mm long steel bolt was dropped from two heights past a CV sensor which was coupled to a counter operating at 100 Hz count rate, this equates to a 50Hz bandwidth. On the lower of the two drops the bolt had a velocity of approximately 3.58 m/sec as it passed the sensor and approximately 5.3 m/sec of the higher of the two drops. The bolt as permanently magnetised along its long axis by being placed in the field of a strong magnet prior to the test.
Results: The results shown are from a drop at the lower height, I.e. the left hand set-up in figure 1.
Figure 2: The scale of this plot is such that the full scale deflection can be seen and is 3500 nT p-p. Note the lack of hysteresis in the data after the bolt has passed the sensor.
Figure 3: This plot shows a portion of plot 2 above, but with a greatly expanded scale, note the base noise level of the data prior to the passing of the bolt is <0.1nT
Figure 4. This shows the impact of the bolt on the ground and demonstrates the sensors high information bandwidth
Figure 5; Shows the data scatter in the period before the bolt was released. As this is an industrial site the measured noise is a combination of several factors, both internal and external to the sensor, although aliased 60 Hz power line noise is clearly visible as the largest component of the measured noise.
The conclusion of this test, made in a noisy urban environment is that the base noise level exhibited by the sensors at this site is <0.1 nT and the bandwidth of the system is such that it is capable of tracking magnetic field changes in the range of thousands of nT over a time period of 1/00th of a second.
Results of typical spin and tumble tests, used to measure the heading error of Caesium Sensors, can be found by following the link here.
