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Technical Note (National Research Council of Canada. Division of Building
Research), 1965-09-01
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Vibrations of a magnetometer
Ward, H. S.
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-DIVISION OF BUILDING RESEARCH
NQ.
NATIONAL RESEARCH COUNCIL OF CANADA
.
452
NOTE
'JrEClHIN lCAL
PREPARED BY H. S. Ward CHECKED BYTDN APPROVED BY NBH
ｾ September 1965
PREPARED FOR Inquiry and record purposes
SUBJECT
VIBRATIONS OF A MAGNETOMETER
The Geomagnetic Division of the Department of Mines and Technical Surveys operates a magnetometer at the Prescott
Highway Laboratory for the purpose of determining the magnetic propertie s of geological specimens. The magnetometer was placed directly on the floor slab of the Laboratory, and occasionally it was impossible to record readings from the instruments because of ground vibrations that caused a galvanometer-type suspension, with a natural period of 20 sec, to oscillate; the optical path of the
recorder was 5 metres, and the oscillations on the scale were often of the order of 2 em (peak to peak). Two possible sources of the ground vibration were traffic on the Prescott Highway, about 500 ft away, and the blasting operations for the tunnel under Dow's Lake, about half a mile from the Laboratory.
The Geomagnetic Division will move the magnetometer to a new location in Blackburn in 1967, and they wished to compare the existing site and the new location with regard to ground
vibrations. At the same time, they wished to determine a toler-able level of vibration for the operation of such an instrument.
Velocity- sensitive transducers were used to measure the ground vibrations at the Laboratory and at the new site. Both vertical and horizontal components of the vibration were measured at the Laboratory, but only the horizontal component was recorded at the new site. In both cases, the horizontal component was
J
..
2
-measured perpendicular to the roads passing near the sites.
The results obtained at the Laboratory are shown in Table I, and those for the new site in Table II. At Blackburn, recordings were taken on the foundation slab of a derelict house approximately
50 ft from the road, and on the ground 50 and 150 ft from the road. The values of the frequencies quoted in Tables I and II are only approximate.
At the Prescott Highway location, the maximum recorded velocities were associated with a lower frequency than the back-ground vibrations, and it was these lower frequency vibrations that
seemed to be re sponsible for the oscillations of the magnetometer
suspension. It was observed that the magnetometer performed
satisfactorily when the vertical component was 0.006 in. / sec peak to peak at 35 cps, but it was not quite satisfactory when the vertical component was 0.003 in./sec peak to peak at 10 cps.
The scale reading on the magnetometer oscillated with an approximate peak-to-peak amplitude of 2 mm when the vertical component of the ground motion was 0.002 in. / sec peak to peak and
an approximate frequency of 10 cps. This was considered to be a
tolerable level of vibration, but it would perhaps be more satisfactory to specify that the ground vibrations should not exceed 0.001 in./ sec peak to peak for the probable range of frequencie s due to local
disturbances (above 5 cps).
A comparison of the vibration environments shown in Tables I and II indicate s that, at the moment, Blackburn is a quieter location,
seismically, than the Prescott Highway Laboratory. Because the
magnetometer is to be placed 900 to 1000 ft from the road at Blackburn, and because it will also be placed on an anti-vibration mount, there should be no difficulty in obtaining a vibration level that is below the tolerance level of 0.001 in. / sec peak to peak.
r
TABLE I
VIBRA TION ENVIRONMENT AT THE PRESCOTT HIGHWAY LABORATORY
Type of Vibration Background
Man jumping in laboratory Maximum vibration recorded due to traffic or blasting
Vertical Component. in./sec peak to peak
0.0005 (20-30 cps) 0.006 (35 cps) 0.003 (10 cps) TABLE II Horizontal Component. in. / sec peak to peak
0.0005 (20-30 cps)
0.002 (5 cps)
VIBRA TION ENVIRONMENT AT BLACKB URN (NUMBERS GIVEN ARE PEAK-TO-PEAK VALUES)
Location where Cause of Vibration
measurements Background Car travelling Car travelling Truck moving
were made at 20 mph at 40 mph along the road
On basement 0.0003 0.0008 0.0007 0.003
slab 50 ft away in. / sec in. / sec in. / sec in. / sec
from the road (15 cps) (10 cps) (20 cps)
On soil 50 ft 0.0002 0.0005 0.0005 0.001
off the road in. / sec in. / sec in./ sec in./ sec
(15-20 cps) (10 cps) (15 cps) (15 cps)
On
soil 150 ft 0.0001 0.0002 0.0004off the road in. / sec in. / sec in. / sec
-(15 cps) (15 cps) (15 cps)
