Robin Connelly, Landon Mutch
EOS 480 – Lab 2
Magnetic survey report
Introduction
Objective
This lab was conducted in order to measure the background magnetic field, locate and measure
the total magnetic field and vertical gradients for three buried targets and their magnetic
anomalies at the UVIC Dog Park (Fig. 1) using a Proton Procession Magnetometer. This
experiment is also to learn how to locate these buried targets using various techniques and
procedures, and how to interpret the data so that the depth of each target could be estimated.
Background/Theory
Objects buried underground can be detected using a proton procession magnetometer to
measure anomalies in Earth’s magnetic field due to a material’s magnetic properties and their
influence on the field being measured. The proton procession magnetometer has a base station
unit that monitors the diurnal changes in the Earth’s magnetic field and a roaming station, or
gradiometer, to measure the magnetic field and vertical gradients of the buried objects. The
base station recordings allow for making corrections to the measured anomaly due to any
changes over time in the magnetic field. The magnetometer has canisters full of a high H proton
fluid that are aligned with Earth’s magnetic field. Each canister has a coil wrapped around it
that when activated induces a polarizing effect that realigns the protons perpendicular to the
magnetic field. When the coils are quickly shut off the protons spiral in proportion to the
direction of Earth’s magnetic field, this in turn induces a current upon the coil which allows the
magnetometer to record the total magnetic strength of the buried materials (Burger, Jones
2006). Along with magnetic anomalies the magnetometer records time, geographic
coordinates, number of satellites, and spacing of survey lines.
Experimental technique
Equipment
Proton precession gradiometer (GSM-19T walking gradiometer, comprising 2 liquid-filled
sensors and console (with charged internal battery)
Backpack for mounting sensors and console
Aluminium staff sections: one half section; two full sections
GPS antenna
GPS antenna cable
Charged battery on belt
Base station proton precession magnetometer (Scintrex Envimag), comprising 1 liquid-filled
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Robin Connelly, Landon Mutch
EOS 480 – Lab 2
sensor and console (with charged battery and spare battery)
Compass (not corrected for declination)
Wooden stakes
2 tape measures
String
Hammer
Experimental Setting
The experiment took place at the UVIC Dog Park. The survey area was located on the North
corner of the park (Fig. 2). The park is mostly flat and covered with grass. There is a ravine
(Mystic Vale) located directly to the West and a chain linked fenced off area in the centre of the
park containing a number of small structures. Dwellings are located along the NE and SE
perimeter of the park. It was a sunny day with very little wind.
Method of Data Collection
Upon arrival to the park the instructor set up the magnetometer base station at an appropriate
location so that the magnetic anomalies of the targets being measured could not interfere with
the diurnal measurements being recorded by the magnetometer. At least 20 minutes of base
station recording time was necessary before the roaming measurements could be recorded.
During this time the Gradiometer was set up. The battery pack, GPS antenna and each sensor
were hooked up to the console and the sensor height was recorded to be 1.76 m. Provided the
sensor is far enough away from the console there should be no great effect while moving with
the magnetometer (Gem Systems Inc.). Total magnetic field was recorded by the bottom sensor
and between both the top and bottom sensor the vertical gradient of the magnetic field was
measured. The instructor gave a brief run through of the menu functions on the console. Survey
parameter were set up in the magnetometer (Table 1)
Table 1: Survey Parameters
File Name
Group B
Datum
84
Survey Mode
Walkgrad
Cycling Rate
0.5sec
AC filter
60Hz/50Hz
Sensor Spacing
56 cm apart
Display Mode
Graph/field nT/Coordinates
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Robin Connelly, Landon Mutch
EOS 480 – Lab 2
Initially the survey was a random walk around to find any spikes in the graph of magnetic
intensity for target one. Once the target was located a N-S line was set up and the axis of the
sensor coils that were wrapped around the containers containing the proton fluid were aligned
E-W, the survey was then conducted walking along N-S oriented lines. This way the coil was
perpendicular to Earth’s magnetic field as the surveyor walked along the line, and when the
current was applied to and then removed from the originally magnetic North aligned protons,
they spiraled back to magnetic North and their frequency was recorded and the magnetic
intensity of any anomaly was acquired. The orientation is important because if the coils are
aligned north south the charge will only enhance the total field strength of the already north
south directed protons.
The same procedure was carried out for locating target two. Both a N-S survey line was walked
and an E-W survey line was walked however the coil was not realigned for the E-W walk which
allowed for error later. Total magnetic field and vertical gradient were recorded for N-S lines
over both targets and over the E-W line for the second target.
For the third target a survey grid was set up and the surveyor changed giving a new antenna
height of 1.73 m. The grid was approximately 20 m x 20 m and initially had corners of the grid
aligned with the N-S, E-W orientation. This had to be adjusted so as to have the edges of the
square lined up correctly so that the survey lines aligned N-S. Twenty one north south lines
were set up across the square at 1 m apart. The grid was then traversed at a slow steady pace
and the magnetic data was recorded.
Figure 1. UVIC dog park,
showing survey area.
N
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Robin Connelly, Landon Mutch
EOS 480 – Lab 2
Figure 2. Plan view map of the survey lines.
Experimental results
A plot of the data obtained from the base station is presented in Fig. 3, including a trend line for
the data over the time period of the survey. To obtain the target profiles, the anomalies were
isolated from the background magnetic field data by subtracting the background magnetic field
(as determined by slope from Fig. 3) from the actual measured total magnetic field:
magneticAnomaly = measTotMagFld – (measTotalMagFld * 7.3491/10000 + 53884)
No further corrections were applied to the data as any diurnal changes in the magnetic field will
be accounted for in the base station data, local terrain is not a factor, and the surveys are small
in extent so distance from the base station does not need to be accounted for.
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Robin Connelly, Landon Mutch
EOS 480 – Lab 2
Figure 3. Slope of the background magnetic field over time as measured by the base
station.
Figure 4. Profile of target 1 from south to north with half-width of ~1m.
Assuming the subsurface body from target 1 (Fig. 4) is a dipole, sphere and taking 70o as the
magnetic inclination (geomag.nrcan.gc.ca), the depth to the top of the body (ztop) can be found
using the half-width (x1/2) method:
Ztop ~= 2 * x1/2 ~= 2 * 1.0 m - 1.76 m = 0.24 m = 0.2m
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Robin Connelly, Landon Mutch
EOS 480 – Lab 2
Figure 5. Profile of target 2 from south to north showing half-width of ~1.5m.
Figure 6. Profile of target 2 from east to west showing half-width of ~1.5m.
Similarly for target 2 (Fig. 5, 6):
Ztop ~= 2 * x1/2 ~= 2 * 1.5 m - 1.76 m = 1.24 m = 1.2m
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Robin Connelly, Landon Mutch
EOS 480 – Lab 2
Figure 7. Contour of the magnetic anomaly obtained from the target 3 grid survey ,
indicating a subsurface feature striking approximated 040 o NE.
From Figure 8, using the half maximum method the depth to the hydrants can be found:
Ztop ~= x1/2 ~= 1.94 m - 1.73 m = 0.21 m
Interpretation
Results
The depths to the top of all targets was found. Target one, the vertical cylinder, was calculated
to have a depth of 0.2 m. Target two, the horizontal cylinder, had a depth of 1.2 m and target 3,
the hydrants, had a depth of 0.21 m and a strike of 040o NE. The measured background total
magnetic field intensity was approximately 53991 nT and the predicted intensity was around
53925.
Discussion
A proton procession magnetometer survey was carried out at the UVIC dog park in order to
locate and estimate the depth of 3 targets from the measured and corrected magnetic
anomalies that these targets possess. Targets 1 (a vertical cylinder) and 3 (buried hydrants)
have similar depths of 0.2 m and 0.21 m respectively, which is a reasonable depth. However
target 2 is calculated to have a slightly greater depth of 1.2. This larger depth could be due to
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Robin Connelly, Landon Mutch
EOS 480 – Lab 2
errors carried out in the survey such as the coil sensor alignment not being changed for the east
to west line walk which would give a larger anomaly than the buried object actual retains.
Other errors in the experiment could be due to many people and dogs walking around and
nearby both the base station and roaming magnetometer. Any ferromagnetic objects on these
people or dogs could skew the measurements recorded and create gradients that are excessive.
The land that the park is now located was once occupied by Indigenous people, and many
buried articles that may have ferromagnetic properties could also affect the readings
measured. During the setup of the target 3 grid the dimensions had to be rearranged leaving
the grid not perfectly lined up north to south. The procession magnetometer also has an
accuracy of 1 nT (Gem Systems Inc.). The measured background total magnetic field intensity
and the predicted intensity differed by approximately 66 nT. This difference could be caused by
the people and animals roaming around the base station making the base station background
higher than the predicted values. Also the quality of the instruments doing the measuring, the
predicted value comes from a higher quality devise.
Conclusion
The objective of this experiment was to learn how to collect magnetic data using a proton
procession magnetometer and to use that data to calculate the depth of a buried object
displaying magnetic anomalies from Earth’s magnetic field. Through this experiment the
magnetic anomalies allowed detection of the targets and their total field strength graphs
provided the necessary information to calculate the depths of a vertical cylinder, a horizontal
cylinder and buried hydrants.
References
Hutchinson, J., and Leonard, L., 2015, EOS 480 Applied Geophysics Lab Manual: Lab 3
Magnetometer Survey: Buried Iron Cylinders and Hydrant/Pipe, University of Victoria, p
1-5.
Burger, Jones and Sheehan, 2006, Introduction to Applied Geophysics, W.W Norton &
Company, New York
Gem Systems Inc., 2010, GSM19T Series Magnetometers – Manual Release 7.4, Ontario,
Canada
Long-term and short-term magnetic storm forecasts
http://www.spaceweather.gc.ca/forecast-prevision/sf-eng.php
Predicted magnetic field components for February 23
http://geomag.nrcan.gc.ca/calc/mfcal-eng.php
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