THE GEOMAGNETIC VARIATION ANOMALY AT KOOTENAY LAKE, B.C.
by
JULES JOSEPH LAJOIE B.Sc, University of Ottawa, 1968
A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF M.Sc.
in the Department of
. GEOPHYSICS
We accept this thesis as conforming to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA April, 1970 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study.
I further agree tha permission for extensive copying of this thesis for scholarly purposes may be granted by the Head of my Department or by his representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.
Department of
The University of British Columbia Vancouver 8, Canada ABSTRACT
The purpose of this thesis was to study the geomagnetic variation anomaly at Kootenay Lake, B.C., first reported by Hyndman in 1963. The anomaly is characterized by a very high correlation between the vertical and horizontal north-south geomagnetic components; this indicates anomalous currents striking magnetic east-west, to the south of Kootenay Lake.
During the summer of 1968, twenty recording stations were set up in the Kootenay Lake area, using four Askania variographs and three fluxgate magnetometers. Spectral analysis shows that the main 'low I - high I' discontinuity was traversed over a relatively short distance on a north-south profile, to the south of Kootenay Lake.
Polarization studies indicate anomalous currents striking magnetic east-west in the same general area of the above discontinuity.
The model proposed is a localized distortion of the main 'low I - high 11 discontinuity, resulting in a well defined conductivity step, striking magnetic east-west, to the south of Kootenay Lake. ii
TABLE OF CONTENTS
Page
I INTRODUCTION 1
A The Anomaly 1 o B Hyndman's Interpretation 2 C Lower Crust and Upper Mantle Structure in We stern Canada 4
II THEORY 7
A Induction In a Step Discontinuity 7 B Parkinson Diagrams, Induction Arrows, and the Mercator Projection . 8 C Power Spectral Analysis 10
III EXPERIMENTAL PROCEDURE 13
A Instrumentation 13 B Field Experiment 14
IV ANALYSIS AND INTERPRETATION 17
A Sections of G.D.S. Magnetograms ' 17 • B Power Spectral Analysis 20 C Magnetic Survey 25 D Polar Diagrams 29 E Interpretation 34
V CONCLUSION 40 Iii
LIST OP FIGURES
Magnetic Record at Boswell, July k, 1968
Location Map of Geomagnetic Depth Sounding and Magneto-Telluric Stations in Western Canada
Petrological Model for Western Canada
Parkinson Diagram
Location Map Showing Askania and Fluxgate Stations
Two Simultaneous Profiles of.G.D.S. Records
Power Spectral Estimates: a Mirror Lake to Rykerts and Newport b Brouse to Salmo and Newport
Power Spectral Ratios: a Mirror Lake to Rykerts and Newport b Brouse to Salmo and Newport
Location Map Showing Magnetic Survey Profile
Filtered Magnetic Survey with Wavelength Cutoffs Shown
Parkinson Diagrams and Corresponding Mercator Projections: a Mirror Lake to Rykerts b Brouse to Salmo
Polar Diagrams and Corresponding Mercator Projections: a Mirror Lake to Rykerts b Brouse to Salmo iv
Page
13 Location Map Showing Anomalous Stations 35
14 Proposed Model Expressed in Depth Contours to Highly•Conducting Layer 37 V
ACKNOWLEDGEMENTS
I wish to acknowledge the. advice, encouragement, and patient supervision of Dr. G.K.C. Clarke during the course of this research.
Special thanks are given to Dr. B. Caner .of the
Dominion Astrophysical Observatory in Victoria, for directing and supervising the field operations, and for many hours of discussions which led to the interpretation of the anomaly.
I would also like to thank the Dominion Observatory in Ottawa, for the use of the three fluxgate units during the survey; and the U.S. Coast and Geodetic Survey for the
Newport Magnetic Observatory magnetograms. 1
I INTRODUCTION
In the summer of 1968, the Department of Geophysics at the University of British Columbia, in cooperation with the.Division of Geomagnetism of the Observatories Branch, at Victoria, sponsored a geomagnetic depth sounding (G.D.S.)
survey to investigate the geomagnetic variation anomaly at
Kootenay Lake, B.C. Because of the high solar flare activity in 1968, this proved to be a most favourable time to conduct the experiment. Four Askania GV-3 variographs and three fluxgate-type continuous recording magnetometers were used.
A total of twenty stations was set up to map the anomalous zone.
A The Anomaly
The anomaly at Boswell, on the eastern shore of
Kootenay Lake, was first reported by Hyndman, on the basis of an east-west G.D.S. profile at latitude 49.5°N (Hyndman,
1963). It is characterized by a strong correlation between the vertical (z) and magnetic north-south (H) intensities.
This is quite evident in a sample magnetogram for this
station (Fig. 1). A similar but weaker correlation was also
reported at Crescent Valley, B.C.
The anomaly is of particular interest because it may be related to a change in upper mantle and/or lower crustal
structure associated with the eastern edge of the western 2 cordillera (Caner and Cannon, 1965). Geologically, the anomaly is at the center of a major tectonic feature known as the Kootenay Arc, which sweeps in a broad curve around the Nelson batholith (Crosby, 1968). The directional dependence of the induced Z field suggests at once a lateral conductivity inhomogeneity striking magnetic east-west, to the south of Boswell, B.C.
12:00 U.T. 15:00 U.T.
D
Fig. 1 Magnetic Record at Boswell, July 4, 1968.
B Hyndman's Interpretation
Hyndman (1963) attempted to fit an Infinite cylinder model to the data. His calculations disclosed, however, that a cylinder, striking magnetic east-west, to the south of
Boswell and Crescent Valley, required unusually large values of conductivity and radius, to satisfy the data. For example, the radius of an infinitely conducting cylinder required it Pig. 2 Location Map of Geomagnetic Depth Sounding and Magneto- Telluric Stations in Western Canada (after Caner, 1970). 4 to extend beyond the surface. Hence, a finite conductivity contrast for the cylinder demanded an unreasonably large radius. Therefore, Hyndman concluded that the Infinite . cylinder could only be a very rough, approximation to the shape of the conductivity zone. This prompted the more detailed G.D.S. survey reported in this thesis, to map the anomaly and resolve the problem.
C Lower Crust and Upper Mantle Structure in Western Canada
Rikitake . (1966) shows that the induced Z field at the surface of an ideal highly conducting 'flat' earth, completely cancels the inducing Z field. For the earth, however, the extent of the attenuation in the Z component depends on the conductivities of the subsurface layers, their depths, and the frequency of the incident E.M. wave.
The parameter generally used to measure this'attenuation is the I ratio defined as
T _ lAZl
1 ~ [(AD) + (AH)]"*
The first extensive G.D.S. profile across the western cordillera was carried out in the U.S.A. by
Schmucker from 1959 to 1962, at latitude 32°N (Schmucker,
1964). The I ratios of stations to the east of Las Cruces and Cornudas, New Mexico, were three times higher than those of stations to the west. This prompted further profiles to be carried out across the cordillera, to determine if this discontinuity was either a local or continental feature. SELKIRK KOOTENAY PURCELL ROCKY MTN. ROCKY PLAINS MOUNTAINS LAKE MOUNTAINS TRENCH MOUNTAINS (KOOTENAY VALLEY)
t \ \ t I0-I5KM\ \ \ \ \ RESISTIVE, NO \ \ \t\ \ X 20 CONDUCTIVE,HYDRATED LOWER RESOLUTION \ 30-35K CRUST (PROBABLY PARTIAL MELTING) A \ \ \ \ * 40 >750°C
60 MODERATELY CONDUCTIVE UPPER MANTLE, a T> 800°C, COMPOSITION UNDEFINED 80 -
Fig. 3 Petrological Model for Western Canada, Latitude 49.3 N. (horizontal scale = vertical scale).(after Caner, 1970) The next two profiles in the United States, at latitudes
34°N (Caner, Cannon, and Livingston, 1967) and 3<3.5°N by
Gough and Reitzel in 1966, indicated that the discontinuity was roughly correlated to the eastern edge of the cordillera.
In southwestern Canada, surveys by Hyndman (1963),
Cannon (1967), Dragert in 19&9, and the Dominion Observatory are integrated into Figure 2, which includes the anomalous station at Kootenay Lake (K00). This figure defines the approximate location of the low I-high I discontinuity in southwestern Canada.
Caner (1970) integrated these results with seismological, aeromagnetic, and heat flow data to construct a petrological and electrical conductivity model of the lower crust and upper mantle for western Canada (Fig. 3).
In the western region, the lower crust is assumed to be highly conductive due to hydration. Water saturation was first proposed by Hyndman and Hyndman (1968) in order to explain the high electrical conductivity found in the lower crust of young, tectonically active, continental areas of the earth. 7
II THEORY
A Induction in a Step Discontinuity
Madden and Swift (1969) studied the magneto-telluric effect in a two-dimensional model of an anomalous upper mantle feature- resembling a poorly defined step discontinuity in conductivity. It included a 10/i-m. sedimentary layer on the surface. They reported that for the case when the electric field was perpendicular to the strike of the feature, the electric field tended to increase when approaching the transition zone from the more resistive side, and to decrease when approaching it from the conductive side. However, there was no noticeable change across the zone when the electric field was parallel to the strike. This indicated that the electric currents induced by the external field were flowing parallel to the strike of the anomalous feature. The shallow sedimentary layer tended to mask the deeper anomalous features at the higher frequencies.
Schmucker (1964) investigated the step discontinuity in conductivity for the steady state case, and showed that the lines of magnetic field converged onto the upper corner of the step. Hence, currents induced in such a conductor by an external time-varying magnetic field, are then Intensified at the corner and flow parallel to it. The more sharply defined the step, the more current will flow.
Weaver (1963) studied a two-dimensional model consisting of'two regions, one being more resistive than the 8
other, representing a land-sea interface. He found that the
ratio |Z| / IYI where Y is the horizontal magnetic field
perpendicular to the strike of the discontinuity, increased
as the boundary was approached from either side. However,
away from the boundary, the ratio diminished much more
rapidly over the sea than on the landward side. The result
over the sea is in agreement with Rikitake's theoretical model for the cancellation of the source vertical component
of magnetic field by induced currents in a semi-infinite
conducting body (Rikitake, 1966).
If a well defined step discontinuity in conductivity
occurred at lower crust and upper mantle depths, lower
frequency E.M. waves would penetrate through the upper crust
and induce currents in the upper corner of the step.
Locally, the resulting induced field would be added to the
primary field at the surface of the earth. The secondary or
induced field would be correlated to the primary field
variations perpendicular to the strike of the discontinuity.
B Parkinson Diagrams, Induction Arrows, and the Mercator Projection.
Let T define the total vector change of the
geomagnetic field in a specified interval of time. Then,
T = \/(AD)2+(AH)2+(AZ)2
and its direction is specified by two angles: 6, the angle
between T\ and the vertical, and (J), the azimuth measured
clockwise from H. © and (J) are conveniently plotted as points 9
N
N
Fig. 4 Parkinson Diagram. on a diagram as shown in Fig. 4 (Parkinson, 1959), where the two circles represents upward and downward changes in the vertical component. The horizontal vector is corrected for declination, so that N in Fig. 4, represents geographic
North. The radial distance from the center of the circle is
6, where 6=90° is at the circumference, and 6=0° is at the center. Hence, the closer a point .is plotted to the center of the circle, the larger is the Z variation of the event.
If, for many events T, points are plotted on the surface of a unit sphere, corresponding to the direction of 10
T from the sphere's center, then it is generally found, at non-polar stations, that the points cluster about a
'preferred plane', cutting the sphere in half. The Parkinson diagram is then a representation of the preferred plane on the horizontal plane.
Usually, the preferred plane is horizontal, defining a normal station. However, in many cases (Parkinson, 1959,
1962; Wiese, 1962; Rikitake, 1964; Simeo and Sposito, 1964;
Whitham and Anderson, 1962), the preferred plane is tilted.
For example, the great circle represented by. the dashed line in the 'UP1 circle of Fig. 4, represents a preferred plane dipping at an angle of 90° - 6' in the (f)1 - l8o° direction.
This implies a correlation between Z and the horizontal
(})1 - l80° vector, and hence a conductivity Inhomogeneity striking perpendicular to the (J)' direction. The induction arrow is defined as a vector whose geographic azimuth is . 90°- e' (p1 and magnitude ^QS—
Another useful method of representation is the
Mercator projection, where the dip of the geomagnetic field change is plotted as a function of geographic azimuth.
C Power Spectral Analysis
i) Continuous Case (Lathi, 1968)
The power spectral density of a finite function fT(t) is defined as: 11
rFrp(w; ) Sf(w) = 11m T^ (1) T-»a> T where, F,p(w) is the Fourier transform of f (t), and,
(f(t) lt| < T/2
fT(t) = (2) I 0 otherwise
An alternate method of obtaining the spectral density is given by the Fourier transform of the time autocorrelation function of f(t):
CO
jWT Sf(w) =/Rf(T)e- dT (3)
-oo where the time autocorrelation function is x
Rf(T) = lim-^-Jf (t)f (t+T)dt GO
ii) Discrete Case (Bendat and Piersol, 1967)
The function fT(t) is now sampled N times at equal intervals h (Nh=T). To avoid aliasing effects, the sampling interval must be small enough so that the cutoff frequency fc=2n~ is at least greater than the largest frequency component in f (t). For simplicity, the N sampled values
of fT(t) are denoted {x-^X^X.^ .
The set {x^ is first transformed such that X-0. The estimated autocorrelation function at the displacement rh is
1
r = Rr = R(rh) = _ ^ VW 0,1,2, ....m (5) where r is the lag number and m is the maximum lag number.
An estimate of the true spectral density in the range 0 ^.f < f is r-1 c c J m+1 estimates are made of S(f) at the discrete frequencies f=kf /m, k=l, 2, ,m.- (6) becomes
The Fourier transform of a finite length of record, f (t), is equivalent to the convolution of F(w), the spectral estimate of the infinite random process f(t) with a sine function. A sine function has the undesirable effect of having considerably large sidelobes, so that the power spectral density using this method can only be a 'raw' estimate of the true spectral density. A better estimate may be obtained by using a Hanning window, since its sidelobes in the frequency domain are much smaller. Although better windows now exist, the Hanning window was retained to obtain results consistent with previous workers.
The Hanning window is defined by
-^-(l+cos(Trr/m)) r=0,1,2 > • • • • > m D(rh) - • (8) 0 r>m
This may be multiplied by the autocorrelation function in (7) to give r m-1 S(kf /m) = 2h'R +2 ]>Z D^cos(7rrk/m) (9) r=l since D =1, D =0 o ' m 13
III EXPERIMENTAL PROCEDURE
A Instrumentation
i) The Askania Variograph
The Askania.variograph records time variations of the earth's magnetic field on film, at the rate of one inch per hour, and sensitivity of about three gammas per mm.
Hourly time marks (using an external chronometer), base-line, and instrument temperature are also recorded. It houses three independent optical systems (D,Z,H) with mirrors mounted on small Alnico-alloy magnets, each system being equipped with calibration Helmhotz coils.
These instruments proved most reliable in the survey, though considerable care was necessary in site selection. The Askania variograph must be housed in a weatherproof building with solid floor, power, and far from magnetic disturbances such as transformers, moving vehicles, etc. Once a suitable site is found, however, the instrument is operational within the hour.
II) The Pluxgate Magnetometer
The Dominion Observatory fluxgate unit consists of a field sensitive head containing three orthogonal magnetic detectors (Serson, 1957), a transistorized electronic assembly, and a chart recorder. The magnetic head may be operated out of doors at the end of a long cable, away from artificial disturbances. Although the quality of the data is generally poorer than that of the photographic variometer, it has the advantage of providing the observer with an 1^ immediate visual record, and site selection is less stringent.
This instrument was intended to supplement the data from the Askania variographs. Unfortunately, the chopper bar type recorder, which was used early in the survey, proved unreliable and resulted in high record loss. Later, better results were obtained with two-channel Hewlett-Packard
(Mosley) recorders.
B Field Experiment
R. Hyndman had postulated a cylindrical body striking east-west to the south of two of his stations at
Boswell and Crescent Valley. In an attempt to cross the anomalous zone, a profile was set up between Sloean Park and Blueberry Creek with intermediate stations at Thrums and Shoreacres (Fig. 5). A fluxgate station was also set up midway between Thrums and Shoreacres.
After three weeks of observation (June 2-21), the anticipated crossover was not found, and one of the
Askanias (Blueberry Creek) was moved to Crescent Valley to recover Hyndman's original anomaly at that site.
On July 2nd, the instruments were moved to a second profile at Kootenay Lake, in an attempt to intercept
Hyndman's anomaly at Boswell (K00 in Fig. 2). Suitable sites were found at La France Creek, Boswell, Sanca, Kuskanook
(fluxgate), and Wynndel (Fig. 5). The anomaly was recovered at all of these stations. On July 13, Sanca and LaFrance 15
117.5 w 1 16.5°V
miles
Schroeder Point
Mirror Lake
t.OLaFrance Ck.
X|*'CRESTON
Askania o _Fluxgate X B.C. IQ Rykerts Roads Idaho
Fig 5 Location Map Showing Askania and Fluxgate Staions. 16
Creek stations were moved to Mirror Lake and Rykerts to outline the boundary of the anomaly.
On July 27, the third and last profile was set up between Brouse and Salmo, with intermediate stations at
Sllverton, Slocan (fluxgate) and Taghum. Two' additional fluxgate instruments were acquired from the Dominion
Observatory (Ottawa) and installed at Schroeder Point and
Howser to extend the second profile. The instruments were operated until.August 27. Shown in Fig. 2, is the location of the Newport Magnetic Observatory, on the Washington-
Idaho border. Records from this observatory were obtained through the courtesy of the U.S. Coast and Geodetic Survey.
The location is of normal 'low I' type, and could be used as a comparison reference for the field recordings.
In an attempt to obtain further information on the boudaries of the anomaly, a static magnetometer survey was conducted between Revelstoke and Bonner's Ferry, Idaho, on
May 1st, 1969, using a McPhar vertical magnetometer. 17
IV ANALYSIS AND INTERPRETATION
A Sections of G.D.S. Magnetograms
In Fig. 6, simultaneous records from two north-south profiles are shown digitized and replotted at equal sensitivity, with Newport as a common reference,
i Mirror Lake - Rykerts and Newport (Fig. 6, right side)
Throughout this profile, the horizontal components are almost identical, except for the latitude effect, implying a uniform source field for these stations. The vertical component, however, changes character between
Sanca (SAN) and Rykerts (RYK). Rykerts shows normal low-Z
(western type) character, identical to that observed at
Newport (NEW). The stations from Sanca to Mirror Lake (MIR) exhibit very high Z amplitudes, strongly correlated to H, as has already been shown for Boswell in Fig. 1. Kuskanook is still highly correlated to H, but with slightly reduced amplitude; at Wynndel (WYN), the anomalous (H-correlated) component is barely perceptible.
Although no simultaneous recordings are available north of Mirror Lake, stations were occupied at a subsequent date; the recordings from Howser (HOW) are shown.on the left side of Fig. 6; they are still strongly anomalous, with
H-correlated Z fluctuations at least twice as large as those obtained at Brouse (BRO), only about 50 km. to the west.
The very short distance of approximately 25 miles over which the discontinuity occurs in a north-south Fig. 6 Two Simultaneous Profiles of G.D.S. Records. 19 direction is now emphasized. On other geomagnetic depth sounding profiles across the eastern edge of the cordillera, the same discontinuity is observed over a larger distance of roughly 150 km. in an east-west direction,
ii Brouse - Salmo and Newport (Fig. 6, left side)
Anomalous (H-correlated) fluctuations in Z are observed on this profile also, but with considerably reduced amplitudes. The simultaneous recording from Howser, from the eastern profile, is shown for comparison to indicate the reduction in amplitude. The stations from Taghum (TAG) to
Brouse show almost identical records; at Salmo (SAL), the anomalous Z amplitudes are slightly reduced, but still clearly large compared to the normal low Z station at Newport.
The transition from 'anomalous' to 'normal western low I' starts at Salmo on this profile. However, since no stations were available south of Salmo, it is not clear if the transition will be as sharp as that observed between
Kuskanook and Rykerts; a closely spaced network of stations between Salmo and Newport would be required to determine this detail. At this stage, it can only be concluded that a) the anomaly on the Brouse - Salmo profile is considerably less pronounced than for the Mirror Lake - Rykerts profile; b) start of the transition occurs at roughly the same latitude, confirming the east-west strike of the structure.
One of the most striking features demonstrated by
Fig. 6 is the difference between the two profiles; the amplitude of the anomaly changes very strongly over an east-west distance of the order of only about 40 km. (for 20 example, Mirror Lake to Sllverton). This indicates a strong
structural change in.this direction as well, i.e. the anomalies cannot be explained in terms of simple two-dimensional structures.
B Power Spectral Analysis
Simultaneous records from the two profiles of the previous section were digitized at a sampling rate of 64 points per hour with the digitizing machine built by
W. Cannon of the Department of Geophysics at U.B.C. The power spectral estimates were computed on the IBM 360 with equation (9) of section II C. The record lengths used were
12 and 1-3 hours, for the Brouse - Salmo and Mirror Lake -
Rykerts profiles, respectively. A 20°/c lag was used in the analysis, which gives a normalized standard error as high as
45^. However, the resulting increase in resolution is preferable since the estimates are used for comparison purposes only.
In both profiles shown in Pig. 7a and Jb, the horizontal spectral estimates (D and H) are closely related, except for a slight latitude effect which becomes progressively more noticeable at the higher frequencies. The
Z estimates of the Mirror Lake - Rykerts and Newport profile
(Fig. 7a) are of special interest here. Mirror Lake and
Sanca are very similar and exhibit high Z power levels.
Rykerts has the same high frequency cutoff as Newport, which is a known low I station. Wynndel is intermediate between the 21
'7C.RM.) .LOG10 (3
MIRROR LAKE SANCA WYNNDEL RYKERTS - NEWPORT
0.02 • 0.06 0.10
FREQUENCY, CRM.
Pig. 7a Power Spectral Estimates, Mirror Lake to Rykerts and Newport. 22
0.02 0.06 0.10
FREQUENCY, CRM,
Fig. 7b Power Spectral Estimates, Brouse to Salmo and Newport. 23
STATION LOCATION
'Fig. 8a Power Spectral Ratios, Mirror Lake to Rykert and Newport. 24
.Fig 8b Power Spectral Ratios, Brouse to Salmo and Newport. 25 two groups. This result is a quantitative expression of the visual comparisons carried out in the previous section.
The Z spectral estimates of the Brouse - Salmo and
Newport profile, do not show a sharp discontinuity (Fig. 7b).
As mentioned in section III A, a more closely spaced G.D.S. profile would have been desirable between Salmo and Newport.
However, stations Brouse to Salmo show evidence of being less anomalous, i.e. Z power levels, and more closely related to western type Newport than to the strongly anomalous stations Mirror Lake to Wynndel.
From the above results, P /P spectral ratios were Z xl found at periods of 20, 35> and 50 min. which showed high coherency. These were normalized with respect to a low I station in each profile and plotted as a function of geographic latitude (Fig. 8a and 8b). This Figure again illustrates the distinct discontinuity on the eastern profile (Mirror Lake - Newport), at all three frequencies; it also shows a frequency dependence for the discontinuity on the Brouse - Newport profile.
C Magnetic Survey
Upon studying an east-west aeromagnetic profile across the United States, passing near Denver, Colorado, a striking difference was found between profiles to the east and to the west of the Rocky Mountains (Pakiser and Zietz,
1966). The eastern profile included both sharp local anomalies and broader regional anomalies whereas the latter 26
Pig. 9 Location Map Showing Magnetic Survey Profile. UJ o UJ cc 1C cc CO UJ _l cr LU i> UJ cc UJ CO o CC cc cc o cc UJ a: CO cc o UJ 1X1 }C o CQ
WIUERED
10.03 MJ
25.00 MI ro
n.oom
•75.00 MI
20 HUES
Pig. 10 Filtered Magnetic Survey with Wave! ength Cutoffs Shown. 28 was absent in the west. This change in character occurred about 100 miles west of Denver, which is approximately the location of the G.D.S. discontinuity (Caner, Cannon, and
Livingston, 1967). This agreement between the pattern of aeromagnetic profiles and G.D.S. discontinuities was also observed in western Canada (Caner, 1970). The smoothing effect in the west may be explained by a rise in the Curie point isotherm (Pakiser and Zietz, 1965) or perhaps by a more silicic lower crust with low magnetite content (Caner,
1969).
Using long aeromagnetic profiles, Caner's work required the filtering of wavelengths up to 100 km. before the magnetic discontinuity became apparent. It would therefore seem rather unlikely that this could be detected in a short ground survey. However, it was thought possible to detect the discontinuity on the Mirror Lake - Rykerts profile because of the relatively short distance over which it occurs.
In the spring of 1969, a ground survey was conducted between Revelstoke, B.C., and Sandpoint, Idaho, using a
McPhar vertical magnetometer (Pig. 9). The data was corrected for time variations, using the Victoria magnetograms, and also for latitude dependence. Low pass filtering in the frequency domain was applied with successive wavelength cutoffs of 10, 25, 50, and 75 miles.
The results shown in Fig. 10 are inconclusive. A smoothing of the longer wavelength anomalies is not apparent 29 south of Rykerts. However, this negative result, i.e. the absence of magnetic sources in the high I region, does not disprove the results of the last section.
D Polar Diagrams
Two types of polar diagrams, which differ in plotting of the dip, were used to investigate the azimuthal dependence of the vertical field component at the following stations, belonging to the two profiles of the previous section: Brouse, Silverton, Taghum, and Salmo; Mirror Lake,
Sanca, Wynndel, and Rykerts. These stations were equipped with Askania variographs. The fluxgate records were not used in this analysis because the three orthogonal components of magnetic field could not all be recorded at these sites, due to recorder limitations.
About one hundred and fifty peak to peak magnetic storm variations were measured by hand from the magnetograms of the above stations. From these events, Parkinson diagrams and Mercator projections, as defined in section II B, were derived, using the plotter output of the IBM 3^0 computer, and are shown in Fig. 11a and lib.
An attempt was made to obtain polar diagrams directly from the digitized data, to eliminate the tedious and time consuming task of measuring magnetic variations by hand. A program was composed to compute the vector changes in
10 minute intervals on the digitized section, and choose the largest 450 of these, to plot on a polar diagram. To obtain O
Fig. 11a Parkinson Diagrams and Corresponding Mercator Projections - Dip (-90° to 90°) vs Geographic Azimuth Measured from North (-180° to l80°) Mirror Lake.to Rykerts. i
Fig. lib Parkinson Diagrams and Corresponding Mercator Projections - Dip (-90° to 90°) vs Geographic Azimuth Measured from North (-l80 to 180 ) Brouse to Salmo. LO ro
Fig. 12a Polar Diagrams and Corresponding Mercator Projections - Dip (-90° to 90°) vs Geographic Azimuth Measured from North (-180° to l80°) Mirror Lake to Rykerts. Fig. 12b Polar Diagrams and Corresponding Mercator Projections - Dip (-90° to 90°) vs Geographic Azimuth Measured from North (-180 to 180 ) Brouse to Salmo. 34
a true horizontal projection of the preferred plane, the
radial distance from the center of the circle was taken to
be cos where § is the dip of the event, as compared to the
Parkinson diagram where the radial distance is -TJ- - This
computational procedure, however, has the disadvantage of
neglecting the phase difference between components during
magnetic events. These results along with the corresponding
Mercator projections are shown in Fig. 12a and 12b, and,
here, are called polar diagrams.
In both types of diagrams, a correlation between dip
and azimuth is evident at the following sites: Mirror Lake,
Sanca, Brouse, Silverton, Taghum, and Salmo. It is debatable
whether Wynndel shows any correlation, and none is evident
at Rykerts.
Induction arrows, as defined in section II B, were
derived from these results, and are shown in Fig. 14, which
is described in the next section.. LaFrance Creek and Boswell
are given induction arrows equal to that of Sanca, since
their magnetograms are identical. The errors, of course, in
such results are very large and are approximately - 15° for
direction and 20% for amplitude. However, the general trend
of the arrows is very helpful in determining the direction
to the inhomogeneity causing the anomaly.
E Interpretation
The anomalous stations are shown in Fig. 13- A few
other stations from previous profiles are included to show UI
Fig. 13 Location Map Showing Anomalous Stations. 36 the approximate boundary of the low I - high I discontinuit
Several magneto-telluric stations which were used to construct the model of Fig. 3 are also included.
The induction, arrows in Fig. 14 show that the anomalous zone is much larger than was previously anticipat suggesting a deeper inhomogeneity. Also, they indicate anomalous currents striking approximately magnetic east-wes to the south of the stations. If the anomaly was caused by an isolated conductor, such as a cylinder in the lower crust, striking east-west, one would expect the induction arrows to reverse upon crossing it with a north-south profile. This is not so. Instead, the low I - high I discontinuity is encountered as a rather sharp step. This then suggests a convolution in the major continental low I - high I discontinuity.
The model proposed is shown in Fig. 14, and is presented in terms of depth contours to the surface of the highly conducting (hydrated?) lower crust (refer to Fig. 3)
The depths shown are based on the magneto-telluric data from the two regions (Caner, 1969). The intermediate contours are of course arbitrary in so far as absolute depth is concerned, but their concentration south of
Wynndel is a graphic representation of the 'cliff-like' discontinuity striking magnetic east-west. Induced currents flowing in the upper corner and/or vertical face of the ste are then responsible for the observed anomaly.
The model presented here is qualitative only. The LO
Fig. 14 Proposed Model Expressed in Depth Contours to Highly Conducting Layer. 38 power spectral analysis served as a mapping technique to locate the low I - high I discontinuity, while the induction arrows indicated the direction to the anomalous currents. The contours shown in Pig-. 14 could be very much in error. Prom
Caner's results (Caner, 1970), the conductivity contrast for the model is at least 50:1.
The lower crust In the anomalous region may be visualized as that of Pig. 3, but with a sinistral fault striking magnetic east-west. The lateral displacement is unknown but it should be of the order of 50 to 100 miles, since the low I - high I discontinuity is recovered again
100 miles west of Denver, Colorado. At the time of writing,
P. Fominoff, of the Department of Geophysics at U.B.C., is constructing an experimental model based on Fig. 14, to investigate the induction effects of this model.
An attempt was made to correlate the offset in the low I - high I discontinuity with surface geology and the tectonics of the area, but without success. However, it may be interesting to note that the direction of the offset, extended eastward, intersects the southern limit of the
Rocky Mountain Trench. Perhaps the feature originated with the Nevadian disturbance in late Jurassic, or the Laramide revolution in late Cretaceous, two of the major disturbances responsible for the. formation of the western cordillera.
The model lends support to the thesis that the high conductivity observed in the lower crust in the west, is caused by hydration rather than a rise in temperature. In 39 the latter case, diffusion would not permit the existence of a well defined step, unless of course the feature originated in geologically recent times. 40
V CONCLUSION
Prom this research, it is concluded that the geomagnetic variation anomaly at Kootenay Lake, B.C., originates in a localized distortion of the main low I - high I discontinuity., resulting in a well defined step in conductivity striking magnetic east-west, to the south of
Kootenay Lake, B.C. The final model in Fig. 14, is qualitative only. It supports the thesis that the high conductivity of the lower crust, in the west, is due to some compositional rather than thermal effect, perhaps hydration as suggested by Caner (Caner, 1969). 41
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