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Polarization of micropulsation sources
EARTH AND PLANETARY SCIENCE LETTERS 3 (1967) 347-350. NORTH-HOLLAND PUBLIStSiNC: COMP., AMSTERDAM
POLARIZATION OF MICROPULSATION SOURCES D.RANKIN and I .K.REDDY
Physics Department, University ofAlberta, Edmonton, Alberta, Canada Received 28 November 1967
Digital processing and harmonic analysis of mag netotelluric data obtained during the geomagnetically quiet years, 1965 and 1966, were made using the Fast Fourier Transform algorithm of Cooley and Tukey 1 ] . In some cases the erratic behaviour of the coherency function made the data inadequate for resistivity analyses . In such cases the study of coherency between the orthogonal pairs of elactric and mag netic field components revealed certain interesting results. Figs . 1 and 2 show plots of the coherency versus period between EX-Hy and EV -Hx, where x and y are geographic North and East directions, for two data sets obtained at two different locations one at the Geophysical Observatory, University of Alberta, which is 30 miles south east of Edmonton and the other at Armada, which is 240 miles further south.
A close examination of these coherency plots indicates that for certain periods a high value of the coherency for one of the E-H pairs is accompanied by a low coherency of the conjugate pair . The phenomenon is observed in other records during periods of low micropulsation activity . It may be noted that the criterion adopted for selecting the data for resistivity analysis is that the coherency should be greater than 0.9 over the whole spectrum under investigation. As an example two coherency curves are shown in fig. 3 . It can be seen that the coherency between Ey-Hx is marginal although two curves are taken from data which were recorded simultaneously. 'The record used in these analyses was approximately two hours in length with the sampling period 32/15 sec. It may be concluded from figs . 1 and 2 that over the considerable length of time of the recording the
0.e u 0.0 Z W ce W S 0.4 O V 0.2 0
10
PERIOD IN SECONDS
Fig. 1. Coherency plot of Eac-Hy and Ey-Hx combinations. Geophysical Observatory, University of Alberta, 19 May 1966 .
D.RANKIN and I.K .REDDY
0.8 r Z o.6 W
ce
0.2
0
ARMADA 10
100
PERIOD IN
MAY 28,1966 L_ .1
1
i
SECONDS
a
.
a
I.i
1000
Fig. 2 . Coherency plot of Ex -H v and Ey-Hx combinations . Armada, Alberta, 28 May 1966 .
0.8 r
v
Ex - Hy "Ey - Hx
W 0 .6 oc
OBSERVATORY
0.2
JUNE 2 , 1966
100
10
PERIOD IN
ç SECONDS
1000
Fig. 3. An er.ample of acceptable coherency. Geophysical Observatory, University of Alberta., 2 June 1966 .
s
"oWSW "" .mo
"""""."" ."""""""""""""""" .. . . .
. . . . . . .. .. . .. ... ..
..... .. .. .. .
c " -89 . E =
0.29
Fig. 4. Traced computer plot of magnetic polarization . Geophysical Observatory, University of Alberta, 19 May 1966 .
POLARIZATION OF MICROPULSATION SOURCES
. .. .. . . ...."" ........ . ...."...... .. . . . .. ............... ........... . . . . ...... ... . . ... .. ....."...... " "
349
""Y
" """ "" ."""""
- -82" E = 03 Fig. 5 . Traced computer plot of magnetic polari/,ation. Geophysical Observatory, University of Alberta, 2 June 1966 .
resultant magnetic vector lies predominantly in one quadrant for certain periods while for other periods it lies in the adjoining quadrant. The basic assumption that low coherency is associated with relatively love micropulsation activity was confirmed by the observation of original analog records . Figs. 4 and 5 :;how examples of magnetic field polarization over periods of two hours from the same portion of the records used for coherency studies . An examination of the analog records shows relatively high Pc 4 activity with periods between 80 and 100 sec. The points in the polarization diagrams in figs. 4 and 5 may be fitted to an ellipse ; the parameters of
the ellipse viz. the azimuth a of the major axis with respect to north, and the ellipticity E are given in each figure. The overall response of the magnetic recording system is shown in fig . 6. This response as well as induction effects arising from anisotropic and/(-,r heterogeneous resistivity structure beneath the recording site have modified the results . Nevertheless a preliminary investigation of these data suggests that although the various period intervals appear related in time, they are produced by sources independently orientated in space .
10 r
FREQUENCY
(Hz)
Fig. 6. The overall response of the magnetic recording system .
100
350
D.RANKIN and IXREDDY
This work was supported by the National Research Council of Canada and the Geological Survey of Canada.
REFERENCE (1 ] J.W .Cooley and J .W .Tukey, An Algorithm for the Machine Calculation of Complex Fourier Series, Math . Computation 19 (1965) 297.
POLARIZATION OF MICROPULSATION SOURCES D.RANKIN and I .K.REDDY
Physics Department, University ofAlberta, Edmonton, Alberta, Canada Received 28 November 1967
Digital processing and harmonic analysis of mag netotelluric data obtained during the geomagnetically quiet years, 1965 and 1966, were made using the Fast Fourier Transform algorithm of Cooley and Tukey 1 ] . In some cases the erratic behaviour of the coherency function made the data inadequate for resistivity analyses . In such cases the study of coherency between the orthogonal pairs of elactric and mag netic field components revealed certain interesting results. Figs . 1 and 2 show plots of the coherency versus period between EX-Hy and EV -Hx, where x and y are geographic North and East directions, for two data sets obtained at two different locations one at the Geophysical Observatory, University of Alberta, which is 30 miles south east of Edmonton and the other at Armada, which is 240 miles further south.
A close examination of these coherency plots indicates that for certain periods a high value of the coherency for one of the E-H pairs is accompanied by a low coherency of the conjugate pair . The phenomenon is observed in other records during periods of low micropulsation activity . It may be noted that the criterion adopted for selecting the data for resistivity analysis is that the coherency should be greater than 0.9 over the whole spectrum under investigation. As an example two coherency curves are shown in fig. 3 . It can be seen that the coherency between Ey-Hx is marginal although two curves are taken from data which were recorded simultaneously. 'The record used in these analyses was approximately two hours in length with the sampling period 32/15 sec. It may be concluded from figs . 1 and 2 that over the considerable length of time of the recording the
0.e u 0.0 Z W ce W S 0.4 O V 0.2 0
10
PERIOD IN SECONDS
Fig. 1. Coherency plot of Eac-Hy and Ey-Hx combinations. Geophysical Observatory, University of Alberta, 19 May 1966 .
D.RANKIN and I.K .REDDY
0.8 r Z o.6 W
ce
0.2
0
ARMADA 10
100
PERIOD IN
MAY 28,1966 L_ .1
1
i
SECONDS
a
.
a
I.i
1000
Fig. 2 . Coherency plot of Ex -H v and Ey-Hx combinations . Armada, Alberta, 28 May 1966 .
0.8 r
v
Ex - Hy "Ey - Hx
W 0 .6 oc
OBSERVATORY
0.2
JUNE 2 , 1966
100
10
PERIOD IN
ç SECONDS
1000
Fig. 3. An er.ample of acceptable coherency. Geophysical Observatory, University of Alberta., 2 June 1966 .
s
"oWSW "" .mo
"""""."" ."""""""""""""""" .. . . .
. . . . . . .. .. . .. ... ..
..... .. .. .. .
c " -89 . E =
0.29
Fig. 4. Traced computer plot of magnetic polarization . Geophysical Observatory, University of Alberta, 19 May 1966 .
POLARIZATION OF MICROPULSATION SOURCES
. .. .. . . ...."" ........ . ...."...... .. . . . .. ............... ........... . . . . ...... ... . . ... .. ....."...... " "
349
""Y
" """ "" ."""""
- -82" E = 03 Fig. 5 . Traced computer plot of magnetic polari/,ation. Geophysical Observatory, University of Alberta, 2 June 1966 .
resultant magnetic vector lies predominantly in one quadrant for certain periods while for other periods it lies in the adjoining quadrant. The basic assumption that low coherency is associated with relatively love micropulsation activity was confirmed by the observation of original analog records . Figs. 4 and 5 :;how examples of magnetic field polarization over periods of two hours from the same portion of the records used for coherency studies . An examination of the analog records shows relatively high Pc 4 activity with periods between 80 and 100 sec. The points in the polarization diagrams in figs. 4 and 5 may be fitted to an ellipse ; the parameters of
the ellipse viz. the azimuth a of the major axis with respect to north, and the ellipticity E are given in each figure. The overall response of the magnetic recording system is shown in fig . 6. This response as well as induction effects arising from anisotropic and/(-,r heterogeneous resistivity structure beneath the recording site have modified the results . Nevertheless a preliminary investigation of these data suggests that although the various period intervals appear related in time, they are produced by sources independently orientated in space .
10 r
FREQUENCY
(Hz)
Fig. 6. The overall response of the magnetic recording system .
100
350
D.RANKIN and IXREDDY
This work was supported by the National Research Council of Canada and the Geological Survey of Canada.
REFERENCE (1 ] J.W .Cooley and J .W .Tukey, An Algorithm for the Machine Calculation of Complex Fourier Series, Math . Computation 19 (1965) 297.