Western Churchill NATMAP
ElectroMagnetic activities

Project leaders.....

EM component leader.....



Project Overview.....

Taken from the WC NATMAP proposal

The principal objective of the western Churchill NATMAP program is to provide modern geological maps of late Archean greenstone belts in a part of the Canadian Shield with great mineral potential, but lacking an adequate geoscientific infrastructure. The maps will underpin an enhanced, multidisciplinary geoscience knowledge base of the scale and scope required to formulate a predictive framework for crustal growth, tectonic evolution and mineral potential, and the establishment of environmental baselines for the western Churchill Province. To that end, a set of integrated multidisciplinary objectives are proposed:

In short, the proposed Western Churchill NATMAP Program's objectives can be summarised as follows: To reveal the character and origin of the Archean greenstone belts of the western Churchill, their mineral wealth, and associated granitic continental crust, by lifting the veil of Paleoproterozoic intracontinental tectonothermal and magmatic events.

EM component overview.....

The teleseismic - magnetotelluric experiment (1998) deployed 8 geophysical instruments along a profile perpendicular to the STZ. Complete analysis of the data will include anisotropy measurements, as well as mapping of sub-horizontal lithospheric boundaries and the seismic and conductivity structure of the lithosphere. According to existing criteria, the test of the age of the lithosphere is as follows: if it is indeed late Archean, then mantle structure should reflect the penetrative nature and orientation of the late Archean crustal structure. Otherwise it may reflect the discontinuous nature of Paleoproterozoic crustal deformation, or show no geometrical correlation with crustal structure at all.

EM activities.....

Eight magnetotelluric stations occupied during July/August/Deptember, 1998, located at the locations shown on the map below:

Geology map Press on map for full sized image (243 Kb)

Recording Schedule (in UT)

Latitude (North)
Longitude (West)
Start Time
End Time

WCH008 65:29:54 (65.4983) 95:33:17 (95.5547) 98-07-18 17:31:00 98-08-14 12:20:00
WCH007 65:16:04 (65.2678) 95:07:41 (95.1280) 98-07-18 13:31:00 98-08-14 13:50:00
WCH006 64:47:01 (64.7836) 95:02:58 (95.0494) 98-07-17 17:00:00 98-08-14 15:35:00
WCH005 64:13:18 (64.2217) 94:48:38 (94.8106) 98-07-14 20:16:00 98-08-14 17:03:00
WCH004 63:55:42 (63.9283) 94:29:18 (94.4883) 98-08-15 19:24:00 98-09-17 00:53:00
WCH003 63:27:13 (63.4536) 94:20:54 (94.3483) 98-08-16 00:31:00 98-09-15 05:33:00
WCH002 62:46:01 (62.7669) 93:37:53 (93.6314) 98-08-18 18:01:00 98-09-18 15:22:00
WCH001 62:26:18 (62.4383) 93:03:23 (93.0564) 98-08-18 21:01:00 98-09-18 18:23:00

Kp indices

A measure of the magnetic activity during the recording interval can be obtained from the Kp indices for the time period June 15th - September 30th.

K indices isolate solar particle effects on the Earth's magnetic field; over a 3-hour period, they classify into disturbance levels the range of variation of the more unsettled horizontal field component. Each activity level relates almost logarithmically to its corresponding disturbance amplitude. Three-hour indices discriminate conservatively between true magnetic field perturbations and the quiet-day variations produced by ionospheric currents.
K indices range in 28 steps from 0 (quiet) to 9 (greatly disturbed) with fractional parts expressed in thirds of a unit. The arithmetic mean of the K values scaled at 13 observatories worldwide (including Ottawa and Meanook) gives Kp.

The histogram of the Kp indices for the interval shows the predominance of values in the range 1-2, and the cumulative histogram shows that half of the data has a Kp index less than 2, 75% less than 3, and almost 90% less than 4. Thus, by discarding 10% of the data we will avoid very strong non-uniform source field effects, and discarding 25% of the data will avoid most non-uniform effects.

Inspecting the Kp indices, there is an interval 69 hours long when the Kp is greater than 3.0, and that is 98-08-26 06:00 - 98-08-29 03:00. The longest interval with Kp equal or less than 2.0 (apart from a short interval) is the 96 hours between 98-09-03 18:00 - 98-09-07 18:00.

The reason that magnetic activity is so important, especially for this survey, is that highly active times may give erroneous MT response functions that will lead to erroneous conclusions about the Earth below. For a theoretical example of this problem, read the abstract from the 1998 SNORCLE transect meeting by Cassels and Jones (1998), and the published paper by Garcia et al. (1997).

A comparison of the MT response functions computed using just the data at WCH004 (without "remote-reference" processing), and in geographic coordinates, for the quite (98-09-03 18:00 - 98-09-07 18:00) and active (98-08-26 06:00 - 98-08-29 03:00) time periods is shown here. The solid symbols are for the quiet period, and the open symbols are for the active period. Note the higher phases for the active period, especially in the range 100 - 1000 seconds, and the lower estimates of apparent resistivity. These are generally as predicted in the theoretical studies given above. (Note: the wavelength of the extenal source fields is period dependent, so the effects on the MT responses will not exhibit the simple form shown in Cassels and Jones, 1998).

MT Time Series

You can view the 5-component MT time series... as well as the magnetic field data for mid-July to end-September from the geomagnetic observatories at Note problems with the Ey data at site WCH002, and the serious clipping of the Ey data a site WCH008. It will not be possible to derive a full MT response for site WCH002 due to the bad Ey data - it is only possible to derive the responses between the Ex component and the horizontal magnetic field components (Hx,Hy).

Information from the time series

An overview of the amount of gross electrical "structure" can be obtained by viewing the vertical magnetic field (Hz) variations across the profile. The graph of Hz variation for the four sites south of Baker Lake show a very uniform vertical magnetic field across the whole region. This is mostly due to the external ionospheric source field. However, if there was a major anomaly it would be visible in this plot. That these data are uniform implies a very uniform electrical structure with little major difference from south (WCH001: bottom line) to north (WCH004: top line). This is the same for the data from the four sites north of Baker Lake. There is a visible increase in Hz amplitude, but this is due to the proximity of the source fields.

One can look at the Hz data in more detail, and especially for quiet days when the source contribution should be small. The Hz data from the southern four sites for day 98-09-10 show a remarkably uniform vertical field across the whole length of the profile. Even at the smaller scale of one hour (98-09-10 18:00-19:00 UT) one sees little difference along the profile. Going to an even smaller time scale of 5 minutes (98-09-10 18:10-18:15 UT) there is now visible differences, with increased amplitudes in the sinusoids with about 30 seconds periodicity, centred at 11 minutes and 13 minutes, for the southernmost site (WCH001) and the site south of Baker Lake (WCH004). The increase in amplitude at WCH001 is likely caused by the coast effect - an increase in magnetic fields caused by currents that flow in the sea water close to the coast. The increase in amplitude at site WCH004 is caused by an anomaly in conductivity beneath, or to the north of, site WCH004. That the increased amplitude is most apparent at the higher frequencies indicates that the anomaly is shallow, probably in the crust.

Inspecting the data from the northern stations in the same manner, the quietest day magnetically is 98-08-02, and the vertical field variations are all very similar along the whole profile. Picking out a two hour very quiet interval, when the source fields should be most uniform, 98-08-02 05:00-07:00 UT shows an increasing amplitude to the north, which is likely a residual effect due to the source fields. An afternoon hour, 98-08-02 19:00-20:00 UT, which is 3 PM local geographic time and about 1:30 PM local geomagnetic time, shows the uniformity across the whole profile.

There is very little large-scale conductive structures in the region visible in the time series.

See Evolution of Archean and Proterozoic subcontinental mantle and lower crust

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Alan G Jones / 22 April 2006 / alan-at-cp.dias.ie