Errors introduced into aeromagnetic survey results because of geomagnetic time variations have been analyzed in two ways. In the first, statistical properties of average magnetic disturbance have been calculated for five locations well distributed in latitude in Canada. The autocorrelation functions of vertical magnetic field fluctuations have been used to estimate errors in aeromagnetic surveying as a function of the length of the base loop and the length of the traverse lines or profiles, for five latitudes. In practical applications, the r.m.s. errors are proportional to the square root of elapsed time. These results can be explained in terms of the shape of the autocorrelation functions, and models of disturbance which might produce them are discussed. In the second part experimental results are presented of a comparison of total field intensity fluctuations measured with recording proton precession magnetometers at two pairs of stations, one pair 26 miles apart and the other 94 miles apart. The measurements were taken in Western Canada south of the auroral zone. The r.m.s. differences in the time variations have been determined quantitatively for each hour for both pairs of stations; statistically, it was found that these differences were proportional to the r.m.s. level of magnetic activity and, in this region, to the separation of the stations. The frequency distributions of the r.m.s. differences between the two pairs of stations for the month of September 1959 have been determined. A comparison of the indirect and direct methods of correction indicates that scientifically, in this region, there is little to choose between them in terms of accuracy. Operational considerations are mentioned.

A three-component airborne magnetometer has been designed and built at the Dominion Observatory. The magnetometer is mechanically linked to a gyro-stabilized platform which is maintained horizontal, independent of the motions of the aircraft. The gyroscopes are precessed at a rate proportional to the time integrals of signals from accelerometers mounted on the platform. The system acts basically as a pendulum with a six-minute period. Damping is provided by phase-advance networks in the control loops. Forced oscillations of the platform are reduced by the addition of automatically computed signals proportional to the aircraft accelerations. The accuracy of the platform is 2 or 3 minutes of arc under normal survey conditions. The azimuth reference for the instrument is provided by a directional gyroscope mounted on the platform, whose drift is determined to an accuracy of 0.2° by astronomical measurements with a periscopic sextant stabilized in azimuth. The magnetometer head contains three orthogonal magnetic detectors of the saturated transformer type, which give direct currents proportional to the fore-and-aft and transverse horizontal components and the vertical component. These and the heading of the aircraft are fed into an analog computer which displays continuously the declination in degrees, and the horizontal and vertical field components in gauss. An alternative display presents automatically the average values of these quantities over successive five-minute intervals. The accuracy of measurement of field components referred to the reference axes established by the stabilization system is estimated to be 0.1° in declination, and 20 gammas in the other components. Sources of error in survey operations are discussed and the reduction of survey results and the determination of the corrections for the magnetic field of the aircraft described. It is concluded that the probable error of a survey observation as plotted on a chart is about 100 gammas in any component, and is principally due to errors in navigation and plotting.

Recent work on prominent magnetic variation anomalies in Canada is reviewed. The first of these is on Ellesmere Island in the Arctic Archipelago, and results from magnetic and magnetotelluric data collected in 1967 are presented. Anomalous effects-i. e., an abnormally high level of magnetic activity coupled with a persistent tendency for the horizontal variation vector to be restricted to a single direction-appear to be confined to a narrow zone nearly 500km long stretching between Alert on the north coast and Eureka on the west. Recent data indicate that the strength of the anomaly is not uniform along its strike, but is somewhat diminished in the central and southern portions. The main features of the anomaly have been explained by postulating the presence of a long narrow conducting body located in the lower part of the crust. The existence of such a conductor would provide a natural channel for currents induced over a much broader area. It may also imply an abrupt upheaval of isotherms underneath Ellesmere Island. Available surface wave dispersion and heat flow information in the Ellesmere Island area will be discussed: the evidence supporting a thermal explanation for the geomagnetic anomaly is unconvincing, but still ambiguous.The Mould Bay anomaly is located in the eastern part of the Arctic Archipelago and is known to extend over large portions of Prince Patrick and Melville Islands. Over this area shorter period fluctuations in the vertical component are very strongly attenuated. The presence of a massive conducting layer deep in the crust is postulated to explain the effect. Seismic, heat flow and gravity data are available in this area, but no clear relation to the geomagnetic anomaly has been found.

Recordings from a crustal seismic experiment, which was conducted in the Yellowknife area in 1966, were used for calibration of the Yellowknife seismic array. In the immediate vicinity of the array the crust is found to be very uniform. A superficial layer with an intercept time of 0.172±0.012 s and unknown velocity is underlain by a crust with a P wave velocity of 6.04 ± 0.01 km s−1 near the top: assuming this velocity constant throughout the second layer, the total thickness of the crust is about 34±2 km. The Mohorovicic discontinuity is horizontal under the array within the resolution of this experiment and the apparent Pn velocity is 8.15 km s−1. At a distance of a few tens of kilometres the crustal uniformity breaks down. The distances are such that, for most teleseismic signals, the effect of these in homogeneities should be negligible.

ABSTRACT In a 30-day period beginning March 29, 1965, some 2026 microearthquakes were recorded at MBC and by the array NPNT. About 10 per cent of them were located. The extent of the epicentral region was 3 sq km and of the hypocentral region 8 cu km. The centroid of the hypocenters was 12.6 km from MBC on an azimuth of 121° and at a depth of 6.8 km, very nearly on the strike of an old fault. Although no foreshock-aftershock series could be distinguished, the events were not independent nor randomly distributed in time. Particle motion diagrams were drawn for 10 events. These identified the longitudinal and transverse waves and confirmed the azimuths and angles of emergence obtained from amplitude data. The array data also confirmed the azimuths and yielded a P-wave velocity of 4.33 km/s near the surface. The magnitudes ranged from − 1.1 to 2.9 and the frequency-magnitude relation was: log N = 2.46 ( ± 0.01 ) − 0.68 ( ± 0.01 ) M L . The coefficient 0.68 suggests tectonic activity. The partition of energy between P and S, and the observed directions of motion (all compressions at MBC) are compatible with the double couple model of focal mechanism and motion on the old fault. However, magma movement cannot be entirely ruled out as there exists geomagnetic evidence of abnormally high upper mantle temperatures. The following energy-magnitude relation was determined from 100 events in the range 0.1 ≦ ML ≦ 1.6: log E = 10.1 ( ± 0.3 ) + 1.91 ( ± 0.03 ) M L .

A number of experimental seismic arrays have been constructed in the past few years. One such array in the form of an asymmetric linear cross has been built at Yellowknife, NWT, by the United Kingdom Atomic Energy Authority in collaboration with the Canadian Department of Mines and Technical Surveys. Data from the 19 individual seismometers are recorded continuously on FM magnetic tape. The analogue tape data are multiplexed into a digital computer at twice the recording speed. The system allows the formation of 168 beams by the delay and sum method: different approaches to this problem and their implications for real time processing are discussed. The correlations between the phased sums of the two lines are calculated and events are detected automatically when the correlation rises above a trigger level for a preset length of time. For each event a selection of logarithmic correlations is output in analogue form, together with other pertinent information.

Three heat flow determinations a were made in M'Clure Strait between Prince Patrick and Banks Islands in the northwestern part of the Arctic Archipelago of Canada. The three stations lie within 55 km of a point some 130 km SSW. of Mould Bay, Prince Patrick Island, and yield a weighted mean heat flow of 0.84 ± 0.09 μcal cm−2 s−1, or 57% only of the worldwide continental average. The measurements were made from sea ice in water depths of some 430 m using a thermal probe and portable equipment carried in a fixed-wing aircraft.Instrumental limitations and errors are discussed, together with environmental factors. The uncertainties in interpreting this result as a truly subnormal equilibrium heat flow are outlined but it is concluded that the calculated systematic errors are unlikely to exceed 25%. Consequently in the absence of any known major perturbing effect, it must be concluded that the structure responsible for the suppression of vertical magnetic held variations at Mould Bay observatory does not extend 130 km to the south, is not produced by an anomalously high near-surface temperature, or is of late-Quaternary origin.

The general problem of analysing the depth of near surface magnetic sources which contribute to the magnetic anomaly spectrum has been analysed in terms of crossings per unit interval. Assuming a random distribution of either magnetic poles or magnetic dipoles aligned in any of three orthogonal directions, explicit functions have been derived expressing the crossings per unit interval for three orthogonal field components directly in terms of either the depth to a thin layer or depth to the top of a thick layer.The statistical weakness of the method is explained: it appears to be most useful in supplementing other statistical techniques. The speed and simplicity of the method, however, makes it an attractive possibility in extracting preliminary information from the large number of World Magnetic Survey aeromagnetic three-component profiles now being acquired.

During the 1963 field season, a magneto-telluric experiment was successfully undertaken at and near Alert, in northern Ellesmere Island. Analysis of the results shows that the electric field is very strongly confined to a direction nearly N–S, that a strongly inhomogeneous or anisotropic situation exists with the electric field in phase (±5°) with the horizontal magnetic field for periods between 240 and 6000 s, and that the electric to magnetic field ratio is constant and very small (0.12(6) mV km−1 γ−1) in the same period range. At Lake Hazen, 150km to the SW approximately, the experimental data is of poorer quality, but the inhomogeneity or anisotropy is weaker, the mean direction of the electric field between 20 and 30° E of N, the phase advance of the electric over the magnetic field nearly 45° and the electric to magnetic field ratio is frequency dependent. The magnetic variations measured at Lake Hazen suggest an extension along strike of the Alert anomaly. The Alert results are not consistent in inhomogeneity, phase and frequency dependence with earlier models used to explain the magnetic variation anomaly observed in northern Ellesmere Island. Equally an interpretation of the magneto-telluric results in terms of a near surface plane conductor with a very large height-integrated conductivity ≃6(.3) × 10−6 emu is not consistent with the earlier magnetic variation results, fails to explain the inhomogeneity or anistropy, and makes no geological or common sense. The inductive situation is very complex and it appears that boundary arguments must be very important and need theoretical elucidation. At the present time no self-consistent hypothesis can be derived : some of the problems in deducing one are demonstrated in the text using illustrative calculations.

Two unusual features of geomagnetic variations have now been discovered in the Arctic Archipelago of Canada. The first anomaly at Alert on Ellesmere Island is characterized by abnormally high levels of irregular magnetic activity and a persistent directional characteristic of the vector horizontal disturbance. An adequate iono-spheric explanation has not been found. Recent papers have given an explanation of the gross characteristics of the Alert magnetograms in terms of induction in a large anomalous conductor in the upper mantle striking parallel to the channel separating north Ellesmere Island from Greenland. Field experiments followed by simple potential and spectral response analyses confirm the existence of a gross conductor. In this paper the uncertainties in and shortcomings of the first-order explanation and minor modifications are stressed, and an alternative solution involving induction in the conducting mantle and a highly conducting circuit examined. The alternative solution appears to be even more inadequate.The second anomaly at Mould Bay, Prince Patrick Island, is characterized by the striking absence of shorter-period vertical field magnetic variations. Assuming the presence of a sheet conductor at depth it has been estimated earlier from the frequency dependence of the power spectrum that a 10-20km thick layer with a conductivity near 1011emu is required near the bottom of the crust or in the upper mantle. The assumptions in deriving this explanation are outlined, and theoretical curves shown which demonstrate that the neglect of reasonable conductivity above and below the anomalous layer appears justified. Results are shown applying the same technique to plane earth induction in a non-anomalous area, and to one magneto-telluric situation of current interest.

The theoretical basis for the interpretation of the Alert anomaly in geomagnetic variations has been reexamined. Under the uniform horizontal inducing field approximation, the electromagnetic response of an infinite elliptical cylinder of infinite conductivity has been derived, and from this the response for the upheaval of a half-elliptical conductor from a highly conducting mantle. The results have been extended to an asymmetric upheaval of the conducting mantle. A theory has also been developed of the electromagnetic response of an elliptic cylinder which is useful for application to the finite conductivity case.The electromagnetic coupling between an infinite circular cylinder and an underlying conducting layer has been investigated as far as the second-order approximation. An exact solution has then been obtained for the infinite conductivity approximation, and useful examples calculated.The situation at Alert has been reexamined following this work which reduces one of the difficulties of the earlier interpretation. An upheaval of about 100 km of the 1400–1500 °C isotherm to within 25–30 km of the surface appears capable of accounting for the gross characteristics of the response but the solution is certainly not unique and may be in error by as much as 60%. The asymmetry in the response is difficult to explain quantitatively. The response parameters thought to be most useful have been clearly defined and the ambiguity in the electromagnetic response solutions illustrated. Suggestions for further progress are made. Brief mention is made of preliminary magnetotelluric and gravitational studies at Alert.

Magnetic variations measured at three stations across the strike of the anomaly in electrical conductivity of the earth's mantle suggested by Whitham and Andersen (1962) near Alert, Ellesmere Island, have been analyzed by simple potential theory. The anomalous internal contributions to the magnetic variation vector show striking confinement consistent with an underground current in a northeast–southwest direction at a depth between 50 and 70 km, a few kilometers southeast of Alert. If the anomalous conductor is approximated by a uniform infinite cylinder, the horizontal field response as a function of frequency can then be explained with a conductivity of [Formula: see text] e.m.u. and a cylindrical radius of about 50 km. The vertical field response is, however, an unsatisfactory fit to this model. A possible asymmetry is noted based on the hourly range data: this could be explained by the dipping of the real conductor under Ellesmere Island.Preliminary earth potential measurements show that, as expected, the electric field variations are abnormally low at Alert.The gravitational consequences of the cylindrical model are discussed and compared with two profiles from the same region. A Bouguer anomaly is found in the predicted region if certain regional gradients are assumed, but its magnitude is smaller than that predicted from the cylindrical model in hydrostatic equilibrium. It is clear that approximate agreement can be obtained with crustal thinning of some 20 km.The significance of the thermal anomaly thought to be responsible for the electrical conductivity anomaly is discussed briefly.

The anomaly, first noted during the I.G.Y., in the level of irregular magnetic activity at Alert on Ellesmere Island, Canada, has been investigated further. Analysis of the characteristics of three-component records and extensive field work in the Arctic Archipelago have shown no evidence that Alert is part of an inner zone of enhanced magnetic activity. A superposed epoch analysis of irregular magnetic activity has indicated it is unlikely that the Alert result is related to the mechanism producing polar cap absorption. An anomalous induced contribution to the observed magnetic fluctuations must therefore be considered. Despite the complexity of magnetic disturbance inside the polar cap, and the paucity of available data, it is shown that the magnitude, directional characteristics and approximate frequency dependence of the variations are consistent with the hypothesis of a large anomalous body striking parallel to the channel separating north Ellesmere Island from Greenland. Appreciable thinning of the crust, and a large heat flow anomaly would be expected. No other geophysical data are available in this logistically difficult region to support or refute this hypothesis. Until sufficient data are available for a satisfactory potential analysis the model must be regarded as quite speculative in view of the tectonic stability of the region.

Hourly ranges in the principal horizontal field component have been measured for sixteen Canadian IGY magnetic observatories and variation stations. The latitudinal variation of disturbance measured by this index has been determined seasonally and as a function of disturbance. One station, Alert, at the northern end of Ellesmere Island, confirms the existence in these longitudes of an apparently narrow zone or area of enhanced magnetic activity, as defined by this measure of disturbance. Semipersistent structure is also apparent in the principal auroral zone in the meridian sections of magnetic activity. Diurnal occurrence patterns, amplitude-frequency plots, the diurnal variation of the mean disturbance field, and the physical significance of this range index have been investigated in an attempt to explain this apparent inner maximum of magnetic activity. More homogeneous very high latitude data are required to determine the morphology of the anomalous region found.

Reviews post-war studies on the secular motion of the dip pole and predicts its position for epoch 1960. 0:74.8 ± 0.3 N., 99.6 ± 1.2 W. The daily track during moderate magnetic disturbance approximates an ellipse with a north-south long axis of ca. 30 mi and an east-west axis of ca. 18 mi. Estimated and observed tracks 1600-1950 are mapped, and sources and magnitudes of errors are discussed. The secular motion, as estimated by various methods, averages 5.5 mi/yr to the north and 0.7 mi/yr to the east.

More than 1,000 pulsations, with approximately constant periods and ranges exceeding 3 gammas, have been studied using Meanook and Agincourt standard run magnetograms for the years 1951 to 1954. This investigation confirms the existence of two separate classes of pulsations, differing in form, time of diurnal occurrence, and mean period. Some additional characteristics reported earlier from Scandinavia are confirmed, but it is now thought that the narrow band Rolf micropulsations are not infrequent ~ 1200 kms. south of the auroral zone in Canada. Very few regular pulsations \1·ere observed on magnetograms from stations north of the auroral zone. Although magnetohydrodynamic waves in the upper parts of the ionosphere provide a possible periodic explanation, the different times of occurrence of the classes in Scandinavia and Canada, and even across Canada, the relationship of the primary sources of pulsations to magnetic disturbance measured by K-indices, and the southern geomagnetic extension in Canada of observable pulsations remain unexplained. Screening effects in the lower ionosphere are considered and provide one explanation of the observed amplitude-period trend.

An investigation of the diurnal and seasonal characteristics of irregular magnetic activity in northern Canada, using two indices of disturbance, shows that in general the K index provides a reliable but smoothed measure of short period activity. Two peaks of activity are evident, a day‐time peak, dominant inside the polar cap, and a night peak, dominant south of the auroral zone. The latter occurs within one hour of local geomagnetic midnight at four Canadian observatories, whereas the day‐time peak occurs progressively later at higher latitudes. The maximum activity occurs in the equinoxes at latitudes near the auroral zone, and during the summer solstice at very high latitudes. Measurements of range at the most disturbed observatory suggest that aeromagnetic surveying in high latitudes is in general feasible, but that for accurate reconnaissance work it is very advantageous to plan operations, so far as conditions permit, taking account of the systematic diurnal variation of disturbance. It is shown that errors of about one half the assumed diurnal change between base lines will occur and sufficient data is given to allow approximate predictions of the optimum length of flight lines.