Short-period Geomagnetic Variations Research Articles

Possibilities of further improvement of 1 s fluxgate variometers

Abstract. The paper discusses the possibility of improving temperature and noise characteristics of fluxgate variometers. The new fluxgate sensor with a Co-based amorphous ring core is described. This sensor is capable of improving the signal-to-noise ratio at the recording short-period geomagnetic variations. Besides the sensor performance, it is very important to create the high-stability compensation field that cancels the main Earth magnetic field inside the magnetic cores. For this purpose the new digitally controlled current source with low noise level and high temperature stability is developed.

The distribution characteristics of short period geomagnetic variations in Shaanxi region

In order to investigate the characteristics of short period geomagnetic variations in Shaanxi region, a temporary measurement profile from Shiquan in the southern part of Shaanxi to Ningxian in Gansu was set up in September of 1996 to carry out simultaneous observation of three-component geomagnetic variations along the profile. The vertical transfer functions, the horizontal transfer functions and the spectral component’s relationship of each period segment at each site in the period range from 3 min to 2 h were statistically calculated. Random data theory was applied to the data processing for reference, which led to a relatively good result. Analysis of the data point out that short period geomagnetic variations are affected by the regional influence of underground anomalous inductive current concentration between Wugong and Qianling in the nearly E-W direction and by the influence of local inductive current in nearby area of measurement site. The main distribution feature is that vertical transfer functions reverse between Wugong and Qianling and frequency responses of the transfer functions have maximum at period about 20 min.

Short-period geomagnetic variations recorded concurrently with an aeromagnetic survey across the Bendigo area, Victoria

A high-resolution aeromagnetic survey was undertaken by the Australian Geological Survey Organisation (AGSO) across the Bendigo 1:250 000 map sheet area during February to April 1994. For part of this period, high-resolution ground magnetometers were also deployed to test for the way that short-period variations of the geomagnetic field (including micropulsations) affect data recorded by the aircraft. These ground instruments consisted of the newly developed Helium base-station for AGSO aeromagnetic surveys, and two ring-core fluxgate magnetometers developed by the School of Earth Sciences at The Flinders University of South Australia. The Helium instrument was deployed at the Bendigo airport, in the south-west corner of the area, and the ring-core instruments were buried in the north-west and north-east corners of the area. Preliminary results indicate that while the phases of continuous Pc3 micropulsations of about 20 s period change little across the map sheet area, the amplitudes can vary significantly. For longer period geomagnetic variations (60 s to 600 s), there are significant differences in the amplitudes and phases of the total-field values measured at different sites. Induction vectors indicate a shallow zone of higher conductivity between the east and west measurement sites. Data from a single base-station magnetometer subtracted from airborne survey data will not adequately remove the short-period geomagnetic variations of the total field recorded during this survey period ? they are an additional noise source in the survey data.

Secular and seasonal changes in transfer function at Guangzhou geomagnetic observatory

Time changes in transfer functions of short period geomagnetic variations in 28 years from 1960 through 1987 are studied, systemically and in detail in this paper. The results indicate that: (1) It is evident that seasonal and secular changes in the transfer functionsA at exist the Guangzhou Geomagnetic observatory. The characteristics for seasonal changes are large in summer and smaller in winter with main cycles of 12 and 6 months. The characteristic for secular change is a descend with a rate of 0.0025 per year. The seasonal and secular changes in transfer functionsB are not evident. (2) The direction of Parkinson vector at Guangzhou geomagnetic observatory is clearly affected by the ocean, and is associated with coastal effect. (3) The values of the transfer function A and B are related to geomagnetic activity ata=0.05. Secular change is not related to geomagnetic activity, maybe it is affected in a great measure by earth conductivity change.

Investigations of tectonomagnetic phenomena in China

Abstract The Tangshan earthquake ( M = 7.8, 28 July 1976) occurred in North China, where there is relatively dense network of geomagnetic observations. Before the earthquake there were anomalies of ∼ 8–10 nT in the total intensity of the geomagnetic field at Ninghe and Dongtietou sites, 50–90 km from the epicenter, and also in the Beijing survey area. Anomalous changes in the vertical component of the geomagnetic field before the earthquake amounted to −12 nT at Tangshan site, and 10 nT at Changli station, which was ∼ 80 km from the epicenter. There were also anomalous short-period geomagnetic variations before and after the earthquake. To investigate tectonomagnetic effects, we measured the magnetic field at 17 sites near an underground nuclear test in 1983 in West China. The total field changed by 2 nT at the time of the explosion, and there were changes of 1–2 nT before and after the explosion. We have also measured the magnetic field around the Miyun reservoir during 1983–1987 and observed a correlation between geomagnetic changes and variations of water level in the reservoir.

Extension of the Eyre Peninsula Conductivity Anomaly

Twenty-two digitally recording fluxgate magnetometers measuring short-period natural time variations of the geomagnetic field were deployed during 1988 at 54 sites to define the northward extension of the Eyre Peninsula Conductivity Anomaly (EPCA). The southern portion of this zone of high electrical conductivity was initially mapped during the early 1980s; it trends inland just to the east of north from the southern extremity of the peninsula, at nearly 90� to the continental shelf edge.The 1988 data show that the axis of the anomaly in the north initially swings to the east, and then turns NNW, following the major structural trends in the Precambrian basement of the Gawler Block. Short-period geomagnetic variations observed in stacked magnetograms display a significant amplitude and phase change in the vertical (Z) and total field (F) components across the anomaly axis, while the horizontal variation fields (D and H) are enhanced along the axis. These features are consistent with a concentration of current in the crust.Transfer-functions relating the anomalous vertical to normal horizontal geomagnetic variation fields are expressed in the form of vectors and hypothetical event contours, and these clearly delineate the surface position of the anomaly axis.The EPCA is coincident with a broad band of seismic activity, and a continuous conductive zone such as this must be formed along a significant structure within the Precambrian crust. The zone probably delineates the boundary between the essentially non-conductive Archean crystalline basement to the west and the multiply deformed Proterozoic metasediments of the Hutchison Group to the east. Its presence is important in understanding the tectonic development of this potentially economic zone.Zones such as the EPCA can also introduce errors of several nT into aeromagnetic surveys; data from closely-spaced magnetometer arrays may be used to accurately predict and eliminate such errors.

Short-period geomagnetic variations at two stations in Sri Lanka and their relation to channelling of induced currents through a conducting region beneath the Palk Strait

Electric currents induced in the deep ocean and channelled through a conducting region beneath the Palk Strait have been invoked by earlier workers to explain anomalies in transient geomagnetic variations such as bays, SSCs and substorms observed at some South Indian coastal stations. The need for confirmation of this idea by measurement on the Sri Lankan side across the Palk Strait has been emphasised by these workers. In the present study, rates of change in the geomagnetic field components in the period range 20–600 s recorded at Kondavil and Hikkaduwa, two stations in the equatorial electrojet belt near the northern and southwestern coasts, respectively, of Sri Lanka, have been analysed and discussed. The results are presented in the form of variation with period of ΔZ/ ΔH and ΔD/ ΔH at Kondavil, ΔZ/ ΔH at Hikkaduwa and of ΔH C/ ΔH H, the ratio of H components at Colombo and Hikkaduwa. They provide strong confirmatory evidence for channelling of induced currents through a conducting region beneath the Palk Strait and could serve as valuable control data in any detailed modelling of the conductivity structure beneath the Palk Strait. A day-time enhancement of ΔZ/ ΔH at Hikkaduwa at all periods in the observed range and a similar enhancement of ΔH C/ ΔH H were observed. These enhancements are possibly due to the effect of the equatorial electrojet on the short-period variations.

Local anomaly in magnetic and electric field variations due to a crustal resistivity change associated with tectonic activity.

Electromagnetic response is examined for a local three-dimensional resistivity anomaly embedded in the earth's crust of uniform resistivity, in order to interpret time-dependence of horizontal amplitudes as well as transfer functions for short-period geomagnetic variations at an observation site in the Izu region, Japan. The results of numerical estimation indicate that the detected enhancement of about ten percent in horizontal amplitude can be well explained by a local decrease, amounting to one order of magnitude, in crustal resistivity, on the condition that the observation site is located over the area of resistivity decrease. In this situation no marked change is expected for transfer functions; this is consistent with the observation. The results of numerical estimation also suggest that electric as well as magnetic field measurements are useful for detecting crustal resistivity changes; for instance, changes are expected in the amplitude of induced electric field, as would be reflected in apparent resistivity changes in magnetotellurics, and also in polarization angle of the electric field. It should be noted, however, that no detectable precursory changes are expected unless observation sites are located close to the area of resistivity change presumably corresponding to an earthquake source area and its vicinity.

Perturbation of induced electric currents by surface conductivity inhomogeneity with special reference to anomalous behavior of short-period geomagnetic variations in the Kanto Plain, Japan.

It has long been considered that anomalous behavior of short-period geomagnetic variations in the Kanto Plain (the Kanto anomaly) reflects a mantle conductivity anomaly and the sedimentary layer as thick as 3, 000m or more plays a rather minor role, only as local perturbation superposed on a regional anomaly. The complicated surface conductivity structure in this region, characterized by the sea existing to the east and the south and the thick sedimentary layer in the central part, has hampered quantitative discussion on the effects of seawater and sediments on short-period geomagnetic variation anomalies. It is shown, however, that such surface effects can be estimated approximately by considering two different polarization cases separately; one for a northwestward geomagnetic variation and the other for a northeastward geomagnetic variation. In the former case, the sea effect is striking as disclosed by numerical calculations of the peninsula effect. In the latter case, the so-called channeling effect due to the presence of the sedimentary layer seems to be a predominant factor. It is concluded that the Kanto anomaly is primarily accounted for by these surface effects, although no significant information on the crustal and mantle conductivity structure could be derived.

An analogue model study of electromagnetic induction for cape and bay coastlines

The electric and magnetic field variations across bay and cape coastlines are determined from scaled laboratory analogue models. The effect on the field response of such features along an otherwise straight coastline is limited to a distance approximately equal to the dimensions (diameter of the bay or cape) of the irregularity. For the E-polarization of the source field the perturbations in H z and H y are more confined, mainly to the immediate region of the bay or cape. The coast effect H z / H y for this polarization, and variations with a 4.2 min. period, is approximately a factor of two larger for the cape than for the bay, while the straight coastline H z / H y is roughly mid-way between the cape and bay values. For H-polarization the coast effect is again approximately a factor of two larger for the cape than for the bay, but both values are larger than the value for a straight coastline. The response of the various field components to the bay and cape features is decreased at longer periods, and as a measure, the coast effect is approximately a factor of 1.2 greater for the cape than for the bay for a 42 min. period as compared with the factor of 2 for a 4.2 min. period. The bay and cape deflect the induced currents flowing in the ocean and modify the currents diffusing into the land. This study demonstrates that data from coastal sites may be unreliable for correcting magnetic surveys or for analyses of short period geomagnetic source-field variations.

Electrical conductivity structure beneath the northwestern philippine sea as inferred from the island effect on Minami-Daito Island.

Observations of short-period geomagnetic variations were carried out at three sites on Minami-daito Island in the northwestern Philippine Sea. The observed results clearly show the island effect; anomalous Z variations on the island, particularly characterized by a reversal of sign in Z variations between northern and southern stations for north-south magnetic fields. Response and current functions are determined from transfer function A obtained for the period range from 7 to 240min.Numerical calculations of the island effect made on the basis of thin-sheet approximation can well account for the anomalous Z variations. Then, the response or current function obtained from the observed results is compared with those determined from the results of numerical calculations for underground conductivity models. It is concluded that a highly conducting layer exists at a likely depth range from 80 to 120km beneath the northwestern Philippine Sea where Minami-daito Island is located. The conductivity of the layer is estimated as 0.5S/m or so.

Changes in the geomagnetic field associated with earthquakes in the Izu Peninsula, Japan.

Intensive observations of the geomagnetic field have been carried out since 1976 in the Izu Peninsula, Japan. An array of observation sites covers the area of anomalously high microearthquake activity persisting since 1975. The array also extends over the region of crustal uplift amounting to 15cm or thereabouts which was revealed at the beginning of 1976. At present an observation system consisting of four types of stations is in operation. The total intensity of the geomagnetic field has been measured continuously by proton precession magnetometers at four stations. At two other temporary stations the total intensity is to be measured whenever earthquake activity becomes high in their vicinity. Three components of geomagnetic field variations have also been measured by a flux-gate magnetometer at another station. Finally, 45 observation sites have been established for repeated surveys of the total intensity. In addition to the repeated surveys at these sites, a small-scale experiment based on a synchronized measurement of difference in the total intensity between station pairs has been under way at an array of 14 sites located near the center of the crustal uplift.During the period from 1976 to 1978, four major earthquakes took place within a distance of 30km from the array: a M5.4 event of August 1976, a M7.0 one of January 1978, a M5.0 one of November 1978, and a M5.4 one of December 1978. In association with these earthquakes, some changes were detected in the total intensity and also in short-period geomagnetic variations. In the case of the largest event of magnitude 7.0, the difference in the total intensity between the two stations, where continuous measurements had been made, underwent a change about two months prior to the shock. The change was very similar in time sequence to that in the electric self-potential observed near one of the above two stations. Amplitudes of short-period geomagnetic variations also changed about two months before the shock. The result of repeated surveys disclosed a striking pattern of spatial distribution of presumably coseismic changes which is quite opposite to that obtained during a post-earthquake period. About two months prior to the M5.0 earthquake which occurred several kilometers east of another station for continuous measurements, the total intensity remarkably decreased by 5nT or so at the station. The field abruptly recovered after its occurrence; a coseismic change amounted to about 5nT. In the case of the M5.4 event of 1978, no coseismic change was detected at a temporary station located a few kilometers west of the aftershock area. Changes in amplitude of short-period geomagnetic variations also appeared before this M5.4 earthquake.

Changes in the direction of magnetic vector of short-period geomagnetic variations before the 1972 Sitka, Alaska, earthquake.

Secular change of the transfer function for short-period geomagnetic variations at Sitka, Alaska, Magnetic Observatory is examined in association with the 1972 Sitka earthquake of magnitude 7.2, the epicentral distance of which was only 40km from the observatory. The analysis is difficult due to the effect of fluctuations of the position of the auroral zone because of the high latitude of the observatory. It seems nevertheless likely that the transfer function underwent a marked change about 1 year prior to the main shock.

Electrical conductivity anomalies in the earth

Anomalies of short-period geomagnetic variations have been found in various regions over the world. It is known that such anomalies arise from electromagnetic induction within an electrical conductivity anomaly or from local perturbation of induced electric currents by a conductivity anomaly. In order to investigate a regional electric state in the Earth, conductivity anomaly (CA) studies based on anomalous behaviors of geomagnetic variations have been extensively undertaken, as well as studies based on magnetotelluries in which induced currents are directly used. Some of the geomagnetic variation anomalies, however, turned out to be caused by surface conductors, such as sea water and sediments. Anomalies of this sort have been intensively studied and classified into coast, island, peninsula, and strait effects in the case of sea effects. Three-dimensional conduction or channelling of induced electric currents is sometimes observed in the cases of sediments and some crustal conductivity anomalies. However, anomalies of such surface origins often provide some information of the underground conductivity structure. Electrical conductivity anomalies can be classified into two types: anomalies originating in the crust and in the upper mantle. Many of crustal anomalies are well correlated with metamorphic belts, fracture zones, and hydrated layers, and magnetic and gravity anomalies are also often found over the conductivity anomalies. Most of mantle anomalies have been interpreted mainly in terms of high temperature and partial melting, since conductivity anomalies coincide well with anomalies in heat flow and seismic wave velocities.

Crustal dilatancy and geomagnetic variations of short period.

Electromagnetic induction in a two-dimensional conductor having a rectangular cross-section embedded in a less conductive half-space is studied. Supposing that such a system of conductors represents an electric model of a dilatant region in the earth's crust, an appreciable enhancement of the horizontal magnetic field is found over the portion of the earth's surface immediately above the higher conductivity region for a certain frequency range of the inducing field.The possibility of monitoring an underground dilatant region by means of an observation of short-period geomagnetic variations is thus suggested.

A model of the geoelectric structure beneath Japan.

A quantitative evaluation of the geoelectric subterranean model beneath Japan is made on the basis of the analyses of short-period geomagnetic variations. For the model it has been surmised that the surface of a highly conducting mantle layer must steeply drop to a depth of 200km or so from a depth of 40-50km beneath the Pacific coast of central Japan. The depression seems likely to become shallower toward the Japan Sea underneath which the depth of the layer again reaches 40km.As the above “depression model” was derived on the basis of a perfect conductor approximation, it is important to see whether the model can account for the frequency characteristics of geomagnetic variation as brought out by intensive observations in recent years.The electromagnetic response of a conductive half-space having a triangular depression on its surface is estimated entirely numerically. Similar studies are also made for a case in which thin conductive sheets representing the sea are added. It turns out that the model seems likely to account for the characteristics of geomagnetic variations having a period range of a few tens of minutes. Although there is no guarantee for uniqueness, the depression model cannot be ruled out as a possible geoelectric structure beneath Japan.

Electrical Conductivity Anomaly and Terrestrial Heat Flow

There are two types in anomalies of short period geomagnetic variations: one (High cut ΔZ type) is characterised by high attenuation of high frequency components of ΔZ and the other (Directional variation type) by unusual directionality in the variation vector. “High-cut ΔZ type” anomaly is expected to be found in areas where the electrically conducting layer is raised and “Directional variation type” is expected to exist at boundaries between areas having different electrical properties. Reported anomalies of geomagnetic variations in various parts of the world are classified into the above two types and compared with the observations of terrestrial heat flow. Most of “High-cut ΔZ type” anomalies are associated with high heat flow, whereas most of “Directional variation type” anomalies are associated with the boundaries of high and low heat flow areas. Such correlations are interpreted in terms of the temperature distribution in the upper mantle. Typical of “Directional variation type” anomalies are found at continental margins, where the effect of sea water should not be overlooked. It is further shown that, from the view-point of both heat flow and the conductivity anomaly, there are two types in continental margins: normal (Atlantic type) and island arc type (Pacific type) continental margins. Finally, a model that appears reasonable is presented for Central Japan anomaly of which complex nature so far rejected theoretical interpretations: Central Japan anomaly is caused by the superposition of peninsula effect and undulation of high conductive layer under the Philippine Sea coastal area of southwestern Honshu where heat flow is higher in the sea than in the land. Unlike northeastern Honshu, southwestern Honshu is not a typical island arc.

The undulation of an electrically conductive layer beneath the islands of Japan

Geomagnetic variation anomalies in Japan, for which a physically plausible explanation has so far been difficult, are approximately accounted for by assuming an undulation in the conducting layer of the mantle. The undulation seems to be correlated with that of a high-temperature isotherm, perhaps 1,200°C. The surface of the conducting layer is shallow in the Japan Sea and off the Pacific coast of western Japan, while it is as deep as 200 km or more beneath the Pacific coast of northeastern Japan. Extreme anomalies which could not previously be explained are likely to be caused by island and peninsula effects on short-period geomagnetic variations.