Journal of Geophysics
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Dispersed shots at optimum depth - an efficient seismic source for lithospheric studies
Get PDFThe case for dispersing charges fired at sea into a number of packages fired simultaneously at optimum depth is outlined and an experiment, carried out to check that linear addition of the signals from separate sources does occur, is described. The advantages of this system for lithospheric studies are demonstrated.
ARK: https://n2t.net/ark:/88439/y018705
Permalink: https://geophysicsjournal.com/article/97
 
Crustal structure of the Aegean Sea and the Hellenides obtained from geophysical surveys
Get PDFIn the period 1971 to 1974 the territory of Greece has been geophysically investigated by Greek and German Institutions. Gravity and magnetic stations have been distributed at a spacing of one station per 25 km2. Five deep seismic sounding cross-sections have been fired and recorded along the lines: Ionian Sea — Peloponnese, Amorgos — Mikonos — Evia, Crete: East-West, Cretan Sea: North-South, Cretan Sea: East-West. The results of the seismic programms revealed a pure continental structure of variable thickness. The most attenuated area is that of the Cretan Sea, with only 22 km depth to the Moho-Discontinuity at the Bouguer gravity maximum of +175 mgal. The very unevenly distributed sedimentary cover of the Cretan Sea is composed mainly of Neogene Sediments with thickness of 3–3,5 km in local basins (Jonsma et al., 1975). The Greek mainland along the Pindos Chains has minimum Bouguer anomalies of –120 to –140 mgal and Moho-Depths between 42–46 km. The Aegean Area builds a large dome and incorporates also a large part of the Taurides, Western Turkey (Makris, 1975).
ARK: https://n2t.net/ark:/88439/y049082
Permalink: https://geophysicsjournal.com/article/153
 
Statistical analysis of the pitch angle distribution of magnetospheric Solar protons during geomagnetic activity
No full textoai:ojs3.journal.geophysicsjournal.com:article/1The pitch angle distributions of energetic Solar protons in the pseudo-trapping region were used in order to examine magnetospheric processes. Temporal and spatial variations of these processes should give insight into the magnetospheric dynamics. Data from two instruments (viewing angle perpendicular and 45° with respect to the local geomagnetic field) on board the low altitude polar orbiting satellite GRS/Azur give a rough measure of the pitch angle distribution of energetic Solar protons (1.5<E<2.7 MeV) over the polar cap. For several Solar particle events the ratios of the counting rates of the two instruments were ordered according to invariant latitude, magnetic local time and geomagnetic activity. The geomagnetic activity was expressed by the time phase relative to the expansion of isolated substorms and the H-component of the magnetic field at a near midnight auroral station. Thus the analysis shows substorm-related effects on energetic particles in the pseudo-trapping region.
ARK: https://n2t.net/ark:/88439/y019155
Permalink: https://geophysicsjournal.com/article/1
 
Magnetic techniques for ascertaining the nature of iron oxide grains in basalts
Get PDFQualitative techniques are described by which the domain nature and magnetite-maghemite oxidation state of the iron oxides in basalts may be rapidly identified through measurements of hysteresis and susceptibility over a wide temperature range. Detailed studies made on different suites of basalts have revealed that their magnetic properties in most cases can be explained only on the basis of single-domain behaviour. Also it has been found that the observed variations in these properties between basalts are usually best explained by differences in the position the iron oxide minerals in the different samples occupy along the magnetite-maghemite oxidation chain. These observations suggest that the role of titanium usually found in association with iron oxides in basalts is to subdivide the grains physically rather than to form solid solutions of titanomagnetites or titanomaghemites. Some implications of these results to basalt formation and the magnetic anomalies such rocks could cause are discussed.
ARK: https://n2t.net/ark:/88439/y055011
Permalink: https://geophysicsjournal.com/article/111
 
Thermal enhancement of magnetic susceptibility
Get PDFThe interpretation of geomagnetic anomalies with deep-seated sources sometimes requires postulating magnetic susceptibilities larger than those measured for common rock types at the Earth's surface. A possible explanation is that rocks buried at depths approaching the Curie point isotherm exhibit enhanced susceptibility due to the Hopkinson effect. In measurements on a sample of single-domain magnetite (0.04 μm particles), the susceptibility increased by a factor 2 between 20 and 500 °C and by a factor 3 at 550 °C. The Hopkinson peak was less pronounced in multidomain magnetites: the relative increase in susceptibility at 550 °C was by a factor 2 in 0.1 μm particles and a factor 1.5 in 0.25 μm particles. Single-domain hematite (0.1–1 μm) gave a spectacular Hopkinson peak, with relative susceptibility enhancement by a factor 5 at 530 °C and a factor 20 at 640 °C. However, rocks containing fine-grained maghemite and magnetite showed an enhancement of 50–70 % at most. The reasons for this variability in the height of the Hopkinson peak are not understood, but the width and shape of the peak are clearly related to the blocking temperature spectrum. Distributed blocking temperatures are associated with a broad peak, while discrete blocking temperatures are accompanied by a sharp susceptibility peak within 50-100 °C of the Curie point. A corollary is that remanent magnetization decreases roughly in inverse proportion to increase in susceptibility, so that the Koenigsberger Qn ratio decreases sharply at high temperature. For this reason, deep-seated anomalies can almost certainly be interpreted in terms of induced magnetization only. Finally, somewhat shallower bodies (temperatures of 200–400 °C) may exhibit thermally enhanced magnetization for two reasons: first, titanomagnetites have widely varying Curie points depending on titanium content, and second, observed anomalies are the result of a geomagnetic field applied over 106 years and viscous magnetization is also known to be enhanced at high temperature.
ARK: https://n2t.net/ark:/88439/y022154
Permalink: https://geophysicsjournal.com/article/162
 
Long-wavelength aeromagnetic anomalies and deep crustal magnetization in Manitoba and Northwestern Ontario, Canada
Get PDFA new type of aeromagnetic anomaly map (a long-wavelength anomaly map with anomaly widths in the range 60 km < λ < 4000 km) is presented for the area. It is believed that this group of anomalies represents a physically distinct field. This field shows considerable correlation with the broad features of deep crustal structure as derived from seismic sounding; linear relationships were found between the field and depths to the bottom of the crust and to the boundary between the upper and the lower crustal layers, as well as to the thickness of the lower crustal layer. A theoretical relationship connecting structure on magnetized layers to magnetic anomalies is given showing that the linear relationships are to be expected. It is shown that the lower crustal layer is the most likely source of the anomalies, with an intensity of magnetization of 5.3 X 10-3 emu/cc. It is also indicated that the upper crustal layer could also be the source, but that a shallow plate of magnetization could not explain the anomalies. Thus deep crustal magnetization must (on all interpretations made in the present paper), be responsible for the long-wavelength anomalies. Also, these anomalies are strongly related to major features in surface geology.
ARK: https://n2t.net/ark:/88439/y012135
Permalink: https://geophysicsjournal.com/article/171
 
Heat flow measurements in Northern Italy and heat flow maps of Europe
Get PDF26 heat flow values were determined in the northern Italian lakes. The mean is calculated to be 1.59 μcal cm-2s-1 and after subtraction of the effect of uplift and denudation of the Alps 1.07 μcal cm-2s-1. With regard to the results of Austria and Hungary a rise in the mean values is observed from west to east. Heat flow maps have been constructed performing a trend analysis of all available European heat flow values. The Rhinegraben and lvrea anomaly having a "local" character are not recognizable on the maps. Simple transient models for the Ivrea anomaly and the two Mediterranean anomalies have been calculated to explain the present surface flow distribution. The existence of a low velocity layer in the crust west of the Ivrea body is not in disagreement with the measured heat flow values. This does not hold for the lvrea region itself if correction of the heat flow values for uplift and denudation is being made. However, it is perceivable that a heat wave caused by the possibly anomalous hot low velocity layer has not yet reached the Earth surface or can not be detected due to the inaccuracy of the measurements.
ARK: https://n2t.net/ark:/88439/y051571
Permalink: https://geophysicsjournal.com/article/278
 
Electromagnetic reflections in salt deposits
Get PDFThe generation of short high frequency electromagnetic pulses in the VHF-band opens up the possibility of applying electromagnetic reflection methods in the geophysical prospecting of dry formations, with high resolution and also over small distances. A method has been developed using a spark transmitter for locating boundaries in salt deposits The transmitter, which is technically very simple in contrast to other pulse transmitters, produces short powerful pulses. The results to date show good quality reflections of salt clay and anhydrite.
ARK: https://n2t.net/ark:/88439/y082408
Permalink: https://geophysicsjournal.com/article/188
 
Crustal structure under the Ionian Sea
Get PDFThe presented crustal section under the Ionian Sea is the result of refractional seismic work on two cruises of the research ship "Meteor" in 1969 und 1971 under the coordination of Prof. Closs, and of three seismic stations on the Peloponnes near the coast in 1971. In that year 25 seismic land stations and four sono buoy sea stations have recorded explosions at sea on a profile line which crossed the Mediterranean Ridge, the Hellenic Trench, and the Peloponnes, and ended in the East near Agina.
ARK: https://n2t.net/ark:/88439/y014115
Permalink: https://geophysicsjournal.com/article/191
 
Palaeomagnetism of tertiary volcanic rocks from the Ethiopian southern plateau and the Danakil block
Get PDFRemanent magnetization of 92 samples (22 sites) collected from Oligocene basalts from the Ethiopian southern plateau (location 9.1°N, 41°E) yields a palaeomagnetic pole at 75.1°N, 170.3°E with A95 = 6.4° after AF-cleaning. This pole agrees with the Eocene-Oligocene pole for the western plateau. Palaeomagnetic data of 125 samples (26 sites) from Early Pliocene basaltic rocks from the Danakil block (12.7°N, 42.5°E) yield a pole at 79.5°N, 258.8°E with A95 = 2.6°. The deviation of this pole position from other Miocene Pliocene poles for Africa is consistent with the hypothesis of 10° counter-clockwise rotation of the Danakil block since the Early Pliocene. On theoretical grounds however this amount of deviation may also be caused by not having completely averaged out secular variation due to insufficient sampling.
ARK: https://n2t.net/ark:/88439/y042442
Permalink: https://geophysicsjournal.com/article/277