Ground Magnetic Measurements Research Articles

Geophysical evidence of deep hydrocarbon flow in Mottled Zone areas, Dead Sea Transform zone

The origin of unusual magnetic initially sedimentary rocks of the Mottled Zone (MZ) in Israel and Jordan remained enigmatic for several decades until integrated characterization of the MZ area was achieved by ground magnetic measurements and geologic observations along representative profiles in parallel with reprocessing and reinterpretation of available aeromagnetic and gravity data. Micromagnetic profiling was combined with gamma-radioactivity measurements, and representative samples were selected to determine rock physical properties. Results of field measurements, reduction and transformation of geophysical fields, anomaly inversion, and forward modeling accompanied by geologic analysis suggest that two types of magnetic anomalies and local gravity minima in the MZ areas are related to the same event, i.e., deep hydrocarbon flow associated with fossil mud volcanism. Physicochemical interaction of deep hydrocarbon flow and surrounding sedimentary rocks caused widespread weak magnetization and corresponding aeromagnetic anomalies. Other scattered heterogeneous magnetization and linked ground magnetic anomalies are common for surface/near-surface local sources and are caused by the burning of combustible gases ejected by mud volcanoes; such origin of the magnetization is confirmed by magnetic measurements of burned rocks in mud volcano areas of the Caucasus. Increased radioactivity of the lower part of the MZ likely indicates mud ejection from a deep uranium-enriched source. Locations of MZ outcrops commonly coincide with residual gravity minima, showing zones of disintegration and possible hydrocarbon accumulations. Seismic prospecting data support the existence of disintegration zones at depth. Results show the need for additional geophysical studies and a potential for revealing hydrocarbon accumulations in the MZ areas, particularly at the Halamish, Nevatim, Ma’ale Adumim, and Nabi Musa sites.

The structure of the ophiolitic beltin Albania inferred from geomagnetic anomalies

The ground magnetic measurements in Albania were used for the compilation of the Total Magnetic Field (TMF) anomaly map of Albania. The magnetic data were processed and interpreted in order to study the structure of the ophiolitic belts of Albania. The ophiolites of Albania are placed at the Mirdita zone and are divided into two parallel alignments which are called the eastern and western ophiolitic belts. They are associated with strong potential field anomalies and their characteristics are considered crucial for a better understanding of the tectonic settings of Albania. The ground TMF data used in this study were acquired over various campaigns (1990-1994) and cover most of Albania's territory. The data were compiled to a map after reduction to the epoch 1990.4. The strongest magnetic anomalies in Albania appear along the known ophiolitic belts which trend NE-SW to the north and NW-SE to the south. Several processing steps were applied to the unified and gridded data in order to obtain information on the distribution of the magnetic sources. The magnetic sources were subsequently modeled using a 2.5D inversion technique. The magnetic properties of the ophiolites determined from laboratory measurements on rock samples, while their lateral extent was calculated from the processing of the magnetic data and used as constraints to the inversion procedure. The bottom of the ophiolitic belts is considered to be predominated by harzburgites. They exhibit lower magnetization than other rocks of the complex, i.e. gabbros, basalt. In fact gabbros are associated with the observed high frequency magnetic anomalies. The modeling results indicate that the thickness of the ophiolites is reduced from east to west. The eastern ophiolitic belt has a maximum thickness of approximately 12 km at its northern section (Kukesi and Lura massifs). The magnetic sources appear with relatively small thickness at the western ophiolitic belt. Boundaries of the eastern ophiolites are characterized by vertical contacts.

The structure of low-latitude Pc3 pulsations observed by CHAMP and on the ground

Abstract. The structure of low-latitude continuous pulsations termed Pc3, which are naturally occurring MHD waves in the Earth's magnetosphere, were studied by comparing ground and satellite magnetic field measurements. Data from two induction magnetometers, located at Hermanus and Sutherland in South Africa were used in conjunction with Challenging Minisatellite Payload (CHAMP) satellite observations to study a Pc3 event observed on 15 February 2003, at a time when CHAMP was passing over the ground stations. We observed a number of discrete frequency oscillations for the fast mode wave, one of which drives a field line resonance (FLR) at characteristic latitude as detected by both ground and satellite measurements. Consequently, our observations confirmed the compressional wave as being the driver of the field line resonance. The toroidal mode frequency observed on CHAMP experienced a Doppler frequency shift due to the rapid motion across the resonance region. Polarization hodograms in the resonance region clearly showed the expected 90° rotation of the field line resonant magnetic field components.

Remanent and induced magnetic anomalies over a layered intrusion: Effects from crystal fractionation and magma recharge

The Bjerkreim-Sokndal (BKS) norite – quartz mangerite layered intrusion is part of the early Neoproterozoic Rogaland Anorthosite Province intruded into the Fennoscandian shield in south Norway at ~ 930 Ma. The BKS is exposed over an area of 230 km 2 with a thickness of ~ 7000 m and is of economic interest for ilmenite, magnetite and apatite deposits. From the point of view of magnetic minerals, in the course of fractional crystallization and magma evolution, the ilmenite becomes less Fe 3+-rich reflected by a change from ilmenite with hematite exsolution to nearly pure ilmenite. Magnetite starts to crystallize relatively late in the intrusive history, but its crystallization is interrupted by influxes of more primitive magma. The variations in aeromagnetic and ground-magnetic anomalies measured over the BKS can be explained in terms of the measured magnetic properties of NRM, susceptibility, and hysteresis presented here, and in terms of mineralogy. Early layers in the intrusion contain hemo-ilmenite. As the magma evolved and magnetite started to crystallize, this caused a distinct change over the layering from remanence-controlled negative anomalies to induced positive anomalies. When new, more primitive magma was injected into the system, hemo-ilmenite returned as the major oxide and the resulting magnetic anomalies are again negative. The most dramatic change in the magnetic signature is in the upper part of the intrusion in MCU IVe, where magnetite became a well established cumulate phase as indicated by susceptibility, but its induced magnetization is overcome by large NRMs associated either with hemo-ilmenite, or with hemo-ilmenite and magnetite exsolved from pyroxenes. The average natural remanent magnetizations change from ~ 3 A/m in MCU IVd, to 15 A/m in MCU IVe, and back to 2 A/m in the overlying MCU IVf, producing a strong negative remanent anomaly that has been followed along strike for at least 20 km by ground-magnetic measurements. The highly varied magnetic properties of this intrusion, caused by varied magmatic crystallization of combinations of opaque minerals, illustrate some of the possibilities to be considered in evaluating crustal magnetic anomalies.

Solar wind time history contribution to the day‐of‐year variation in geomagnetic activity

The day‐of‐year (DOY) variation in the average value of a solar wind driver of geomagnetic activity has been shown to explain only a minority of the observed amplitude of DOY variation in geomagnetic activity. The proxies for solar wind driving used to show this are averages of a solar wind measurement in the same hour or 3‐hour interval as the geomagnetic activity measurement. This model of solar wind driving of geomagnetic activity does not account for the observation that the solar wind state in a given time interval can have an effect on activity that lasts many hours. In this work we show a model that includes the solar wind time history predicts a much higher DOY variation in both auroral zone and midlatitude geomagnetic activity. This model is used to estimate the solar wind contribution in two geomagnetic activity measures that exhibit a semiannual DOY variation: the am index and postmidnight ground magnetic field measurements in the auroral zone. The estimated solar wind driver contribution to the DOY variation in the am index is 75%, which is approximately twice the amount of previous estimates. Solar wind driving is estimated to explain 40–60% of the DOY variation in the magnetic field measured by auroral zone ground magnetometers in the postmidnight sector, where previous estimates were near zero.

Thermospheric nitric oxide at higher latitudes: Model calculations with auroral energy input

[1] The nitric oxide (NO) density in the lower thermosphere has been calculated by a photochemical model for NOx and compared with measured NO densities from Student Nitric Oxide Explorer (SNOE). At higher latitudes the most important contributor for NO density increases is energetic electron precipitation. The electron energy is estimated in two ways, from auroral ultraviolet (UV) and X-ray measurements obtained from Ultraviolet Imager (UVI) and Polar Ionospheric X-ray Imaging Experiment (PIXIE) on board the Polar satellite and from ground magnetometer measurements. For the time intervals when the Polar satellite was not above the northern hemisphere, a parameterization of the electron energy flux from ground magnetic measurements was used. This parameterization was based on data from the SuperMAG database compared to UVI/PIXIE derived electron energy fluxes. The negative perturbation in the northward ground magnetic component is found to be linearly related to the precipitating electron energy flux. The 4-day period studied is from 30 April (day 120) until 4 May 1998, where the onset of a geomagnetic storm occurred 2 May (day 122). The results of the comparisons show an overall larger modeled nitric oxide density at auroral latitudes than what was measured by SNOE. The largest discrepancies were for the day of the storm onset, when the background atmosphere was more distorted by Joule heating. The next day the agreement between the model and the observations was far better, which might be due to less amount of Joule heating this day.

Geomagnetic signatures during the intense geomagnetic storms of 29 October and 20 November 2003

Solar cycle 23 in its declining phase witnessed the most pronounced space weather events during October–November 2003. A series of powerful solar flares and associated geoeffective Coronal Mass Ejections (CMEs) travelling at ∼2000 km/s drove shock fronts that impacted the Earth's magnetic field consecutively on 29 and 30 October, resulting in intense geomagnetic disturbances during 29–31 October. Another intense geomagnetic storm activity occurred during 20–21 November, resulting from a solar flare that had an associated geoeffective CME travelling at a speed of ∼1100 km/s. Digital ground magnetic field measurements from the equatorial and low-latitude locations in the Indian longitude zone, in conjuction with the interplanetary solar wind and magnetic filed parameters, are used to study the characteristics of these storms. Maximum magnitude of the total magnetospheric energy injected into the magnetosphere during the mainphase of the three major storm amounts to approximately 4.5×10 13, 3.6×10 13, 2.8×10 13 W. Another salient feature brought out is the close correspondence between the magnitude of the peak of the southward component of the interplanetary magnetic field B z and the strength of the storm intensity as inferred from the D st and low-latitude digital magnetic records.

Magnetic mapping, ground penetrating radar surveys and magnetic susceptibility measurements for the study of the archaeological site of Serra di Vaglio (southern Italy)

In this work we present the results of a multidisciplinary study for characterising the archaeological site of Serra di Vaglio (Basilicata Region, Southern Italy). The site includes an ancient native settlement, arisen between the 8th and the 3rd century BC. The investigation has been performed by means of the integrated use of three different high resolution and no invasive geophysical techniques: magnetic mapping, Ground Penetrating Radar profiling (GPR) and magnetic susceptibility measurements. The integrated approach allows us to detect buried archaeological structures. In particular, our results helped to define spatial pattern of the buried remains, to define the geometry of the anthropogenic settlements and to obtain detailed information about the composition and the manufacturing processes of different building materials. The archaeological excavations, carried out in one of the investigated sites, confirm the usefulness and the reliability of the integrated employment of different geophysical techniques for the archaeological study of ancient buried structures.

Evaluation of the tail current contribution to Dst

The Dst index is produced using low‐latitude ground magnetic field measurements and frequently is used as an estimate of the energy density of the ring current carried mainly by energetic (∼ 10 – 200 keV) ions relatively close to the Earth. However, other magnetospheric current systems can cause field perturbations at the Earth's surface: for example, dayside magnetopause currents are known to contribute to the Dst index. It has also been suggested that the nightside tail current sheet can significantly affect the Dst index during high magnetic activity periods when the currents are intense and flow relatively close to the Earth. In this study, several disturbed periods are input into Tsyganenko magnetic field models. From the time series of the external and internal fields an artificial Dst index is computed using the same procedure followed in the actual Dst calculation. A tail region in the magnetosphere is explicitly defined and the T96 and T89 models are used to calculate the effect of current within this tail region on ground measurements and therefore on Dst. The results are then compared with the measured Dst to determine the tail current contribution to Dst. It is found that for a geomagnetic storm and a storm‐time substorm with Dst of ∼ 80 nT the tail current contribution is between 22 and 26 nT. The same analysis is also applied to several isolated non‐storm‐time substorms, yielding a nearly linear relationship between Dst and the tail current contribution. This contribution is approximately one quarter of Dst.

Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities

Abstract. Giant pulsations are nearly monochromatic ULF-pulsations of the Earth's magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30-90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = - 21 ± 4, is in accord with the value m = - 27 ± 6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f / ∂W > 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ∂f / ∂W > 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed.Key words. Magnetospheric physics (energetic particles , trapped; MHD waves and instabilities) · Space plasma physics (wave-particle interactions).

Concerning the generation of geomagnetic giant pulsations by drift-bounce resonance ring current instabilities

Giant pulsations are nearly monochromatic ULF-pulsations of the Earth's magnetic field with periods of about 100 s and amplitudes of up to 40 nT. For one such event ground-magnetic observations as well as simultaneous GEOS-2 magnetic and electric field data and proton flux measurements made in the geostationary orbit have been analysed. The observations of the electromagnetic field indicate the excitation of an odd-mode type fundamental field line oscillation. A clear correlation between variations of the proton flux in the energy range 30-90 keV with the giant pulsation event observed at the ground is found. Furthermore, the proton phase space density exhibits a bump-on-the-tail signature at about 60 keV. Assuming a drift-bounce resonance instability as a possible generation mechanism, the azimuthal wave number of the pulsation wave field may be determined using a generalized resonance condition. The value determined in this way, m = - 21 ± 4, is in accord with the value m = - 27 ± 6 determined from ground-magnetic measurements. A more detailed examination of the observed ring current plasma distribution function f shows that odd-mode type eigenoscillations are expected for the case ∂f / ∂W > 0, much as observed. This result is different from previous theoretical studies as we not only consider local gradients of the distribution function in real space, but also in velocity space. It is therefore concluded that the observed giant pulsation is the result of a drift-bounce resonance instability of the ring current plasma coupling to an odd-mode fundamental standing wave. The generation of the bump-on-the-tail distribution causing ∂f / ∂W > 0 can be explained due to velocity dispersion of protons injected into the ring current. Both this velocity dispersion and the necessary substorm activity causing the injection of protons into the nightside magnetosphere are observed.Key words. Magnetospheric physics (energetic particles , trapped; MHD waves and instabilities) · Space plasma physics (wave-particle interactions).

Global convection electric field and current: Comparisons between model's predictions and data from STARE, Saint‐Santin, and magnetometers

Ionospheric electric field and ground magnetic field measurements at high, middle and low latitudes are used to study the global pattern of the convection electric field on March 26, 1979. The IMF Bz component was southward and relatively steady over a prolonged period of time. The data set is thus representative of a steady magnetospheric convection. A semi‐analytical time‐dependent convection model is used to interpret the observations. During the daytime the model steady state results roughly reproduces the mean observed convection pattern. Fluctuations of the observed electric field from the model predictions are interpreted as resulting from (1) the transient responses of the ionosphere to small time variations of the IMF, (2) substorm effects, (3) neutral wind disturbances. Only the first type of fluctuations can be reproduced by the initial state prediction of the convection model.

Relationship between ionospheric electric field and ground magnetic pulsations in the PC 3 domain at midlatitude

At middle latitudes, measurements of electric field fluctuations in the F region, in the Pc 3 frequency range is difficult. In France, with the incoherent scatter velocity measurements, three experiments have been performed. Technical performances and difficulties are described. Specific signal processing methods are presented, and pulsations in the ionosphere are described in time and in frequency form. They are identified by comparison with ground magnetic measurements through the use of autospectrum and coherency coefficient estimations. In the ionosphere we have found few events with amplitude waves higher than the detection level. When there was correlation between ionosphere and ground it was possible to infer the impedance of the ionosphere plus ground for the transfer between the electric field in the F region and the magnetic field on the ground. The deduced value of this impedance is approximatively 10 Ω. According to Hughes and Southwood's theory this means that the signals have a very short horizontal spatial scale.

General overview of geophysical studies at Cerro Prieto

Geophysical investigations by the CFE in the Mexicali Valley near the Cerro Prieto volcano began nearly 20 years ago. Initially, gravity and seismic refraction methods were used for structural information related to faults and basement configuration. Geophysical emphasis shifted to d.c. resistivity surveys in the early 1970's after it was determined that a large area of low resistivity coincided with the thermal manifestations south of the volcano and the known high temperature zone at depth. Supplemented by ground magnetic and gravity measurements, the resistivity data are being interpreted to yield a detailed picture of the structure concealed by valley fill and to identify promising areas for future exploration. Under the DOE/CFE agreement, the LBL has been conducting various types of electrical, passive seismic and precision gravity surveys over the field. This work is less concerned with geothermal exploration than the CFE geophysical program. It involves applying exploration methods to reservoir-related questions; namely, can the reservoir area be delineated and can changes in reservoir characteristics be detected and monitored by means of surface measurements? As the second question involves a long time scale, repeated measurements and correlations to production data, only baseline information is presently being obtained. Dipole-dipole resistivity, magnetotelluric (MT) and self-potential (SP) surveys have been completed and results will be discussed. The large dipolar SP anomaly found over the field has encouraged further use of the method in the Mexicali Valley. Careful interpretation of dipole-dipole and Schlumberger data set yields a subsurface resistivity model which provides a surprising amount of useful detail. Permanent current electrodes have been emplaced and repeat measurements will be made at later times to determine whether changes in resistivity occurred within the reservoir area. Reference magnetic MT technique, developed recently at U.C. Berkeley, was applied at Cerro Prieto. Earlier tests have shown that in the presence of electro-magnetic noise, such as cultural noise, this technique yields more accurate subsurface information than conventional MT. Regional seismicity has been studied for many years. Recently, the CICESE has maintained a four station network in the Northern Mexicali Valley. The LBL passive seismic results show epicenters within the reservoir boundary associated with right-lateral strike-slip movement. V P/V S data from these events indicate a Poisson's ratio of 0.4 for the upper 2–3 km of the field. Although limited, P-wave velocity and attenuation data imply a local low near the western edge of the field superimposed upon a slight high for the entire production region. Beginning in the fall of 1978, continuous monitoring by the LBL and CICESE will provide more complete data on field characteristics. The first phase of precision gravity surveys at 60 monuments is nearly complete. Repeated gravity measurements coupled with repeated first-order leveling by the CFE will be needed to distinguish subsidence or tectonic induced gravity changes from the small changes (80–100 μgal) that might be associated with mass depletion due to fluid withdrawal. A five-station seismic array is being installed in shallow holes so that the long-term changes in seismic activity and relationship to subsidence and geothermal fluid production can be studied.

General structure of the Skye Tertiary igneous complex and detailed structure of the Beinn an Dubhaich Granite from magnetic evidence

Synopsis On the Isle of Skye, a series of Tertiary igneous rocks, including a central layered gabbro and associated granites occurs in an area of basement gneiss overlain by sediments of Cambrian to Cretaceous age. Two-dimensional computer modelling of aeromagnetic data (this paper) agrees reasonably well with the three-dimensional gravity model of Bott and Tuson (1973). Both models indicate that the gabbro pluton is a large, slightly cone-shaped mass, with a width of approximately 12 km at the top and 18 km at the base, penetrating to a depth of about 14 km, and that the granites are a shallow, near-surface phenomenon (less than 2.5 km deep), comprising about five to ten per cent of the total mass of the igneous assemblage. The Beinn an Dubhaich granite is approximately 0.8 km wide and 3.2 km long in outcrop and contains numerous patches of limestone. Ground magnetic measurements and the relationship between the granite-limestone contact and the topography indicates that the granite is a stock which extends to depth and underlies the limestone. Further, the granite surface beneath the limestone is not smooth but rather consists of many lobes which intrude the limestone, possibly along pre-existing fractures.

SOME APPLICATIONS OF IP‐TECHNIQUE FOR DIFFERENT GEOPHYSICAL PROSPECTING PURPOSES *

AbstractThe usefulness of IP‐technique for different geophysical purposes is discussed on the basis of field examples. IP and resistivity results are not always correlated, and the IP‐information may be unique in prospecting for very low grade sulphide mineralizations. The IP‐technique seems to be useful sometimes in prospecting for disseminated Ni‐Cu‐ mineralizations. A massive sulphide mineralization could be localized using IP.The disturbances from different kinds of geological objects other than prospecting targets generally are smaller in IP‐work than in resistivity work. It was possible to discriminate between apparent resistivity anomalies caused by shear zones and by sulphide mineralizations by means of IP‐measurements. The influence of magnetite on IP‐ measurements is briefly discussed. The relation between ground magnetic measurements ΔZ and the IP‐parameter is in many areas weak and does not seriously influence the interpretation of the IP‐measurements. A high correlation between ΔZ and IP‐measurements has been found on a titaniferous iron ore. This ore body gives strong IP‐anomalies indicating that it is a good IP‐target. Finally, the application of small moving arrays in very detailed IP‐measurements is found useful for detailed mapping of a mineralized zone under a thin overburden and for determination of the dip of mineralized zones.

Magnetic features of fracture zones

During interpretation of low-altitude aeromagnetic and ground magnetic measurements linear magnetic minima are frequently observed. They are often attributed to linear fracture zones. Detailed petrophysical studies have been made on an outcropping fracture zone showing a negative magnetic anomaly. The magnetic susceptibility has been measured in situ and the variable content of opaque minerals has been studied in polished sections. It was found that the observed linear magnetic minima are caused by a combination of a fault graben and the oxidation of magnetite to hematite. The latter effect is dominating and the alteration process (martitization) is shown on a susceptibility—Q-value diagram.