Strong Magnetic Anomalies Research Articles

Chinook salmon and green sturgeon migrate through San Francisco Estuary despite large distortions in the local magnetic field produced by bridges.

Empirical evidence exists that some marine animals perceive and orient to local distortions in the earth’s main static geomagnetic field. The magnetic fields produced by undersea electric power cables that carry electricity from hydrokinetic energy sources to shore-based power stations may produce similar local distortions in the earth’s main field. Concerns exist that animals migrating along the continental shelves might orient to the magnetic field from the cables, and move either inshore or offshore away from their normal path. We have studied the effect of the Trans Bay Cable (TBC), an 85-km long, high voltage, direct current (DC) transmission line leading underwater from Pittsburg, CA to San Francisco, CA, on fishes migrating through the San Francisco Estuary. These included Chinook salmon smolts (Oncorhynchus tshawytscha) that migrate downstream through the San Francisco Estuary to the Pacific Ocean and adult green sturgeon (Acipenser medirostris), which migrate upstream from the ocean through the estuary to their spawning habitat in the upper Sacramento River and return to the ocean after spawning occurs. Based on a detailed gradiometer survey, we found that the distortions in the earth’s main field produced by bridges across the estuary were much greater than those from the Trans Bay Cable. The former anomalies exceeded the latter by an order of magnitude or more. Significant numbers of tagged Chinook salmon smolts migrated past bridges, which produced strong magnetic anomalies, to the Golden Gate Bridge, where they were recorded by dual arrays of acoustic tag-detecting monitors moored in lines across the mouth of the bay. In addition, adult green sturgeon successfully swam upstream and downstream through the estuary on the way to and from their spawning grounds. Hence, the large anomalies produced by the bridges do not appear to present a strong barrier to the natural seasonal movement patterns of salmonid smolts and adult green sturgeon.

Effect of magnetic anisotropy on the natural remanent magnetization in the MCU IVe' layer of the Bjerkreim Sokndal Layered Intrusion, Rogaland, Southern Norway

AbstractA strong negative magnetic anomaly, caused by an intense natural remanent magnetization (NRM) approximately opposite today's geomagnetic field, is observed above the MCU IVe' unit of the Bjerkreim Sokndal Layered Intrusion. The anomaly is strongest in the east, close to Heskestad, and decreases when following the layer toward the north and west. This study investigates how the NRM changes along the layer and how its direction and intensity are affected by magnetic fabrics in the intrusion. NRM, low‐field anisotropy of magnetic susceptibility, and anisotropy of anhysteretic remanence have been measured on 371 specimens from 46 sites. The orientation of both the magnetic fabrics and the NRM changes for different locations along the layer, and it appears that the NRM is tilted away from the mean paleofield and toward the direction of maximum susceptibility and maximum anhysteretic remanence. When NRM directions are corrected for magnetic fabrics, the angle between the NRM and mean paleofield direction generally decreases for specimens with a single‐component NRM. No correlation was found between the NRM intensity and the directional relationship between NRM, magnetic fabric, and mean paleofield.

Magnetic exploration of a low-temperature ultramafic-hosted hydrothermal site (Lost City, 30°N, MAR)

A 2003 high-resolution magnetic survey conducted by the Autonomous Underwater Vehicle ABE over the low-temperature, ultramafic-hosted hydrothermal field Lost City reveals a weak positive magnetic anomaly. This observation is in direct contrast to recent observations of strong positive magnetic anomalies documented over the high-temperature ultramafic-hosted hydrothermal vents fields Rainbow and Ashadze, which indicates that temperature may control the production of magnetization at these sites. The Lost City survey provides a unique opportunity to study a field that is, to date, one of a kind, and is an end member of ultramafic-hosted hydrothermal systems. Our results highlight the key contribution of temperature on magnetite production resulting from serpentinization reactions. Whereas high temperature promotes significant production and partitioning of iron into magnetite, low temperature favors iron partitioning into various alteration phases, resulting in a magnetite-poor rock. Moreover, the distribution of magnetic anomalies confirms results of a previous geological survey indicating the progressive migration of hydrothermal activity upslope. These discoveries contribute to the results of 25 yrs of magnetic exploration of a wide range of hydrothermal sites, from low- to high-temperature and from basalt- to ultramafic-hosted, and thereby validate using high-resolution magnetics as a crucial parameter for locating and characterizing hydrothermal sites hosting unique chemosynthetic-based ecosystems and potentially mineral-rich deposits.

Mars MOURA magnetometer demonstration for high-resolution mapping on terrestrial analogues

Abstract. Satellite-based magnetic measurements of Mars indicate complex and very strong magnetic anomalies, which led to an intensive and long-lasting discussion about their possible origin. To make some progress in the investigation of the origin of these anomalies the MOURA vector magnetometer was developed for in situ measurements on Mars. In this work we propose the utilisation of such an instrument for future planetary on-ground surveys. The proof of its suitability is seen through testing it on various terrestrial analogues characterised by the distinct magnetic anomalies of their basement rocks: (1) a magnetite body of EL Laco (up to +110 000 nT) and its transition to surrounding andesites ( < +2000 nT) in the northern Andes of Chile showing the highest local magnetic anomalies. The magnetite-bearing ore body has highly variable local anomalies due to its complex formation history where a significant dispersion in palaeo-orientations has been previously reported, while our vector data show relatively uniform and probably induced declinations. (2) A basaltic spatter cone of the Pali Aike volcanic field, in southern Chile, was characterised by very strong magnetic anomalies along the crater rim (up to +12 000 nT), controlled by the amount of single domain magnetites in the ground mass of the basalts. Due to their strong remanent signature, palaeo-declinations of the lavas and reorientations of collapsed blocks could be constrained by the vector data. (3) The Monturaqui meteorite crater (350 m diameter), in northern Chile, shows significant variations of its anomalies (from −2000 to > +6000 nT) in restricted areas of several square metres along its crater rim related to unexposed iron-bearing fragments of the impactor while its granitic and ignimbritic target rocks exhibit only very weak anomalies. (4) An area with several amphibolitic dykes, which cross-cut a Cretaceous granitoid in the southernmost Andes, where a decimetre-scale mapping was performed. In this case, pyrrhotite is the only magnetic carrier. It was formed during hydrothermal processes within the dykes. Very low (+40 to +120 nT) positive magnetic anomalies clearly depict the amount of 1–4 vol % pyrrhotite in these dykes, which is important as a mineralogical indicator as well as to detect associated gold and copper enrichment.

Seismic Sensor Misorientation Measurement UsingP‐Wave Particle Motion: An Application to the NECsaids Array

Seismic sensor orientation is one of the most critical parameters for modern three‐component seismological observation. However, this parameter is easily subject to error imposed by strong magnetic anomalies near the station or by human error in declination calibration. It is therefore very important to inspect and correct for sensor misorientation before utilizing three‐component waveform data. In this study, we measured the epoch‐dependent sensor misorientation for our temporary seismic array (NorthEast China Seismic Array to Investigate Deep Subduction, or NECsaids) by analyzing P ‐wave particle motions. We applied principal component analysis and the minimizing transverse energy method to study earthquakes with epicentral distance between 5° and 90° to estimate the sensor misorientation. Our results show high consistency with the direct gyrocompass measurements, with a correlation coefficient of 0.95. Our statistical analysis suggests that we can estimate robust sensor misorientation utilizing 10 earthquakes with high signal‐to‐noise ratio records and highly linear P ‐wave polarizations. We also find that the influence of anisotropy or a dipping interface produces a periodical pattern with back azimuth and is relatively small for our misorientation estimation. By analyzing the amplitude change of synthetic seismograms due to misorientation and taking into account the influence of anisotropy and dipping interfaces, as well as the measurement errors, we expect engineers to be able to orient seismic sensors with error smaller than 3°.

Geopotential field anomalies and regional tectonic features – two case studies: southern Africa and Germany

Abstract. Maps of magnetic and gravity field anomalies provide information about physical properties of the Earth's crust and upper mantle, helpful in understanding geological conditions and tectonic structures. Depending on data availability, whether from the ground, airborne, or from satellites, potential field anomaly maps contain information on different ranges of spatial wavelengths, roughly corresponding to sources at different depths. Focussing on magnetic data, we compare amplitudes and characteristics of anomalies from maps based on various available data and as measured at geomagnetic repeat stations. Two cases are investigated: southern Africa, characterized by geologically old cratons and strong magnetic anomalies, and the smaller region of Germany with much younger crust and weaker anomalies. Estimating lithospheric magnetic anomaly values from the ground stations' time series (repeat station crustal biases) reveals magnetospheric field contributions causing time-varying offsets of several nT in the results. Similar influences might be one source of discrepancy when merging anomaly maps from different epochs. Moreover, we take advantage of recently developed satellite potential field models and compare magnetic and gravity gradient anomalies of ∼ 200 km resolution. Density and magnetization represent independent rock properties and thus provide complementary information on compositional and structural changes. Comparing short- and long-wavelength anomalies and the correlation of rather large-scale magnetic and gravity anomalies, and relating them to known lithospheric structures, we generally find a better agreement in the southern African region than the German region. This probably indicates stronger concordance between near-surface (down to at most a few km) and deeper (several kilometres down to Curie depth) structures in the former area, which can be seen to agree with a thicker lithosphere and a lower heat flux reported in the literature for the southern African region.

Geological and geophysical characteristics of massive sulphide deposits: A case study of the Lirhanda massive sulphide deposit of Western Kenya

An integrated geophysical ground survey was conducted on an airborne electromagnetic (EM) anomaly located in Kakamega forest of Western Kenya. The purpose of the study was to establish the existence of massive sulphides and identify suitable optimal geophysical method(s) for the investigation of similar anomalies. The study was also expected to provide information on the geological and geophysical characteristics of the deposit.Field work involved electromagnetic methods: Vertical Loop (VLEM), Horizontal Loop (HLEM), TURAM EM and potential field methods: gravity and magnetics. Geochemical sampling was carried out concurrently with the geophysical survey.All the geophysical methods used yielded good responses. Several conductors conforming to the strike of the geology were identified. TURAM EM provided a higher resolution of the conductors compared to VLEM and HLEM. The conductors were found to be associated with positive gravity anomalies supporting the presence of bodies of higher density than the horst rock. Only the western section (west of 625W) of the grid is associated with strong magnetic anomalies. East of 625W strong EM and gravity anomalies persist but magnetic anomalies are weak. This may reflect variation in the mineral composition of the conductors from magnetic to non-magnetic. Geochemical data indicates strong copper anomalies (upto 300 ppm) over sections of the grid and relatively strong zinc (upto 200 ppm) and lead (upto 100 ppm) anomalies. There is a positive correlation between the location of the conductors as predicted by TURAM EM and the copper and zinc anomalies.A test drill hole proposed on the basis of the geophysical results of this study struck massive sulphides at a depth of 30m still within the weathered rock zone. Unfortunately, the drilling was stopped before the sulphides could be penetrated. The drill core revealed massive sulphide rich in pyrite and pyrrhotite.An attempt has been made to compare characteristics of the Lirhanda massive sulphide deposit with those of better documented massive sulphides. Despite the fact that very little is known about Lirhanda, there are several similarities on the characteristics compared. These include evidence of back arc regional environment, calc-alkaline volcanic associations, conformity of anomalies to the structural trend of the host rock, proximity of synvolcanic rift, dispersive anomalies of copper and zinc in soils, presence of gossan and association of the deposit with strong EM anomalies.

3D gravity modelling reveals off-axis crustal thickness variations along the western Gakkel Ridge (Arctic Ocean)

Near-orthogonal ultra-slow (13.3mmyr−1 to 6.5mmyr−1) sea floor spreading in the absence of large transform faults make the Arctic Gakkel Ridge ideally suited for the study of magmatic processes. To enable this, we generated a three-dimensional gravity model of crustal thickness over the ridge and parts of the adjacent Nansen and Amundsen basins west of 65° E. The model shows that oceanic crust accreted prior to chrons C5/C6 is generally very thin (1–3km). Magnetic anomalies over this thin crust are highly variable both parallel and perpendicular to the ridge axis. This is the result of amagmatic or weakly volcanic spreading that started with the opening of the basins 56Ma ago. The separation of Greenland from Svalbard at chron C5/C6 led to the inflow of North Atlantic mantle into the western Eurasia Basin leading to a change in the mantle convection system and the establishment of a magmatic dichotomy along the Gakkel Ridge. Robust magmatism was established in the Western Volcanic Zone (6° 30′ W–3° 30′ E), leading to creation of a 6.6km thick igneous crust, characterized by a strong positive axial magnetic anomaly, numerous volcanic cones, and widespread thick mid-ocean ridge basalts. The transition to the neighbouring Sparsely Magmatic (3° 30′ E–29° E) and Eastern Volcanic (29° E–85° E) zones is sharp. Peridotites cover the central valley and the inner rift flanks, the central magnetic anomaly vanishes and crustal thickness decreases to 1–4km. Transverse basement ridges, extending for as much as 100km into the adjacent basins, intersect the central valley. Although partly of tectonic origin, the transverse ridges are also an expression of long-living magmatic centres, as revealed by increased magnetic anomaly intensities and local thickening of the crust to values as great as 5.9km.

Residual Magnetic Anomalies Mapping from CHAMP Observations

CHAMP high-quality vector magnetometer observations collected from July 2000 to September 2010 have been used to map the residual vector magnetic anomaly fields. This field is so called the lithospheric magnetic field which is the result of two contributions of the induced and the remanent magnetization. It is therefore essential to study the magnetic properties of the crustal rocks. Isolating this field from the other contributions, interpreting and even defining are however difficult and still debated. We investigate how to identify and separate the lithospheric vector magnetic field ΔX, ΔY and ΔZ from other contributions. For this purpose we use selected night magnetic data from which we remove a model field of degree 16 and external model field of degree 2 developed by spherical harmonics analysis. Concerning the induced lithospheric field which is assumed to be aligned with the internal dipole was also removed. To minimize the secular variation effects, we calculated internal models for each two months. The method developed here has been successfully applied to isolate lithospheric field produced by remanent magnetizations from CHAMP satellite data. The resolution and altitude measurements make it very hard to map short wavelength crustal magnetic anomalies. The large-scale strong magnetic anomalies detected using this technique are in agreement with previous global magnetic maps. These anomalies appear with an amplitude of about 10 nT at satellite altitude such as Bangui’s anomaly.

Magnetic signature of North-East Greenland, the Morris Jesup Rise, the Yermak Plateau, the central Fram Strait: Constraints for the rift/drift history between Greenland and Svalbard since the Eocene

A compilation of several aeromagnetic surveys across North Greenland and the Fram Strait area is presented. Across North Greenland, the data are related to the known onshore geology. The correlation between the geological structure and the large-scale magnetic anomalies is excellent. Along the North Greenland coast, the data indicate that the Late Mesozoic Kap Washington volcanics might have a larger areal extent than known from onshore geological mapping. The submarine Morris Jesup Rise is characterized by strong positive magnetic anomalies, and the anomalies across the plateau indicate the presence of distinct volcanic centers. This massive magmatism has occurred latest in conjunction with the Oligocene initial stages of plate divergence between Svalbard and North Greenland that led to the development of Fram Strait. Both plateaux, the Morris Jesup Rise and the Yermak Plateau, are interpreted to be of continental origin and, in the initial rift stage of the Eurasia Basin around 56Ma, to have been contiguous with the Lomonosov Ridge. More toward the east in the Fram Strait interpretations of seismic and bathymetric data indicate the presence of an active segment of mid-ocean ridge, the Lena Trough. This approximately 280-km-long segment terminates in the south at the NW–SE striking Spitsbergen Fracture Zone, while in the north it bends to connect with the Gakkel Ridge. A shallow water pathway along the line of the trough might have existed between those times and ~21Ma, above the propagating rift system in the north and, further south, over extending and subsiding continental crust of the Svalbard and North-East Greenland margins. Magnetic anomaly identifications indicate that the Lena Trough has remained an ultraslow spreading system, or possibly even all of the magnetic lineations may be signals of serpentinized exhumed mantle rocks rather than of basaltic crust.

MARSIS remote sounding of localized density structures in the dayside Martian ionosphere: A study of controlling parameters

AbstractEnhanced topside electron densities in the dayside Martian ionosphere have been repetitively observed in areas of near‐radial crustal magnetic fields, for periods of tens of days, indicating their long‐term spatial and temporal stability despite changing solar wind conditions. We perform a statistical study of these density structures using the ionospheric mode of the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) onboard Mars Express. We estimate the apparent extents of these structures relative to the altitude of the surrounding ionosphere. The apex of the density structures often lies higher than the surrounding ionosphere (median vertical extent of 18 km), which indicates upwellings. These structures are much wider than they are high, with latitudinal scales of several degrees. The radar reflector regions are observed above both moderate and strong magnetic anomalies, and their precise locations and latitudinal extents match quite well with the locations and latitudinal extents of magnetic structures of given magnetic polarity (oblique to vertical fields), which happen to be regions where the field lines are open part of the time. The majority of the density structures occur in regions where ionospheric plasma is dominant, indicating closed field regions shielded from shocked solar wind plasma.

Hybrid simulation of the interaction of solar wind protons with a concentrated lunar magnetic anomaly

AbstractUsing a two‐dimensional hybrid simulation, we study the physics of the interaction of the solar wind with a localized magnetic field concentration, or “magcon,” on the Moon. Our simulation treats the solar wind protons kinetically and the electrons as a charge‐neutralizing fluid. This approach is necessary because the characteristic scale of the magcon is of the same order or smaller than the proton inertial length—the characteristic scale in the hybrid simulation. Specifically, we consider a case in which the incident solar wind flows exactly normal to the lunar surface, and the magcon is represented by a simple dipole whose moment is parallel to the surface, with a center just below it. We find that while the magcon causes the solar wind to be deflected and decelerated, it does not completely shield the lunar surface anywhere. However, protons which impact the surface in the center of the magnetic anomaly have energies well below the solar wind ram energy. Thus, in this region, any backscattered neutral particles resulting from the interaction of solar wind protons with the lunar regolith would have energies lower than that of the solar wind. Moreover, very few neutrals, if any, would emanate from within the magcon with energies comparable to the solar wind energy. This may explain recent observations of lunar energetic neutral atoms associated with a strong crustal magnetic anomaly. Our study also finds that a significant fraction of the incoming solar wind protons are reflected back into space before reaching the surface. These particles are reflected by a strong electrostatic field which results from the difference in the proton and electron inertia. The reflected particles are seen at very high altitudes above the Moon, over 200 km, and over a much broader spatial scale than the magcon, several hundred kilometers at least. Our simulation also revealed a second population of reflected particles which originate from the side of the magcon where the interplanetary and magcon magnetic fields are directed opposite to one another, leading to a magnetic topology much like magnetic reconnection. As previously reflected particles move through this region, they are deflected upward, away from the surface, forming a second component. Our simulation has a number of similarities to recent in situ spacecraft observations of reflected ions above and around magcons.

Variation of the Earth’s magnetic field strength in South America during the last two millennia: New results from historical buildings of Buenos Aires and re-evaluation of regional data

The causes of the systematic decay of the Earth’s Magnetic Field strength since eighteen century have been a matter of debate during the last decade. It is also well known that such variations may have completely different expressions under an area characterized with strong magnetic anomalies, such as the South Atlantic Magnetic Anomaly. To fully understand these atypical phenomena, it is crucial to retrieve the past evolution of Earth’s magnetic field beyond the observatory records. We report on detailed rock-magnetic and archeointensity investigations from some well-studied historical buildings of Buenos Aires city, located at the heart of the South Atlantic Magnetic Anomaly. Samples consist of bricks, tiles, fireplaces and pottery, which are considered as highly suitable materials for archaeointensity studies. The dating is ascertained by historical documents complemented by archeological constraints. Eighteen out of 26 analyzed samples yield reliable absolute intensity determinations. The site-mean archaeointensity values obtained in this study range from 28.5 to 43.5μT, with corresponding virtual axial dipole moments (VADMs) ranging from 5.3 to 8.04×1022Am2. Most determinations obtained in the present study are in remarkable agreement with the values predicted by the time varying field model CALS10k.1b (Korte et al., 2011). For the older periods the recently available SHA.DIF.14 model (Pavon-Carrasco et al., 2014) seems to have greater resolution.South American archaeointensity database now includes absolute intensities from 400 to 1930 AD based on 63 selected archaeointensity determinations. The data set reveals several distinct periods of quite large fluctuations of intensity. However, most data are concentrated into a relatively narrow interval from AD 1250 to AD 1450. At the beginning of the record, values between 400 AD and 830 AD match well with ARCH3k.1 model. Some general features may be detected: the time intervals from about AD 400 to 950 and 1150 to 1280 are characterized by a quite monotonic decrease of geomagnetic intensity, while some increase is observed from AD 950 to AD 1250. In contrast, a systematic intensity decay is detected from 1550 to 1930 in excellent agreement with the model prediction. No firm evidence of correlation between the climate changes over multi-decadal time scales and geomagnetic intensity was found for South America.

Crustal magnetic anomaly in the Ordos region and its tectonic implications

In this paper, the NGDC-720-V3 model is utilized to investigate the distribution of crustal magnetic anomaly and Curie surface in the Ordos block and its surrounding areas. The Curie surface is compared with the regional heat flow, Bouguer anomalies, Moho, and seismicity. The interior block has strong wide-distributed magnetic anomalies, well corresponding to the basement lithofacies. The surrounding areas have weak anomalies of various strikes and intensities. The magnetic anomaly confirms the geological finding that the Yabulai and Bayanhot Basins are small blocks detached from the northern Ordos block. The magnetic anomalies at the satellite altitude reveal that deep materials flow northward from the northeast margin of Tibet Plateau to the Qilian fold belt. The depth of Curie surface ranges from 24 to 42km, and the Ordos block is dominated by Curie surface depth over 32km and average heat-flow value of 44mW/m2. The distribution of Bouguer anomalies is consistent with that of magnetic anomalies. The well correlation between the Curie surface and the seismicity is also explained.

Imaging the South Pole–Aitken basin in backscattered neutral hydrogen atoms

The lunar surface is very efficient in reflecting impinging solar wind ions as energetic neutral atoms (ENAs). A global analysis of lunar hydrogen ENAs showed that on average 16% of the solar wind protons are reflected, and that the reflected fraction can range from less than 8% to more than 24%, depending on location. It is established that magnetic anomalies reduce the flux of backscattered hydrogen ENAs by screening-off a fraction of the impinging solar wind. The effects of the surface properties, such as porosity, roughness, chemical composition, and extent of weathering, were not known.In this paper, we conduct an in-depth analysis of ENA observations of the South Pole–Aitken basin to determine which of the surface properties might be responsible for the observed variation in the integral ENA flux. The South Pole–Aitken basin with its highly variable surface properties is an ideal object for such studies. It is very deep, possesses strikingly elevated concentrations in iron and thorium, has a low albedo and coincides with a cluster of strong magnetic anomalies located on the northern rim of the basin. Our analysis shows that whereas, as expected, the magnetic anomalies can account well for the observed ENA depletion at the South Pole–Aitken basin, none of the other surface properties seem to influence the ENA reflection efficiency. Therefore, the integral flux of backscattered hydrogen ENAs is mainly determined by the impinging plasma flux and ENA imaging of backscattered hydrogen captures the electrodynamics of the plasma at the surface. We cannot exclude minor effects by surface features.

An extremely high-altitude plume seen at Mars' morning terminator.

The Martian limb (that is, the observed 'edge' of the planet) represents a unique window into the complex atmospheric phenomena occurring there. Clouds of ice crystals (CO2 ice or H2O ice) have been observed numerous times by spacecraft and ground-based telescopes, showing that clouds are typically layered and always confined below an altitude of 100kilometres; suspended dust has also been detected at altitudes up to 60kilometres during major dust storms. Highly concentrated and localized patches of auroral emission controlled by magnetic field anomalies in the crust have been observed at an altitude of 130kilometres. Here we report the occurrence in March and April 2012 of two bright, extremely high-altitude plumes at the Martian terminator (the day-night boundary) at 200 to 250 kilometres or more above the surface, and thus well into the ionosphere and the exosphere. They were spotted at a longitude of about 195° west, a latitude of about -45° (at Terra Cimmeria), extended about 500 to 1,000 kilometres in both the north-south and east-west directions, and lasted for about 10days. The features exhibited day-to-day variability, and were seen at the morning terminator but not at the evening limb, which indicates rapid evolution in less than 10hours and a cyclic behaviour. We used photometric measurements to explore two possible scenarios and investigate their nature. For particles reflecting solar radiation, clouds of CO2-ice or H2O-ice particles with an effective radius of 0.1micrometres are favoured over dust. Alternatively, the plume could arise from auroral emission, of a brightness more than 1,000 times that of the Earth's aurora, over a region with a strong magnetic anomaly where aurorae have previously been detected. Importantly, both explanations defy our current understanding of Mars' upper atmosphere.

Composite Geophysical Study Comprising Gravity, Magnetic, and Resistivity Surveys to Delineate Basement Salt Deposits Near Jabbar Nala East of Kherwa Gorge, Pakistan

A composite geophysical survey has been carried out comprising gravity, magnetic, and resistivity surveys for delineation of salt deposits, thickness of overburden and basement depth over an area of 631 acres near Jabbar Nala east of Khewra Gorge, Sodi, Pakistan. The survey is carried out on the girded pattern of 100 x 100 meters; however, some variability in coverage tends to exist due to the existence of high hills topography. Present research revealed that salt deposits exist within the depth of 75~100 meters at specific locations in the extreme northeast and northwest. Most of the anomalous zones fall in this area is analogous when these three surveys are explored and comprehensive analysis performed. Existing excavated area of salt mine also exists in the northeast where the strong Bouguer gravity, magnetic, and resistivity anomalies have been superimposed, however, anomalous zone extents further in the northeast. Gravity, magnetic, modeling has been carried out along the anomalous zones comprising cross-sections of AA’, BB’ and CC’. However, for further confirmation of the results, a test bore of 90~120 meters depth needs to be drilled at the recommended anomalous zones (Zone B and Zone C).

A spherical harmonic model of the lithospheric magnetic field of Mars

AbstractWe present a model of the lithospheric magnetic field of Mars which is based on Mars Global Surveyor orbiting satellite data and represented by an expansion of spherical harmonic (SH) functions up to degree and order 110. Several techniques were applied in order to obtain a reliable and well‐resolved model of the Martian lithospheric magnetic field: A modified Huber‐Norm was used to properly treat data outliers, the mapping phase orbit data was weighted based on an a priori analysis of the data, and static external fields were treated by a joint inversion of external and internal fields. Further, temporal variabilities in the data which lead to unrealistically strong anomalies were considered as noise and handled by additionally minimizing a measure of the horizontal gradient of the vertically down internal field component at surface altitude. Here we use an iteratively reweighted least squares algorithm to approach an absolute measure (L1 norm), allowing for a better representation of strong localized magnetic anomalies as compared to the conventional least squares measure (L2 norm). The resulting model reproduces all known characteristics of the Martian lithospheric field and shows a rich level of detail. It is characterized by a low level of noise and robust when downward continued to the surface. We show how these properties can help to improve the knowledge of the Martian past and present magnetic field by investigating magnetic signatures associated with impacts and volcanoes. Additionally, we present some previously undescribed isolated anomalies, which can be used to determine paleopole positions and magnetization strengths.

What causes low magnetization at basalt-hosted hydrothermal sites? Insights from inactive site Krasnov (MAR 16°38′N)

High-resolution magnetic surveys acquired near the seafloor show that active basalt-hosted hydrothermal sites are associated with zones of lower magnetization. This observation may reflect the thermal demagnetization of a hot hydrothermal zone, the alteration of basalt affected by hydrothermal circulation, and/or the presence of thick, nonmagnetic hydrothermal deposits. In order to discriminate among these inferences, we acquired vector magnetic data 50 m above inactive hydrothermal site Krasnov using the Remotely Operated Vehicle (ROV) Victor. This deep hydrothermal site, located 7 km east of the Mid-Atlantic Ridge (MAR) axis at 16°38′N, is dissected by major normal faults and shows no evidence of recent hydrothermal activity. It is therefore a perfect target for investigating the magnetic signature of an inactive basalt-hosted hydrothermal site. Krasnov exhibits a strong negative magnetic anomaly, which implies that the lower magnetization observed at basalt-hosted hydrothermal sites is not a transient effect associated with hydrothermal activity, but remains after activity ceases. Thermal demagnetization plays only a secondary role, if any, in the observed magnetic low. Forward models suggest that both the nonmagnetic hydrothermal deposits and an altered zone of demagnetized basalt are required to account for the observed magnetic low. The permanence of this magnetic signature makes it a useful tool to explore midocean ridges and detect inactive hydrothermal sites.

Distribution of the crustal magnetic field in Sichuan-Yunnan region, southwest China.

Based on the new and higher degree geomagnetic model NGDC-720-V3, we have investigated the spatial distribution, the altitude decay characteristics of the crustal magnetic anomaly, the contributions from different wavelength bands to the anomaly, and the relationship among the anomaly, the geological structure, and the geophysical field in Sichuan-Yunnan region of China. It is noted that the most outstanding feature in this area is the strong positive magnetic anomaly in Sichuan Basin, a geologically stable block. Contrasting with this feature, a strong negative anomaly can be seen nearby in Longmen Mountain block, an active block. This contradiction implies a possible relationship between the magnetic field and the geological activity. Completely different feature in magnetic field distribution is seen in the central Yunnan block, another active region, where positive and negative anomalies distribute alternatively, showing a complex magnetic anomaly map. Some fault belts, such as the Longmen Mountain fault, Lijiang-Xiaojinhe fault, and the Red River fault, are the transitional zones of strong and weak or negative and positive anomalies. The corresponding relationship between the magnetic anomaly and the geophysical fields was confirmed.