Interpretation of magnetic anomalies using a genetic algorithm

A genetic algorithm (GA) is an artificial intelligence method used for optimization. We applied a GA to the inversion of magnetic anomalies over a thick dike. Inversion of nonlinear geophysical problems using a GA has advantages because it does not require model gradients or well-defined initial model parameters. The evolution process consists of selection, crossover, and mutation genetic operators that look for the best fit to the observed data and a solution consisting of plausible compact sources. The efficiency of a GA on both synthetic and real magnetic anomalies of dikes by estimating model parameters, such as depth to the top of the dike (H), the half-width of the dike (B), the distance from the origin to the reference point (D), the dip of the thick dike (δ), and the susceptibility contrast (k), has been shown. For the synthetic anomaly case, it has been considered for both noise-free and noisy magnetic data. In the real case, the vertical magnetic anomaly from the Pima copper mine in Arizona, USA, and the vertical magnetic anomaly in the Bayburt–Sar?han skarn zone in northeastern Turkey have been inverted and interpreted. We compared the estimated parameters with the results of conventional inversion methods used in previous studies. We can conclude that the GA method used in this study is a useful tool for evaluating magnetic anomalies for dike models.     

A New Approach for Border Detection of the Dumluca (Turkey) Iron Ore Area: Wavelet Cellular Neural Networks

Anomaly analysis is used for various geophysics applications such as determination of geophysical structure's location and border detections. Besides the classical geophysical techniques, artificial intelligence based image processing algorithms have been found attractive for geophysical anomaly analysis. Recently, cellular neural networks (CNN) have been applied to geophysical data and satisfactory results are reported. CNN provides fast and parallel computational capability for geophysical image processing applications due to its filtering structure. The behavior of CNN is defined by two template matrices that are adjusted by a properly supervised learning algorithm. After training stage for geophysical data, Bouguer anomaly maps can be processed and analyzed sequentially. In this paper, CNN learning and processing capability have been improved, combining Wavelet functions and backpropagation learning algorithms. The new architecture is denoted as Wavelet-Cellular Neural networks (Wave-CNN) and it is employed to analyze Bouguer anomaly maps which are important to extract useful information in geophysics. At first, Wave-CNN performance is tested on synthetic geophysical data, which are created by a computer environment. Then, Bouguer anomaly maps of the Dumluca iron ore field have been analyzed and results are reported in comparison to real drilling results.     

Quantitative Interpretation of Self-Potential Anomalies of Some Simple Geometric Bodies

We have developed a new numerical method to determine the shape (shape factor), depth, polarization angle, and electric dipole moment of a buried structure from residual self-potential (SP) anomalies. The method is based on defining the anomaly value at the origin and four characteristic points and their corresponding distances on the anomaly profile. The problem of shape determination from residual SP anomaly has been transformed into the problem of finding a solution to a nonlinear equation of the form q = f (q). Knowing the shape, the depth, polarization angle and the electric dipole moment are determined individually using three linear equations. Formulas have been derived for spheres and cylinders. By using all possible combinations of the four characteristic points and their corresponding distances, a procedure is developed for automated determination of the best-fit-model parameters of the buried structure from SP anomalies. The method was applied to synthetic data with 5% random errors and tested on a field example from Colorado. In both cases, the model parameters obtained by the present method, particularly the shape and depth of the buried structures are found in good agreement with the actual ones. The present method has the capability of avoiding highly noisy data points and enforcing the incorporation of points of the least random errors to enhance the interpretation results.     

A Least-squares Minimization Approach to Depth Determination from Magnetic Data

—We have developed a least-squares minimization approach to depth determination from magnetic data. By defining the anomaly value T(0) at the origin and the anomaly value T(N) at any other distance (N) on the profile, the problem of depth determination from magnetic data has been transformed into finding a solution to a nonlinear equation of the form f(z)=0. Formulas have been derived for a sphere, horizontal cylinder, dike, and for a geologic contact. Procedures are also formulated to estimate the effective magnetization intensity and the effective magnetization inclination. A scheme for analyzing the magnetic data has been formulated for determining the model parameters of the causative sources. The method is applied to synthetic data with and without random errors. Finally, the method is applied to two field examples from Canada and Arizona. In all cases examined, the estimated depths are found to be in goodagreement with actual values.     

Reflection of the solar wind parameters in the CR geomagnetic cutoff rigidity before a strong magnetic storm in November 2003

The time variations in the CR geomagnetic cutoff rigidity and their relation to the interplanetary parameters and the Dst index during a strong magnetic storm of November 18–24, 2003, have been analyzed. The Tsyganenko (Ts03) model of a strongly disturbed magnetosphere [Tsyganenko, 2002a, 2002b; Tsyganenko et al., 2003] have been used to calculate effective geomagnetic thresholds with the help of the method for tracing CR particle trajectories in the magnetospheric magnetic field. The geomagnetic thresholds have been calculated using the method of global spectrographic survey (GSS), based on the data from the global network of CR stations, and the results have been compared with the effective geomagnetic cutoff rigidities. The daily anisotropy of effective geomagnetic thresholds during the Dst variation minimum have been estimated. The relation of the theoretical and experimental geomagnetic thresholds, obtained using the GSS method, to the interplanetary parameters and Dst variation is analyzed. The Dst variations, IMF B _z , and solar wind density are most clearly defined in the geomagnetic thresholds during this storm. The correlation between B _y and experimental geomagnetic thresholds is higher than such a correlation between this parameter and theoretical thresholds by a factor 2–3, which suggests that a real dawn-dusk asymmetry during this storm was stronger than such an asymmetry represented by the Ts03 model.     

Interpretation of Magnetic Anomalies and Estimation of Depth of Magnetic Crust in Slovakia

The magnetic map of Slovakia used in the paper was compiled as part of a project titled Atlas of Geophysical maps and profiles in 2001. The residual magnetic data were analyzed to produce Curie point estimates. To remove distortion of magnetic anomalies caused by the Earth’s magnetic field, reduction to pole transformation was applied to the magnetic anomalies using the magnetization angle of the induced magnetization. Anomalies reduced to the pole tend to be better correlated with tectonic structures. We applied a 3-km upward continuation to the residually compiled magnetic anomalies in order to remove effects of topography. The depth of magnetic dipoles was calculated by an azimuthally averaged power spectrum method for the entire area. Such estimates can be indicative of temperatures in the crust, since magnetic minerals lose their spontaneous magnetization according to Curie temperature of the dominant magnetic minerals in the rocks. The computed Curie point depths in the Slovakia region vary between 15.2 km and 20.9 km. Heat flow higher than 100 mWm⁻² occurs at the central volcanics and eastern part of Slovakia, where the Curie point depths values are shallow. The correlation between Curie point depths, heat flow and crust depth was investigated for two E-W cross sections. Heat flow and Curie point depth values are correlated with each other however, these values could not be correlated with crust depth. The Curie point isotherm, which separates magnetic and non-magnetic parts of the crust, is represented in two cross sections.     

Chromite in the Paricutin lava flows (1943–1952)

Small euhedral chromite crystals are found in olivine macrophenocrysts (Fo_80–84) from the basaltic andesites (150 ppm Cr) erupted in 1943–1947, and in orthopyroxene macrophenocrysts of the andesites (75 ppm Cr) erupted in 1947–1952. The majority of the chromite octahedra are 5–20 μm in diameter, and some are found in clusters and linear chains of three or more oriented chromite crystals. The composition of the majority of the chromite grains within olivine and orthopyroxene macrophenocrysts is Fe²⁺/(Fe²⁺+Mg)=0.5–0.6, Cr/(Cr+Al)=0.5–0.6 and Fe³⁺/(Fe³⁺+Al+Cr)=0.2–0.3. The chromite crystals in contact with the groundmass are larger, subhedral, and grade in composition from chromite cores to magnetite rims. Comparison of the composition of chromite with those of other volcanic rocks shows that the most primitive Paricutin chromite is richer in total iron and higher in Fe³⁺/(Fe³⁺+Al+Cr) than primary chromite in most lavas. The linear chains of oriented chromite octahedra are found in olivine and orthopyroxene macrophenocrysts, and in the groundmass. These chromite chains are thought to result from diffusion-controlled crystallization because of the very high partition coefficient (1000) of Cr between chromite and melt. We conclude that chromite was a primary phase in the lavas at the time of extrusion and that magnetite only crystallized after extrusion during cooling of the lava flows. The presence of chromite microphenocrysts in andesitic lavas containing as little as 70 ppm Cr can be explained by dissolved H₂O in the melt depressing the liquidus temperature for orthopyroxene such that chromite becomes a liquidus phase. The influence of dissolved H₂O can also explain the lack of plagioclase macrophenocrysts in most of the lavas and the relatively high partition coefficient (20) of Ni between olivine and melt and the high partition coefficient (40) of Cr between orthopyroxene and melt. The liquidus temperature of the basaltic andesite is estimated to have been less than 1140°C, assuming H₂O>1 wt.%, and the log f_O2 to have been above that of the QFM buffer. The chromite and orthopyroxene liquidus temperature of the andesites, assuming H₂O>1 wt.%, is estimated to have been 1100°C or less. The derivation of the later andesites from the earlier basaltic andesites has been explained by a combination of fractional crystallization of olivine, orthopyroxene and plagioclase, and assimilation of xenoliths. The significantly lower Cr, Ni and Mg of the andesites may have been in part due to the separation of olivine macrophenocrysts plus enclosed chromite crystals from the earlier basaltic andesites.     

Nomograms for rapid evaluation of magnetic anomalies over long tabular bodies

The magnetic anomaly due to a long tabular body usually consists of a maximum and a minimum. The distances and the amplitudes of the maximum and the minimum, when defined in dimensionless quantities, may be used as characteristics of the source. In this paper, a method based on the positions of the maximum and the minimum on the magnetic anomaly due to a long tabular body has been presented. Characteristic ratios,D andA involving the distances and amplitudes of the maximum and the minimum points on the anomaly curve are defined. Nomograms showing the variations ofD andA with the parameters of (1) the dike and (2) the vertical fault models are presented. The parameters of the causative source are evaluated from the two ratiosD andA and the nomograms, using some simple analytical relations presented here. From the nomograms, it is observed that (a) for a thick dike,A is always greater thanD, (b)A=D for a thin sheet and (c) for a vertical fault,A is always less thanD. Thus from the characteristic ratiosD andA it is possible to evaluate the source parameters and also to distinguish whether the source is a dike, sheet or a vertical fault. The method is fast and is applicable for the magnetic anomalies either in total, vertical or horizontal component. The method has been applied on two field examples and the results are found to be in close agreement with those obtained by using other methods. A simple method of locating the origin on the anomaly curve is included. The limitations of the method are also discussed.     

Dynamics of magnetospheric current systems during magnetic storms of different intensity

The dynamics of the magnetospheric magnetic field during the magnetic storms of different intensity has been studied. The magnetic field variations on the Earth’s surface were calculated using the paraboloid model of the magnetosphere, taking into account the induction currents flowing in the diamagnetically conductive Earth. Dst and its components have been calculated for ten magnetic storms. It has been indicated that relative contributions of magnetospheric sources to Dst change depending on the storm power. For weak and moderate storms, the contribution of the magnetotail current sheet can reach values comparable with the ring current contribution and, sometimes, can even exceed this contribution. For strong storms, the ring current field dominates over the tail current field, the absolute value of which does not exceed 150 nT (also achieved during less intense storms). For storms with minimum Dst exceeding-200 nT, the tail current field is saturated, whereas the ring current can continue developing.     

Disturbance daily variation of electrojet current at Indian longitude sector

Using the hourly mean data of the horizontal (H) and vertical (Z) components of the geomagnetic field at the set of nine observatories in India, it has been found that the Disturbance Daily Variation (SD) of H shows a prominent midday depression over the magnetic equator of the H field and a midday increase of Z field at stations near the northern fringe of the equatorial electrojet belt. The magnetic disturbance introduces a westward electric field over the equatorial region, causing a band of westward ionospheric current over the magnetic equator during the day time hours. The latitudinal extent of the disturbance time counter electrojet current seems to be larger than that of quiet time normal electrojet current. This suggests a systematic westward electric field superposed on the normal Sq field at low latitude ionosphere during disturbed periods, the source of which has to be clearly defined. Further correlative data analysis is required to isolate these sources of the disturbed equatorial electric fields.     

A transfer pathway modeling template for radiological environmental risk assessment

As radioactive human doses are calculated through food chain and exposure pathways, it is important to take a pathway analysis to determine all pathway contributions for further radiological environmental risk assessment. A challenge of contaminant fate and environmental pathway analysis is to handle a large number of environmental components and to assign proper calculation models to linked component pairs. This paper presents a model template called transfer pathway model template (TPMT) that stores transfer models to environmental class pairs. When a site-specific calculation scenario is built, a source–receptor transfer matrix (SRTM) is defined by the scenario. Then the calculation loops through all source–receptor component pairs in SRTM and apply proper models stored in TPMT to calculate the transfer factors. TPMT can be used as the component pathway verification template and model allocation template. This structure is used in an environmental risk assessment (ERA) application called IMPACT that has been applied for a number of nuclear power plants and uranium mines in Canada.     

An extended structure-function model and its application to the analysis of solar wind intermittency properties

An extended structure-function model is developed by including the new effect in the p-model of Meneveau and Sreenivasan which shows that the averaged energy cascade rate changes with scale, a situation which has been found to prevail in nonfullydeveloped turbulence in the inner solar wind. This model is useful for the small-scale fluctuations in the inner heliosphere, where the turbulence is not fully developed and cannot be explained quantitatively by any of the previous intermittency turbulence models. With two model parameters, the intrinsic index of the energy spectrum <alpha>, and the fragmentation fraction P ₁, the model can fit, for the first time, all the observed scaling exponents of the structure functions, which are calculated for time lags ranging from 81 s to 0.7 h from the Helios solar wind data. From the cases we studied we cannot establish for P ₁ either a clear radial evolution trend, or a solar-wind-speed or stream-structure dependence or a systematic anisotropy for both the flow velocity and magnetic field component fluctuations. Generally, P ₁ has values between 0.7 and 0.8. However, in some cases in low-speed wind P ₁ has somewhat higher values for the magnetic components, especially for the radial component. In high-speed wind, the inferred intrinsic spectral indices (<alpha>) of the velocity and magnetic field components are about equal, while the experimental spectral indices derived from the observed power spectra differ. The magnetic index is somewhat larger than the index of the velocity spectrum. For magnetic fluctuations in both high- and low-speed winds, the intrinsic exponent <alpha> has values which are near 1.5, while the observed spectral exponent has much higher values. In the solar wind with considerable density fluctuations near the interplanetary current sheet near 1 AU, it is found that P ₁ has a comparatively high value of 0.89 for V _x . The impact of these results on the understanding of the nature of solar wind fluctuations is discussed, and the limitations in using structure functions to study intermittency are also described.     

Permeability Prediction for Low Porosity Rocks by Mobile NMR

Estimating permeability from NMR well logs or mobile NMR core scanner data is an attractive method as the measurements can be performed directly in the formation or on fresh cores right after drilling. Furthermore, the method is fast and non-destructive. Compared to T ₁ relaxation times, commonly measured T ₂ distributions are influenced by external and internal magnetic field gradients. We performed two-dimensional T ₁ and T ₂ relaxation experiments on samples of Rhaetian sandstone, a rock with low porosity and small pore radii, using a mobile NMR core scanner which operates within a nearly homogeneous static magnetic field. Because small pore sizes are associated with high internal magnetic field gradients, standard methods from NMR logging in the oil industry cannot be applied for accurate permeability prediction. Therefore, a new model theory was developed, which describes the pore radius dependence of the surface relaxivity ρ ₂ by both an analytical and a more practical empirical equation. Using corrected ρ ₂ values, permeability can be predicted accurately from the logarithmic mean of the T ₂ distribution and the physically based Kozeny-Carman equation. Additional core plug measurements of structural parameters such as porosity, permeability, specific inner surface area and pore radius distributions supported the NMR results.     

磁暴二倍法预报唐山7.8级地震

磁暴二倍法预报地震的依据是,应变能在运移过程中导致地壳膨胀和压缩形成一个周期,在时间上呈有二倍关系．唐山７．８级地震是我国７０年代最大地震,用磁暴二倍法推出有４组磁暴能对应这次地震．每组磁暴前后都发生了地震,由于这些地震释放的剩余能量在聚积过程中,地壳发生膨胀和压缩,地磁场就受到影响形成了磁暴．因此磁暴与地震能量变化有相关性．可见,磁暴二倍法可以预报地震．     

Determination of the maximum-depth to potential field sources by a maximum structural index method

A simple and fast determination of the limiting depth to the sources may represent a significant help to the data interpretation. To this end we explore the possibility of determining those source parameters shared by all the classes of models fitting the data. One approach is to determine the maximum depth-to-source compatible with the measured data, by using for example the well-known Bott–Smith rules. These rules involve only the knowledge of the field and its horizontal gradient maxima, and are independent from the density contrast.Thanks to the direct relationship between structural index and depth to sources we work out a simple and fast strategy to obtain the maximum depth by using the semi-automated methods, such as Euler deconvolution or depth-from-extreme-points method (DEXP).The proposed method consists in estimating the maximum depth as the one obtained for the highest allowable value of the structural index (N_max). N_max may be easily determined, since it depends only on the dimensionality of the problem (2D/3D) and on the nature of the analyzed field (e.g., gravity field or magnetic field). We tested our approach on synthetic models against the results obtained by the classical Bott–Smith formulas and the results are in fact very similar, confirming the validity of this method. However, while Bott–Smith formulas are restricted to the gravity field only, our method is applicable also to the magnetic field and to any derivative of the gravity and magnetic field. Our method yields a useful criterion to assess the source model based on the (∂f/∂x)_max/f_max ratio.The usefulness of the method in real cases is demonstrated for a salt wall in the Mississippi basin, where the estimation of the maximum depth agrees with the seismic information.     

Variations in the geomagnetic field and auroras during the main phase of a large magnetic storm of November 20, 2003

The substorm characteristics during the main phase of a large magnetic storm of November 20, 2003, are studied based on the data of TV observations of auroras and auroral absorption at Tixie Bay station and at the global network of magnetic stations. The contribution of auroral particles, responsible for the emission of discrete auroras, has been estimated based on an analysis of the spatial-time variations in the auroral luminosity intensity. This contribution accounted for ～40% of the total luminous flux, which is approximately twice as large as was previously observed in substorm disturbances. Responses of the solar wind and IMF parameters in substorms and variations in the magnetic indices, characterizing geomagnetic activity in the northern polar cap and ring current (PCN, ASY-H and SYM-H), have been detected. The spatial-time distribution of the equivalent ionospheric currents has been constructed, and the total value of these currents along the meridian has been determined based on the [Popov et al., 2001] method and using the IMAGE magnetic data. It has been obtained that the maximal total equivalent ionospheric current in the premidnight sector (～2000 MLT) leads the minimal value of the SYM-H index by ～1.5 h.     

Modelling Potential Field Sources in the Gelibolu Peninsula (Western Turkey) Using a Markov Random Field Approach

In this study, a Markov Random Field (MRF) approach is used to locate source boundary positions which are difficult to identify from Bouguer gravity and magnetic maps. As a generalized form of Markov Chains, the MRF approach is an unsupervised statistical model based algorithm and is applied to the analysis of images, particularly in the detection of visual patterns or textures. Here, we present a dynamic programming based on the MRF approach for boundary detection of noisy and super-positioned potential anomalies, which are produced by various geological structures. In the MRF method, gravity and magnetic maps are considered as two-dimensional (2-D) images with a matrix composed of N₁ × N₂ pixels. Each pixel value of the matrix is optimized in real time with no a priori processing by using two parameter sets; average steering vector (θ) and quantization level (M). They carry information about the correlation of neighboring pixels and the locality of their connections. We have chosen MRF as a processing approach for geophysical data since it is an unsupervised, efficient model for image enhancement, border detection and separation of 2-D potential anomalies. The main benefit of MRF is that an average steering vector and a quantization level are enough in evaluation of the potential anomaly maps. We have compared the MRF method to noise implemented synthetic potential field anomalies. After satisfactory results were found, the method has been applied to gravity and magnetic anomaly maps of Gelibolu Peninsula in Western Turkey. Here, we have observed Anafartalar thrust fault and another parallel fault northwest of Anafartalar thrust fault. We have modeled a geological structure including a lateral fault, which results in a higher susceptibility and anomaly amplitude increment. We have shown that the MRF method is effective to detect the broad-scale geological structures in the Gelibolu Peninsula, and thus to delineate the complex tectonic structure of Gelibolu Peninsula.     

Mineral magnetic variation of the Jingbian loess/paleosol sequence in the northern Loess Plateau of China: Implications for Quaternary development of Asian aridification and cooling

A high-resolution mineral magnetic investigation has been carried out on the Jingbian loess/paleosol sequence at the northern extremity of the Chinese Loess Plateau. Results show that the magnetic assemblage is dominated by large pseudo-single domain and multidomain-like magnetite with associated maghemite and hematite. Variations in the ratios of SIRM_100mT/SIRM, SIRM_100mT/SIRM_30mT and SIRM_100mT/SIRM_60mT (SIRM is the saturation isothermal remanent magnetization; SIRM_nmT represents the residual SIRM after an n mT alternating field demagnetization) have been used to document regional paleoclimate change in the Asian interior by correlating the mineral magnetic record with the composite δ¹⁸O record in deep-sea sediments. The long-term up-section decreasing trend in those ratios in both loess and paleosol units has been attributed to a long-term decrease in the relative contributions of eolian hematite during glacial extrema and of pedogenic hematite during interglacial extrema, respectively, which reveals a long-term decreasing trend in chemical weathering intensity in both glacial-stage source region (the Gobi and deserts in northwestern China) and interglacial-stage depositional area (the Loess Plateau region). We further relate this long-timescale variation to long-term increasing aridification and cooling, during both glacial extrema in the dust source region and interglacial extrema in the depositional area, over the Quaternary period. Changes in those ratios are most likely due to Quaternary aridification and cooling driven by ongoing global cooling, expansion of the Arctic ice-sheet, and progressive uplift of the Himalayan–Tibetan complex during this period.     

Scaling Analysis of Time Distribution between Successive Earthquakes in Aftershock Sequences

The earthquake inter-event time distribution is studied, using catalogs for different recent aftershock sequences. For aftershock sequences following the Modified Omori’s Formula (MOF) it seems clear that the inter-event distribution is a power law. The parameters of this law are defined and they prove to be higher than the calculated value (2–1/p). Based on the analysis of the catalogs, it is determined that the probability densities of the inter-event time distribution collapse into a single master curve when the data is rescaled with instantaneous intensity, R(t; M _th ), defined by MOF. The curve is approximated by a gamma distribution. The collapse of the data provides a clear view of aftershock-occurrence self-similarity.     

Quantifying and specifying the solar influence on terrestrial surface temperature

This investigation is a follow-up of a paper in which we showed that both major magnetic components of the solar dynamo, viz. the toroidal and the poloidal ones, are correlated with average terrestrial surface temperatures. Here, we quantify, improve and specify that result and search for their causes.We studied seven recent temperature files. They were smoothed in order to eliminate the Schwabe-type (11 years) variations. While the total temperature gradient over the period of investigation (1610–1970) is 0.087 °C/century; a gradient of 0.077 °C/century is correlated with the equatorial (toroidal) magnetic field component. Half of it is explained by the increase of the Total Solar Irradiance over the period of investigation, while the other half is due to feedback by evaporated water vapour. A yet unexplained gradient of ?0.040 °C/century is correlated with the polar (poloidal) magnetic field. The residual temperature increase over that period, not correlated with solar variability, is 0.051 °C/century. It is ascribed to climatologic forcings and internal modes of variation.We used these results to study present terrestrial surface warming. By subtracting the above-mentioned components from the observed temperatures we found a residual excess of 0.31° in 1999, this being the triangularly weighted residual over the period 1990–2008.We show that solar forcing of the ground temperature associated with significant feedback is a regularly occurring feature, by describing some well observed events during the Holocene.