2 ¾ dimension modeling of the aeromagnetic anomaly of Volcán de Colima, western Mexico

"Se presentan los resultados de un modelo 2 ¾ D de la anomalía aeromagnética del Volcán de Colima ubicado en la porción sur del Complejo Volcánico de Colima. El modelo estructural documenta la presencia de una cámara magmática de forma alargada con una longitud mayor a los 10 km en la dirección norte–sur y que se extiende más de 6.6 km al sur del Volcán de Colima. El máximo espesor de la cámara magmática está hacia el ESE del cráter del Volcán de Colima y tiene una extensión del orden de los 6.9 km. Hacia los bordes la cámara magmática presenta espesores del orden de los 820 m a 620 m al sur y norte respectivamente. Bajo el Volcán de Colima de localiza a una profundidad entre 4.9 y 1.2 km bajo el nivel del mar. El modelo geofísico es consistente con las propuestas de la migración norte–sur de la actividad en el Complejo Volcánico de Colima.""Results of 2 ¾ D modeling of the aeromagnetic anomaly of Volcán de Colima, in the southern sector of the Colima Volcanic Complex are presented. The structural model documents the presence of a magma chamber with an elongated shape and a length greater than 10 km in a north–south direction extending over 6.6 km south of Volcán de Colima. The maximum thickness of the magma chamber, around 6.9 km, is located to the ESE of the volcano. The edges of the magma chamber have thicknesses ranging from 820 m to 620 m to the south and north, respectively. The magma chamber under Volcán de Colima is located at depths of 4.9 and 1.2 km below sea level. The geophysical model is consistent with a proposed north–south migration of activity in the Colima Volcanic Complex.

Geofísica para la localización de agua subterránea en ambientes volcánicos áridos de la Mesa Central. Caso La Dulcita, Villa de Ramos, San Luis Potosí, México

"Uno de los problemas más importantes en zonas áridas es el relacionado con la exploración y aprovechamiento de agua subterránea. En este trabajo se presenta el método magnético aéreo y terrestre como herramienta para la localización de zonas asociadas con permeabilidad (fracturamiento) mediante la interpretación de lineamientos y dominios magnéticos aéreos y terrestres. A partir de estas interpretaciones, se emplean métodos eléctricos por medio de secciones y sondeos eléctricos verticales para verificar si las zonas se correlacionan con humedad en el subsuelo. En los casos en que se cumplen ambas condiciones de permeabilidad y humedad, se consideran zonas con altas probabilidades para la localización de agua subterránea en las zonas áridas de la Mesa Central Mexicana. Se presenta la aplicación de estos métodos en una zona rural del Estado de San Luis Potosí, en donde existen problemas con la localización de estos recursos.""One of the most important problems in arid zones is the one related to the exploration and exploitation of groundwater. This paper presents the air and ground magnetic method as a tool for locating zones associated with permeability (fracturing), by means of the interpretation of alignments and of air and ground domains. Based on these interpretations, electrical methods by sections and vertical electrical soundings are used to verify if the zones are correlated to the moisture underground. If both permeability and moisture are met, they are considered as zones with high probability of locating groundwater in the Mexican Central Mesa arid zones. The application of this methodology is presented in a rural population in the State of San Luis Potosí, where problems with the location of these resources exist.

The Magnetometry—A Primary Tool in the Prospection of Underground Water

One of the most important problems in arid and semi-arid zones in the Mexican Mesa Central is the one related to the exploration and exploitation of groundwater. It is found at depths over 200 m, and movement is primarily through fractures. This work presents a geophysical methodology, which shows the potential of combining natural and induced methods to locate confined aquifers in fault zones. The study begins by interpreting the aeromagnetic survey, mainly by searching alignments associated with low magnetic anomalies, which are correlated with faults zones, and/or fractures and/or geologic contacts where ferromagnetic minerals have undergone oxidation due to their association with recharged zones. These aeromagnetic alignments are confirmed on land by a ground magnetic survey. Based on these interpretations, electrical methods include sections and vertical electrical sounding are used to verify if the zones are correlated to the underground moisture. If both permeability and moisture are met together, then they considered as zones with a high probability of locating ground water in the Mexican Mesa Central

A review of iron oxide transformations, rock magnetism and interpretation of magnetic anomalies: el Morro Mine (Brazil), a case study

"Se presenta una revisión de la mineralogía de los óxidos de hierro y su relación con las propiedades magnéticas. Se discuten los procesos magnéticos y modelos de una gran área alrededor de la mina El Morro, Brasil en el depósito alcalino-carbonatítico Jacupiranga. Propiedades magnéticas de las rocas y observaciones de minerales opacos sugieren que la titanomagnetita es la principal fase mineral. La anomalía magnética sugiere cuerpos mineralizados con una tendencia general este-oeste. El cuerpo mas grande esta en la parte norte del área y mide al menos 5 por 2 km. Todos los cuerpos están ubicados en un dominio aeromagnético de forma semi-elipsoidal y rumbo general noreste-suroeste. Se propone que la zona anómala continua hacia el norte y que el área de interés económico total puede extenderse aproximadamente 8 x 4 km en dirección norte-sur. Se modelaron dos perfiles con direcciones N-S y E-O y el mejor ajuste corresponde a varios grandes cuerpos irregulares. Los cuerpos mineralizados mas importantes están ubicados debajo del alto magnético hacia el norte; esta inferencia es apoyada por datos gravimétricos. Otros cuerpos están ubicados debajo del bajo magnético y se extienden hacia el sur. El mejor ajuste de la anomalía calculada a la observada se obtuvo usando principalmente la magnetización inducida.""A review of iron oxide mineralogy and its relationship with rock magnetic properties is presented. Magnetic processes and models of a large area around the El Morro, Brazil mine in the Jacupiranga alkaline-carbonatitic deposit is discussed. Rock magnetic properties and opaque mineral observations suggest that titanomagnetite is the main magnetic phase. The magnetic anomaly suggests ore bodies of an east-west general strike. The largest body is in the northern part of the area. It measures at least 5 by 2 km. All bodies are located in an aeromagnetic domain of semi-ellipsoidal shape and northeast-southwest general strike. It is proposed that the anomalous zone is continuous to the north and that the whole area of economic interest may extend roughly 8x4 km in the north-south direction. We modeled two N-S and E-W profiles and the best fit corresponds to several large irregular bodies. The most important mineralized bodies are located beneath the northward continuation of the magnetic high; this inference is supported by a gravimetric data. Other bodies are located under the magnetic low and extend to the south. The best fit of calculated to observed anomaly is obtained by using mostly induced magnetization.

Internal architecture of the Tuxtla volcanic field, Veracruz, Mexico, inferred from gravity and magnetic data

"The Tuxtla Volcanic Field (TVF) is a basaltic volcanic field emerging from the plains of the western margin of the Gulf of Mexico in the Mexican State of Veracruz. Separated by hundreds of kilometers from the Trans-Mexican Volcanic Belt to the NW and the Chiapanecan Volcanic Arc to the SE, it stands detached not only in location but also in the composition of its rocks, which are predominantly alkaline. These characteristics make its origin somewhat puzzling. Furthermore, one of the large volcanoes of the field, San Martin Tuxtla, underwent an eruptive period in historical times (CE 1793). Such volcanic activity conveys particular importance to the study of the TVF from the perspective of volcanology and hazard assessment. Despite the above circumstances, few investigations about its internal structure have been reported. In this work, we present analyses of gravity and aeromagnetic data obtained from different sources. We present the complete Bouguer anomaly of the area and its separation into regional and residual components. The aeromagnetic data were processed to yield the reduction to the pole, the analytic signal, and the upward continuation to complete the interpretation of the gravity analyses. Three-dimensional density models of the regional and residual anomalies were obtained by inversion of the gravity signal adding the response of rectangular prisms at the nodes of a regular grid. We obtained a body with a somewhat flattened top at 16 km below sea level from the inversion of the regional. Three separate slender bodies with tops 6 km deep were obtained from the inversion of the residual. The gravity and magnetic anomalies, as well as the inferred source bodies that produce those geophysical anomalies, lie between the Sontecomapan and Catemaco faults, which are proposed as flower structures associated with an inferred deep-seated fault termed the Veracruz Fault. These fault systems along with magma intrusion at the lower crust are necessary features to understand the origin and structure of the TVF.

Estudio aeromagnético del complejo volcánico de Colima, occidente de México - implicaciones tectónicas y estructurales

"Se reporta la interpretación cualitativa y cuantitativa de anomalías aeromagnéticas del complejo volcánico de Colima (CVC) y sus alrededores. La zona se caracteriza por anomalías magnéticas de distintas amplitudes y longitudes de onda asociadas con estructuras volcánicas y cuerpos intrusivos, incluyendo al CVC. Se identifican 24 dominios aeromagnéticos definidos por patrones característicos de anomalías en un área de aproximadamente 11,500 km2 alrededor del CVC. El Volcán de Colima y el Nevado de Colima se caracterizan por anomalías dipolares de gran amplitud. Se investiga la estructura profunda del complejo volcánico utilizando un modelo magnético dimensional 2¾ - D, encontrando que la fuente asociada a las anomalías de los volcanes se relaciona con una posible cámara magmática de forma alargada, con una longitud mayor a los 6.8 km en dirección N-S y espesor máximo de 5.6 km. En el Volcán de Colima, ésta se localiza a una profundidad de 4.8 km. Este cuerpo se extiende por una distancia del orden de 5.6 km hacia el sur del CVC con un espesor promedio del orden de 0.54 km. Bajo el volcán Nevado de Colima se interpreta una posible cámara magmática colapsada de forma alargada con una longitud en dirección N-S mayor a 5.6 km y con espesor máximo de 1.2 km con una profundidad del orden de los 5.2 km bajo el cráter del Nevado. Se documentan, por primera vez, nuevas estructuras geológicas asociadas a anomalías magnéticas sepultadas por productos volcánicos, comofallas y cuerpos intrusivos. En la mayor parte del mapa magnético reducido al polo, se observan anomalías monopolares de grandes dimensiones, amplitudes altas y longitudes de onda largas, asociadas a rocas intrusivas. A 47 km al NW de los volcanes Colima y Nevado de Colima se puede apreciar un cinturón formado por tres de estas anomalías. En la zona también existe una serie de anomalías magnéticas relacionadas con la presencia de yacimientos de mineral de hierro, como la mina El Encino ubicada a 48 km al SE y el área mineralizada de la Sierra de Manantlán a 42 km al ESE del volcán de Colima.""Results of qualitative and quantitative interpretation of aeromagnetic anomalies over the Colima volcanic complex (CVC) and surrounding areas are presented. The area is characterized by magnetic anomalies of distinct amplitude and wavelength associated with volcanic structures and intrusive bodies, including the CVC. We identified 24 aeromagnetic domains in an area of 11,500 km2 around the CVC. We identify, for the first time, new geological structures buried under volcanic products, like faults and intrusive bodies. High amplitude magnetic dipolar anomalies characterize the volcanic structures of Colima and Nevado de Colima volcanoes. CVC deep structure was investigated by using a 2¾ D magnetic model, finding that the source is related to a possible magmatic chamber of elongated shape, >6.8 km long, in north-south strike with maximum thickness of 5.6 km, which is located at 4.8 m depth beneath the Colima volcano. The body extends to 5.6 km south of the Colima volcano, with average thickness of about 0.54 km. Under Nevado de Colima volcano, we interpreted a collapsed magmatic chamber, with elongated shape, >5.6 km long and 1.2 km of maximum thickness at a depth of around 5.2 km below the Nevado summit. In most of the reduced to the pole magnetic anomaly map, we observed large monopolar anomalies, high amplitude, and long wavelengths associated to the intrusive bodies. Three of these anomalies form a 47 km long belt to the NW of the CVC. There are several magnetic anomalies related to iron-ore deposits, i.e., El Encino mine (48 km SE of CVC) and Sierra de Manantlán (42 km ESE of CVC).

Full vector magnetic dating of some pyroclastic rocks associated to the Colima volcano, western Mexico

"Se presentan los resultados de un estudio paleomagnético y de magnetismo de rocas realizado en rocas de flujos piroclásticos y algunos fragmentos de cerámica pre-Colombina asociados al complejo Volcánico de Colima. Con la finalidad de estimar la mineralogía magnética y determinar tanto la magnetización remanente característica, así como la intensidad absoluta, se realizó un conjunto de experimentos y mediciones magnéticas. Se reporta una edad absoluta determinada mediante el fechamiento magnético de dos erupciones volcánicas asociadas al volcán Colima (volcán de Fuego). Esta es la primera vez que el modelo de predicción de campo magnético SHADIF.14K, basado en el vector completo (declinación, inclinación y paleointensidad absoluta), es utilizado en depósitos de flujos piroclásticos intercalados con cerámicas. Los resultados obtenidos concuerdan con las edades radiométricas (C14) disponibles y constituyen una determinación más precisa de la edad del momento de enfriamiento de los flujos de lava.""A combined rock-magnetic and paleomagnetic researches were carried out on some pyroclastic rocks and pre-Columbian pottery fragments associated to the Colima Volcanic Complex. A full battery of magnetic measurements was performed in order to estimate magnetic mineralogy and to determine both the directions and absolute intensities of the characteristic remanent magnetization. In this paper we report absolute age determination using magnetic dating for two volcanic eruptions associated to the Colima volcano (Volcán de Fuego). The geomagnetic field prediction model SHA.DIF.14K, based on full vector (declination, inclination and absolute paleointensity), was used for first time in pyroclastic flows deposits and intercalated potteries. The results obtained agree reasonably well with available radiometric (C14) data, them provide a more accurate age determination of the cooling moment for the lava flows.

Paleomagnetism and aeromagnetic survey from Tancitaro volcano (Central Mexico) - paleo-secular variation at low latitudes during the past 1 Ma

"El volcán Tancítaro (TV) forma parte del campo volcánico monogenético MichoacánGuanajuato (MGVF) en el sector centro-oeste del Eje Volcánico Trans Mexicano (TMVB). Los resultados de un estudio paleo magnético de flujos de lava del volcán Tancítaro fechados radiométricamente, se utilizaron para investigar la variación paleo secular (PSV) y el campo promedio temporal (TAF) para latitudes bajas. El rango de fechado Ar-Ar fue de 70 a 960 kaños considerando los Chrones de polarización de Brunhers y Matuyama. Todas las muestras arrojaron una polaridad de magnetización normal bien definida. Dos flujos se correlacionaron con la polaridad del Evento Jaramillo, lo cual provee un marcador útil para la actividad volcánica en el MGVF. Para los análisis del PSV y TAF las paleo direcciones se combinaron con resultados anteriores de alta precisión. El estudio aeromagnético alrededor del volcán Tancítaro se caracterizó por una serie de anomalías tanto positivas como negativas. El Volcán Tancítaro presenta una gran anomalía positiva que sugiere la presencia de una gran fuente subterránea. El análisis espectral de este campo anómalo da una estimación promedio hacia la parte alta de los cuerpos que la originan entre 2-3 km.""The Tancitaro volcano (TV) is part of the Michoacan-Guanajuato monogenetic volcanic field (MGVF) in the central-western sector of the Trans-Mexican Volcanic Belt (TMVB). Results of a paleomagnetic study of radiometrically dated lava flows from Tancitaro volcano were used to investigate the paleosecular variation (PSV) and time averaged field (TAF) at low latitudes. Ar-Ar dates range from ~70 to 960 kyr spanning the Brunhes and Matuyama polarity chrons. All samples yielded well defined normal polarity magnetization. Two flows are correlated to the Jaramillo polarity event, which provide a useful marker for the volcanic activity in the MGVF. For the PSV and TAF analysis, paleodirections were combined with previously reported high standard results. The aeromagnetic survey around the Tancitaro volcano was characterized by a trend of regional anomalies over the volcanic structures. The residual field showed several positive and negative anomalies. The Tancitaro volcano is marked by a broad positive anomaly suggesting the presence of a large underground source. Spectral analysis of this anomaly field gives an average estimate to the top of the source bodies between 2-3km.

Caracterización magnética de polvo urbano y plantas de la Zona Metropolitana del Valle de México

"Investigaciones recientes han demostrado que los gases automotores contienen elementos potencialmente tóxicos, los cuales se acumulan sobre las superficies del suelo urbano (asfalto, cemento, empedrado, etc.) y se mezclan con el polvo urbano (PU). En este estudio se determinan los valores de umbrales magnéticos por medio de muestras de PU depositado en calles y hojas de plantas provenientes de zonas con diferentes tipos de usos del suelo de la Zona Metropolitana del Valle de México. Se colectaron 89 muestras de PU y un mismo número de muestras de hojas de Ficus benjamina de zonas con diferentes tipos de uso de suelo (habitacional, equipamiento, habitacional-comercial, espacios abiertos e industria). Los parámetros magnéticos determinados fueron: susceptibilidad magnética específica másica (χlf), porcentaje de la susceptibilidad dependiente de la frecuencia (χdf%), magnetización remanente isotermal a 700 mT (MRI0.7) y adicionalmente la concentración de Pb y Zn. Las curvas de magnetización vs campo magnético presentaron una saturación de la magnetización para campos magnéticos entre 200 a 300 mT y la fracción S-200 (S-200 = MRI-0.2/MRI0.7) presentó valores entre 0.7 a 1.0; este comportamiento es característico de muestras con minerales de baja coercitividad magnética (ferrimagnéticos). Las curvas termomagnéticas (κ vs T) de PU indican que la principal fase magnética es alrededor de 580 °C, lo que sugiere la presencia de titano-magnetita pobre en Ti y magnetita. Las concentraciones de Pb y Zn presentan correlación estadísticamente aceptable con los parámetros magnéticos. Los análisis discriminantes en ambos tipos de muestras (agrupación por tipos de uso del suelo, las variables χlf, χdf%, MRI0.7 y las concentraciones de Pb y Zn) dan un resultado menor al 50% en ambos casos. Por tanto, no es posible establecer diferencias significativas entre los tipos de uso de suelo, los parámetros magnéticos medidos y la concentración de Pb y Zn. La determinación de los valores umbrales magnéticos se obtuvo del polvo urbano con las menores concentraciones de Pb y Zn. Los valores umbrales para el PU fueron: MRI0.7 = 58 mAm2 kg-1 y χlf = 4.5 mm3 kg-1, en tanto que para las plantas fueron: MRI0.7 = 1.6 mAm2 kg-1 y χlf = 0.13 mm3 kg-1. Las representaciones espaciales de los valores umbrales de MRI0.7 de PU y plantas revelaron que las zonas de la parte central y sureste de la Zona Metropolitana del Valle de México son potencialmente peligrosas por su alta concentración de Pb o Zn.""Recent investigations have demonstrated that motor-vehicle gas contains particles with potentially toxic elements. These contaminating particles accumulate on the soil surface or overlying material (asphalt, stone and concrete pavements) and they are usually mixed with urban dust. In this work we determine the magnetic threshold values using urban dust and leave samples collected from different type of urban land-use soil along the metropolitan area of Mexico City and identify potentially dangerous areas considering potentially toxic elements. 89 urban dust samples deposited on the streets and same number of Ficus benjamina leaves were collected from different land-use zones (residential, equipment, residentialcommercial, open space and industry). The magnetic parameters determined were: mass especific magnetic susceptibility (χ lf), percentage frequency dependent susceptibility (χ df%), isothermal remanent magnetization at 700 mT (IRM0.7). Additionally the concentrations of Pb and Zn were also obtained. Isothermal remanence acquisition curves for dust samples yielded a saturation between 200 and 300 mT and S-200 (S-200= MRI-0.2/MRI0.7) fraction values were found between 0.7 to 1.0 - characteristic of samples with low magnetic coercivity minerals (ferrimagnetics). The κ vs T curves indicate the main magnetic phase about 580 °C which attests the presence of Ti-poor titanomagnetite or almost pure magnetite. The Pb and Zn concentrations show acceptable statistically correlation with magnetic parameters. Discriminant analyzes in both types of samples gave values low than 50% in both cases. Therefore, it is not possible to attest the significant differences between measured magnetic parameters, concentration of Pb and Zn and soil type. The threshold values for urban dust were MRI0.7 = 58 mAm2 kg-1 and χlf = 4.5 mm3 kg-1 while the plant threshold values were: MRI0.7 = 1.6 mAm2 kg-1 and χlf = 0.13 mm3 kg-1. Spatial representations of these threshold values revealed that zones from central and southeast of Valle de Mexico Metropolitan Zone are the most potentially dangerous areas considering their high concentration of Pb and Zn.

Statistically coherent calibration of X-Ray Fluorescence Spectrometry for major elements in rocks in minerals.

We applied both the ordinary linear regression (OLR) and the new uncertainty weighted linear regression (UWLR) models for the calibration and comparison of a XRF machine through 59 geochemical reference materials (GRMs) and a procedure blank sample. )e mean concentration and uncertainty data for the GRMs used for the calibrations (Supplementary Materials) (available here) filewere achieved from an up-to-date compilation of chemical data and their processing from well-known discordancy and significance tests. )e drift-corrected XRF intensity and its uncertainty were determined from mostly duplicate pressed powder pellets. )e comparison of the OLR (linear correlation coefficient r∼0.9523–0.9964 and 0.9771–0.9999, respectively, for before and after matrix correction) and UWLR models (r∼0.9772–0.9976 and 0.9970–0.9999, respectively) clearly showed that the latter with generally higher values of r is preferable for routine calibrations of analytical procedures. Both calibrations were successfully applied to rock matrices, and the results were generally consistent with those obtained in other laboratories although the UWLR model showed mostly narrower confidence limits of the mean (slope and intercept) or lower uncertainties than the OLR. Similar sensitivity (∼2.69–46.17 kc·s1·%1 for the OLR and ∼2.78–59.69 kc·s1·%1 for the UWLR) also indicated that the UWLR could advantageously replace the OLR model. Another novel aspect is that the total uncertainty can be reported for individual chemical data. If the analytical instruments were routinely calibrated from the UWLR model, this action would make the science of geochemistry more quantitative than at present