Sources of cultural noise are one of the main reasons why seismic methods fail in study areas near or within large cities, such as Mexico City. Under certain conditions, electrical and electromagnetic methods might
Antecedentes historicos de la geometria analytical yahoo dating a suitable substitute or complement to seismic exploration, as they respond to different physical properties of the subsurface.
To evaluate the applicability of such methods, Antecedentes historicos de la geometria analytical yahoo dating we present two recent case studies from the desiccated lakes Chalco and Xochimilco both located in urban areas within the Basin of Mexico where electrical resistivity tomography Antecedentes historicos de la geometria analytical yahoo datingtransient electromagnetic TEM and magnetotelluric MT soundings were conducted for the characterization of lake basement and lacustrine sediments.
In both examples, the results of the geophysical exploration contributed valuable stratigraphic information for the following drilling and core recovery, which highlights the potential of electrical and electromagnetic methods in paleolimnological studies. Geophysics, electrical resistivity tomography, electromagnetic methods, paleolimnology, lacustrine sediments, Mexico. Lakes are important sentinels and integrators of the effects of climate and environmental change Schindler,and lacustrine sediments are natural archives of these past changes.
Stratigraphic analysis of biological and non-biological indicators in lacustrine sediment cores provides information on direct and indirect climate-lake interactions in the past Fritz, A few water bodies, however, are much older and were shown to have consistently held water during the late Pleistocene, e. Recovery of such long sedimentary sequences requires specialized equipment and personnel, which are generally expensive Dean, Therefore, basin surveys need to be conducted prior to drilling operations for a careful planning and an efficient use of the drilling budget, even for smaller scale projects Cohen, Essential data comprises but is not limited to sediment thickness and composition, depth to bedrock and possible heterogeneities within the lake sediments Last and Smol, Ideally, these parameters are available on the lake-basin scale to allow a well-founded selection of the drilling site.
However, even smaller-scale information on the subsurface conditions in the vicinity of the selected site can aid drilling operations. If no data is available from collocated boreholes, geophysical methods are an efficient alternative to gain information on the e. A great advantage of most geophysical exploration methods is that they are non-invasive, relatively low-cost and able to quickly provide quantitative results.
While a wide range of applied geophysical methods exists for near-surface studies, seismic reflection methods are most widely used to assist the planning and execution of drilling projects for paleolimnological studies e. Due to the typically large contrast between the mechanical properties, i. Amongst other, limiting factors can be 1 insufficient contrasts among the seismic velocities of geologic materials, 2 the presence of gas-charged lake-floor sediments that act as a strong reflector and mask underlying sediments e.
Particularly in densely populated urban areas, where e. In some cases, geoelectrical e. Binley and Kemna, and electromagnetic e. In order to make these techniques available to the broader community of paleolimnological researchers, in this paper we briefly introduce some of the most commonly used geoelectrical and electromagnetic methods.
In particular, we present the basic relations between the electrical resistivity and the key petrophysical parameters of geologic materials.
The electrical resistivity i. In order to evaluate the possibilities and limitations of these geophysical methods applied to paleolimnological studies we present two case histories from the Basin of Mexico.
The first example presents the results of an ERT survey at the former Lake Xochimilco, with a maximum exploration depth of m that was carried out prior to retrieving a 47 m sediment core. Both study sites have a particular geological context, where lake sediments are in contact with different volcanic materials, such as basalt layers and thick pyroclastic deposits.
These materials, which are typical for the Basin of Mexico, are characterized by sufficiently contrasting electric properties, which facilitates the success of this type of geophysical exploration. Furthermore, the availability of stratigraphic data from the collocated drilling at both sites allowed us to evaluate and discuss the potential of the applied geophysical methods. Sediments and sedimentary rocks are composed of solid mineral grains of different size and the pore space between the grains, which is occupied by varying fractions of air and water.
Except for rarer metallic minerals, most rock-forming minerals are poor electrical conductors and do not contribute significantly to the electrical conductivity of these media e. Instead, the electric current is carried by ions dissolved in the pore water electrolytic conduction and even a thin film of water at the mineral surface of an interconnected pore network can increase the electrical conductivity by orders of magnitude.
Consequently, in partially water saturated media the electric conductivity is directly proportional to the electrical conductivity of the pore water and increases with the porosity of the material volume fraction of free pore space and the water saturation.
Generally, clay minerals have high cation exchange capacities leading to the build-up of electrical double layers at the water-mineral interface e. Waxman and Smits, The resulting high excess ion concentration accumulated at the further increases the electrical conductivity of clay-bearing sediments.
The connectivity between pores is typically low in metamorphic and igneous rock such as basalt but also decreases in sedimentary rocks with the degree of compaction and cementation of the sediments e. While the first three properties are directly linked to lithology, and thus the measured resistivity offers an indirect access to stratigraphic information, varying water saturation and water conductivity can occasionally dominate the electrical response of the subsurface materials and mask lithological contacts.
Antecedentes historicos de la geometria analytical yahoo dating measurements two electrodes are firmly attached to the surface in order to inject an electric current into the ground, while another pair of electrodes is used to measure the resulting voltage Figure 1.
The measured voltage varies with the geometrical configuration of the four electrodes but also depends on the electrical resistivity of the subsurface. From the voltage to current ratio and considering the geometric configuration of the electrodes, an apparent resistivity of the measurement can be calculated.
For a homogeneous subsurface, this apparent resistivity is equal to the homogeneous electrical resistivity. In order to assess vertical resistivity variations, the distances between the electrodes can be increased stepwise, which allows the electrical current to penetrate increasingly larger depths.
To determine lateral contacts only, the four-point measurement must be repeated at different positions along a profile maintaining fixed distances between the electrodes.
For a tomographic measurement these two approaches are combined and typically hundreds of four-point measurements with varying electrode apertures are collected at different positions along the line e. Binley and Kemna, Modern tomographic measurement systems automatically control hundreds of electrodes and measure thousands of four-point configurations. Depending on the survey objective, different geometric arrays are commonly used.
Figure 1 illustrates three typically deployed electrode configurations known as dipole-dipole, Wenner, and pole-dipole array, employed in the first case study of this work.
This task is realized by inversion algorithms, which adjust an approximate block model of electrical resistivities that reproduces the measured apparent resistivities within a certain margin of error. The results of this processing are 2D-sections of the electrical resistivity beneath the profile, which represent the spatial variation of the electrical resistivity in the subsurface.
Wenner topdipole-dipole centreand pole-dipole bottom array. Transient electromagnetic TEM soundings, also known as Antecedentes historicos de la geometria analytical yahoo dating electromagnetic soundings, can be carried out in many different ways e.
For the most commonly applied TEM soundings an electric current is injected into a horizontal loop consisting of a simple cable extended on the terrain Figure 2. The current carrying loop acts as a magnetic coil, which generates a Antecedentes historicos de la geometria analytical yahoo dating magnetic field that extends into the subsurface. Then the transmitter current is abruptly interrupted
Antecedentes historicos de la geometria analytical yahoo dating the decay of the magnetic field, which by means of electromagnetic EM induction causes eddy currents to flow in the subsurface.
Within milliseconds, the induced current system diffuses into the subsurface moving downwards and extending itself laterally. The temporal variation of this current system in turn generates a secondary magnetic field, the evolution of which can be measured at the surface.
Typically, a smaller receiver coil does this, but it can also be realized with the transmitter loop itself single-loop TEMwhich reduces the effort in the field. Figure 2 shows the variation of the time derivative of the secondary magnetic field, also known as impulse response, for different simple layered-earth models.
As the magnitude and the temporal evolution of the secondary magnetic field depend on the subsurface resistivity, sounding data can be used to reconstruct the geometric distribution of stratigraphic units with different electrical properties.
Similar to ERT measurements, the field data must be inverted in order to obtain resistivity models representing the subsurface materials. Consequently, large loops up to m x m square loops are generally used for deep soundings and small loops a few decimetres to meters for shallow investigations.
As a rule of thumb, Antecedentes historicos de la geometria analytical yahoo dating investigation depth of a TEM sounding is about 2—3 times the loop diameter. A direct current is injected into the transmitter loop. After current shut-off, eddy currents are induced that diffuse into the subsurface. The receiver coil or loop placed in the centre of the transmitter loop detects the transient voltage induced by the decaying secondary magnetic field.
However, instead of using an active source, the MT method makes use of EM waves originating from ionospheric and magnetospheric currents, electrical storms and long-wave radio transmitters.
The temporal variations of the magnetic components of these waves induce electrical currents in the subsurface, which in turn generate secondary magnetic fields. For MT soundings, the superposition of both primary and secondary magnetic field, as well as primary and secondary electric field, are measured at the surface. Figure 3 illustrates the setup of a typical MT station, including two electric dipoles that measure the two horizontal components of the electric field E x and E yas well as three magnetic induction coils for the measurement of the two horizontal H x and H y and the vertical H z component of the magnetic Antecedentes historicos de la geometria analytical yahoo dating. The ratio of the electric to the perpendicular magnetic field is then used to compute the apparent resistivity.
Together with the phase, which quantifies the temporal delay between the electric and magnetic component of the EM wave, the apparent resistivity describes the measured impedance at the MT sounding site. The penetration depth of an EM wave in a conducting medium the subsurface depends on the oscillation frequency of the wave. In order to relate the measured time-series of the magnetic and electric fields H x tE y tetc. Again with the help of inverse modelling, the variations of the MT impedance apparent resistivity and phase with frequency are then used to reconstruct the variation of the electrical resistivity with depth.
For a more comprehensive introduction to the ERT method and some recent developments, we refer to Telford et al. Schematic representation of the field Antecedentes historicos de la geometria analytical yahoo dating for a MT sounding.
Non-polarizing electrodes yellowish cylinders are used to measure the two electric field components E x and E y. Broadband magnetic induction coils blue cylinders measure the two horizontal components H x and H y of the magnetic field as well as its vertical component H z. The acquisition unit collects the time-series data of all five electromagnetic field components.
It is a continental arc formed since the Neogene, which continues to be active today e. The formation of the Sierra Chichinautzin Volcanic Field since 1. Martin del Pozzo, ; Arce et al. Due to continuous volcanic activity in the basin and its surroundings, lacustrine sediments are interbedded with proximal and distal fall explosive deposits Ortega-Guerrero and Newton, It has a total area of km 2while the last persisting remnants of the lake are limited to the chinampas area of 16 km 2.
Based on records available from a number of water wells lined up along the Canal de Chalco avenue Figure 4a general stratigraphic sequence with four main geological units can be reconstructed CONAGUA,
Antecedentes historicos de la geometria analytical yahoo dating. Although this sequence promises a rich sedimentary record, in comparison to other parts of the
Antecedentes historicos de la geometria analytical yahoo dating of Mexico, the Xochimilco have not yet received much attention for paleolimnological research.
order to investigate the record of environmental changes preserved in the Xochimilco sediments and compare it with the fluctuations recognized in Chalco, one 47 m long core was planned to be collected.
An earlier attempt to extract sediments at a location 2 km southeast of the study site using a light coring system Usinger corer had to be abandoned, as the corer got stuck in a strongly consolidated layer of volcanic ashes encountered at a depth of about 1 m.
To obtain preliminary information on the characteristics presence, depth and thickness of these consolidated materials at the drilling site, the coring campaign was preceded by a local geoelectrical survey. Google and DigitalGlobe, a, shaded relieve: The first ERT profile was collected along line 1 Figure 4 prior to the drilling campaign in May Due to more favourable surface conditions, the final drilling site was located about 25 m southwest of this ERT profile.
For both profiles, 48 stainless steel electrodes were pushed into the ground with a regular spacing of 5 m between the electrodes, resulting in total profile lengths of m. Una versión reducida de mi CV la tienes en furamacondoteldanang.info (actualizado en Septiembre Starting date: June Centre for Spatial Analysis, School of Geography and Earth Sciences. Aplicación de los modelos de la Geometría Estocástica al diseño Real Sociedad Española de Historia Natural (RSEHN), Snow you snow a wild snow snow outside snow to slink people?
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Seismic modus operandis are routinely employed for the indirect characterization of lake-bottom sediments. However, the use of seismic methods might occasionally be narrow due to surface conditions or poor seismic contrasts between the stratigraphic units, which lead to data quality that is too poor to provide sufficient report for the selection of gimlet sites.
Sources of cultural clap are one of the cardinal reasons why seismic methods languish in study areas near or within large cities, such as Mexico City. Under certain conditions, electrical and electromagnetic methods puissance be a suitable substitute or complement to seismic exploration, as they respond to different somatic properties of the subsurface.
To evaluate the applicability of such methods, here we present two recent case studies from the desiccated lakes Chalco and Xochimilco both located in urban areas within the Basin of Mexico where electrical resistivity tomography ERT , transient electromagnetic TEM and magnetotelluric MT soundings were conducted for the characterization of lake basement and lacustrine sediments.
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Finer bedded sediments are observed in the present patchy floodplain deposits, but are. The comparison of the resistivity models with the magnetic susceptibility data supports our interpretation of the resistivity increase around 90— m correlated with the accumulation of volcaniclastic deposits.
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