Método del Campo Eléctrico Natural

| 28 octubre, 2010 | 1 Comentario
En esta oportunidad quiero compartir con ustedes material bibliográfico sobre el método del Campo Eléctrico Natural ( Apuntes, lista de artículos y tesis), se trata de toda la recopilación que he realizado hasta el momento, espero contar con sus aportes sobre este tema.

Sobre el Método del Campo Eléctrico Natural

Sinónimos: Método del potencial espontáneo, método de la polarización espontánea o método de autopotencial.
En Ingles: Self-potencial, Spontaneous potential.
Abreviado: PE, siglas en ruso de “campo natural” y en español de “potencial o polarización espontánea”.
En ingles: SP, siglas en ingles de “self potencial o Spontaneous potential
Cuando hablamos de Potencial o Polarización espontánea se entiende dos cosas: Primero, sobre el fenómeno físico como tal; segundo, se utiliza también esta expresión para designar el método de prospección que se basa en la medición de fenómeno.

Introducción

El método del Potencial o Polarización espontánea es de campo natural por lo que no precisa de circuito de emisión alguno.  Destaca también este método por ser el más antiguo, ya que su origen remonta a 1815 descubierto por el Ingles Robert Fox quien sugirió el uso de este fenómeno para la prospección de yacimientos minerales, por lo que se le ha considerado (Kunetz, 1966) como “el abuelo de los Geofísicos”, aunque los primeros resultados positivos no se obtuvieron hasta 1913.

Se basa este método en que, en determinadas condiciones, ciertas heterogeneidades conductoras del subsuelo se polarizan, convirtiéndose en verdaderas pilas eléctricas, que originan en el subsuelo corrientes eléctricas. Estas corrientes producen una distribución de potenciales observable en la superficie del terreno, y que delata la presencia del cuerpo polarizado (Ernesto Orellana, 1992).   Si se clavan en el terreno dos electrodos impolarizables, y se conectan a los terminales de un voltímetro sensible, se observará que entre ellos existe una diferencia de potencial.
A pesar de su antigüedad este método sigue siendo utilizado en la actualidad  por ser simple en equipamiento y de fácil ejecución en el campo.  Se utiliza para el descubrimiento de cuerpos conductores, especialmente de yacimientos de sulfuros.

GENERALIDADES
Título: Potencial Espontâneo
Universidade Federal do Espírito Santo – UFES/ALEGRE
Prof. Welitom Borges – 1º Semestre/2009
Contenido:
1. Introdução
2. Ocorrência dos potenciais espontâneos
3. Origem dos potenciais espontâneos
Ejemplos
4. Bombeamento de poços
5. Fluxo de água subterrâneo
6. Deslizamientos
7. Convecção geotérmica
8. Injeção de água
9. Áreas contaminadas
10. Áreas mineralizadas

Título: Metodos Geoeléctricos del campo Natural

Cátedra de Geofísica Aplicada
Facultad de Ciencias Naturales – UNPSJB
17 de Abril, 2009
Contenido:
1. Geoelectricidad
1.1 Prospección Geoelétrica
1.2 Clasificación general de los métodos prospectivos
2. Propiedades electricas de las rocas
2.1 tipos de conductividad
a. Conductividad Electrónica
b. Conductividad iónica
3. Magnitudes eléctricas medibles
4. Resistividad eléctrica de los suelos
5. Comportamiento eléctrico de las rocas
6. Metodos de Campo Natural
6.1 Corrientes Telúricas
6.2 Corrientes magnetotelúricas
6.3 Potencial espontáneo (Self Potential)
a. Origen de los potenciales espontáneos
b. Medición del potencial en superficie
7. Cuestionario básico

Título:Potencial Espontáneo

Contenido
1. Introducción
2. Objetivo del método
3. Principios teóricos básicos
3.1 Potencial Electrocinético
3.2 Estudio del potencial en terrenos no consolidados
3.3 Estudio del Potencial en medios fisurados
4. Equipo requerido
5. Metodología de campo
5.1 Tipos de Configuraciones en la captura de datos
5.1.1 Configuración de gradiente
5.1.2 Configuración de base fija
5.1.3 Configuración multielectródica
5.2 Fenómenos que contaminan nuestras medidas de potencial
5.3 Planificación de la campaña de reconocimiento
5.4 Proceso e interpretación de los datos
6. Ventajas y limitaciones del método

Título: O método do potencial espontâneo (SP) – uma revisão sobre suas causas, seu uso histórico e suas aplicações atuais

Autor: José Domingo Faraco Gallas
Revista Brasileira de Geofisica, Vol. 23(2), 2005
Abstract. This paper synthesizes the results of a compilation and integration of the most important papers and textbooks dealing with the self potencial method (SP), also taking into account the author experienceon the subject. Futhermore, it introduces survey and processing techniques as well as presentation and interpretation modes.
A short description of the SP generation processes, related to both mineral exploration and ground water flow studies, is presented. In the former case, the usually negatives SP anomalies are ascribed to the presence of masive sulphide bodies (electrical conductors), in the case of environmental or engineering studies, the main aplication of the SP method to determine the sense of the gronund water flow.
The occurrence of noises, and the way of eliminating or minimizing them, are considered. the distinct applications of the SP method as well as expected results from diferent situations and possible are also discussed.
Keywords: Self potencial, SP, applied geophisycs.

Título: Spontaneous Potential

UNIVERSITY OF OSLO
Faculty of Mathematics and Natural sciences

Título: The Spontaneous Potential Log

Autor: Dr. Paul glover
Petrophysiscs MSc Course Notes
Contenido
1. Introduction
2. Principles
2.1 Electrochemical Components
a. The diffusion potential (sometimes called the liquid-junction potential)
b. The membrane potential (sometimes called the shale potential)
2.2 Electrokinetic Components
a. The mudcake potential
b. The shale wall Potential
2.3 The Combined Spontaneous Potential Effect
3. Measurement Tools
4. Log Presentation
5. Vertical Resolution and Bed Resolution
6. The Amplitude of the SP Deflection
7. Uses of the Spontaneous Potential Log
7.1 Permeable beds
7.2 Correlation and Facies
7.3 Mineral Recognition
7.4 Calculation of Rw
7.4.1 The Quick-Look Method – Procedure
7.4.2 The Quick-Look Method – Example
7.4.3 The Single Chart Method – Procedure
7.4.4 The Single Chart Method – Example
7.4.5 The Smits Method – Procedure
7.4.6 The Smits Method – Example
7.5 Calculation of Shale Volume

Título: Well Log Interpretation – SP Log

Earth & Environmental Science
University of Texas at Arlington


RELACIÓN DE ARTÍCULOS

Self-potential signals associated with pumping  tests experiments

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, B10203, doi:10.1029/2004JB003049, 2004
E. Rizzo,1  B. Suski, and A. Revil
S. Straface and S. Troisi
Received 26 February 2004; revised 30 June 2004; accepted 12 July 2004; published 7 October 2004.

Abstract. The flow of groundwater during a pumping test experiment is responsible for a measurable electrical field at the ground surface owing to the electrokinetic coupling between the Darcy velocity and the electrical current density. This electrical field can be measured passively with a network of nonpolarizable electrodes connected to a digital multichannel multimeter with a high internal impedance (>10 Mohm). These so-called self-potential signals can be used to track the pattern of groundwater flow in the subsurface. A field test was performed using a set of 53 Pb/PbCl2  electrodes plus an additional electrode used as a unique reference in the field and a set of five piezometers to monitor the position of the piezometric surface. Using appropriate Green’s functions, the electrical response is analyzed in terms of piezometric head distribution. This new methodology, which we call ‘‘electrography,’’ allows visualization of preferential fluid flow pathways and the distribution of heads during pumping test experiments. Using a conditioning technique, this method could allow inversion of the hydraulic conductivity distribution around a pumping well.
INDEX TERMS: 0925 Exploration Geophysics: Magnetic and electrical methods; 1832 Hydrology: Groundwater transport; 5104 Physical Properties of Rocks: Fracture and flow; 5109 Physical Properties of Rocks: Magnetic and electrical properties; 5114 Physical Properties of Rocks: Permeability and porosity; KEYWORDS: self-potential, pumping test, transmissivity
Citation:    Rizzo, E., B. Suski, A. Revil, S. Straface, and S. Troisi (2004), Self-potential signals associated with pumping tests experiments, J. Geophys. Res., 109, B10203, doi:10.1029/2004JB003049.

Self-potential  tomography  applied  to the determination  of cavities

GEOPHYSICAL RESEARCH LETTERS, VOL. 33, L13401, doi:10.1029/2006GL026028, 2006
A. Jardani,  A. Revil,  and J. P. Dupont
Received 19 March 2005; revised 26 April 2006; accepted 2 May 2006; published 1 July 2006.
Abstract. A  3D  tomography  algorithm  of  self-potential (SP) signals is applied for the first time to the localization of subsurface cavities. A  specific application is  made  to  a marl-pit in Normandy (North-West of France). A SP map with a total of 221 (5 m-spaced) measurements shows a negative anomaly with an amplitude of    8 mV associated with the position of the marl pit. To explain these data, we solved the boundary-value problem for the coupled hydro- electric problem associated with the presence of the cavity using  a  finite-element code.  The  numerical  simulations point  out  the  role of  open  conduits in  electrical charge accumulation near the roof of the cavity and the resistivity contrast between the cavity and the surrounding formation. We  applied  successfully  a  SP  tomography  algorithm showing that the roof of the cavity was associated with a monopole charge accumulation due to the entrance of the ground   water  flow   in   a   network   of   open   cracks. Citation:    Jardani,  A.,  A.  Revil,  and  J.  P.  Dupont  (2006), Self-potential   tomography   applied  to   the   determination   of cavities,   Geophys.  Res.  Lett.,  33,   L13401,   doi:10.1029/
2006GL026028.

Detection and localization of hydromechanical  disturbances  in a sandbox using the self-potential method

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 113, B01205, doi:10.1029/2007JB005042, 2008
A. Crespy, A. Revil, N. Linde, S. Byrdina, A. Jardani, A. Boléve, and P. Henry
Received 12 March 2007; revised 8 September 2007; accepted 5 November 2007; published 30 January 2008.
Abstract. Four sandbox experiments were performed to understand the self-potential response to hydro-mechanical disturbances in a water-infiltrated controlled sandbox. In the first two experiments,   0.5 mL of water was abruptly injected through a small capillary at a depth of 15 cm using a syringe impacted by a hammer stroke. In the second series of experiments,    0.5 mL of pore water was quickly pumped out of the tank, at the same depth, using a syringe. In both type of experiments, the resulting self-potential signals were measured using 32 sintered Ag/AgCl medical electrodes. In two experiments, these electrodes were located 3 cm below the top surface of the tank. In two other experiments, they were placed along a vertical section crossing the position of the capillary. These electrodes were connected to a voltmeter with a sensitivity of 0.1 mV and an acquisition frequency of 1.024 kHz. The injected/pumped volumes of water produced hydro- mechanical disturbances in the sandbox. In turn, these disturbances generated dipolar electrical anomalies of electrokinetic nature with an amplitude of few microvolts. The source function is the product of the dipolar Green’s function by a source intensity function that depends solely on the product of the streaming potential coupling coefficient of the sand to the pore fluid overpressure with respect to the hydrostatic pressure. Numerical modeling using a finite element code was performed to solve the coupled hydro-mechanical problem and to determine the distribution of the resulting self-potential during the course of these experiments. We use 2D and 3D algorithms based on the cross-correlation method and wavelet analysis of potential fields to show that the source was a vertical dipole. These methods were also used to localize the position of the source of the hydromechanical disturbance from the self-potential signals recorded at the top surface of the tank. The position of the source agrees with the position of the inlet/outlet of the capillary showing the usefulness of these methods for application to active volcanoes.
Citation:    Crespy, A., A. Revil, N. Linde, S. Byrdina, A. Jardani, A. Boléve, and P. Henry (2008), Detection and localization of hydromechanical disturbances in a sandbox using the self-potential method, J. Geophys. Res., 113, B01205, doi:10.1029/2007JB005042.

Tomography  of the Darcy velocity from self-potential measurements

GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L24403, doi:10.1029/2007GL031907, 2007
A. Jardani, A. Revil, A. Boléve, A. Crespy, J.-P. Dupont, W. Barrash,
and B. Malama
Received 1 September 2007; revised 31 October 2007; accepted 8 November 2007; published 21 December 2007.
Abstract. An algorithm is developed to interpret self-potential (SP) data in terms of distribution of Darcy velocity of the ground water. The model is based on the proportionality existing  between  the  streaming  current  density  and  the Darcy  velocity. Because  the  inverse problem  of  current density determination from SP data is underdetermined, we use Tikhonov regularization with a smoothness constraint based on  the differential Laplacian operator and  a  prior model. The regularization parameter is determined by the L-shape method.  The  distribution of  the  Darcy velocity depends on the localization and number of non-polarizing electrodes and information relative to the distribution of the electrical resistivity of the ground. A priori hydraulic information can be introduced in the inverse problem. This approach is tested on two synthetic cases and on real SP data  resulting  from  infiltration  of  water  from  a  ditch. Citation:    Jardani,  A.,  A.  Revil,  A.  Bole`ve,  A.  Crespy,  J.-P. Dupont, W. Barrash, and B. Malama (2007), Tomography of the Darcy velocity from self-potential measurements, Geophys. Res. Lett., 34, L24403, doi:10.1029/2007GL031907.

Advanced Noninvasive Geophysical Monitoring Techniques

Roel Snieder, Susan Hubbard, Matthew Haney,  Gerald Bawden, Paul Hatchell, André Revil, and DOE  Geophysical Monitoring  Working  Group.
Key Words. time-lapse, deformation,  fluid flow, biogeochemical  processes
Abstract. Geophysical methods can be used to create images of the Earth’s in- terior that constitute snapshots at the moment of data acquisition, In many applications, it is important to measure the temporal change in the subsurface, because the change is associated with deformation, fluid flow, temperature  changes, or changes in material properties, We  present  an overview of how noninvasive geophysical methods can be used for this purpose,  We  focus on monitoring   mechani- cal properties,  fluid transport,  and biogeochemical  processes, and present  case studies that illustrate  the use of geophysical methods for detecting time-lapse changes in associated properties.
First published online as a Review in Advance on February 1, 2007

Redox potential distribution inferred from self-potential measurements
associated with the corrosion of a burden metallic body

Geophysical  Prospecting, 2008, 56, 269–282 doi:10.1111/j.1365-2478.2007.00675.x
J. Castermant,  C.A. Mendonc¸ a, A. Revil,  F. Trolard,   G. Bourrié
and N. Linde
received July 2007, revision accepted October  2007

Abstract. Negative self-potential  anomalies can be generated at the ground  surface by ore bod- ies and ground  water  contaminated   with organic  compounds.  These anomalies  are connected  to the distribution   of the redox  potential  of the ground  water.  To study the relationship  between  redox  and  self-potential  anomalies,  a controlled  sandbox experiment  was performed.  We used a metallic iron bar inserted in the left-hand  side of a thin Plexiglas sandbox  filled with a calibrated  sand infiltrated  by an electrolyte. The self-potential  signals were measured  at the surface of the tank  (at different time lapses) using a pair of non-polarizing  electrodes. The self-potential,  the redox poten- tial, and the pH were also measured inside the tank on a regular grid at the end of the experiment.  The self-potential  distribution  sampled after six weeks presents a strong negative anomaly in the vicinity of the top part of the iron bar with a peak amplitude of −82 mV. The resulting  distributions   of the pH,  redox,  and  self-potentials  were interpreted  in terms of a geobattery  model combined  with a description  of the elec- trochemical  mechanisms  and reactions  occurring  at the surface of the iron bar.  The corrosion  of iron yields the formation  of a resistive crust of fougerite  at the surface of the bar. The corrosion  modifies both the pH and the redox potential  in the vicinity of the iron bar. The distribution  of the self-potential  is solved with Poisson’s equation with a source term given by the divergence of a source current  density at the surface of the  bar.  In turn,  this  current  density  is related  to  the  distribution   of the  redox potential  and electrical resistivity in the vicinity of the iron bar. A least-squares  inver- sion method  of the self-potential  data,  using a 2D finite difference simulation  of the forward  problem,  was developed to retrieve the distribution  of the redox potential.

Inner  structure  of  La Fossa di Vulcano (Vulcano Island, southern Tyrrhenian   Sea, Italy) revealed by high-resolution electric resistivity tomography  coupled with self-potential, temperature,  and CO2  diffuse degassing measurements

JOURNAL  OF GEOPHYSICAL RESEARCH, VOL.  113, B07207, doi:10.1029/2007JB005394, 2008

A. Revil, A. Finizola, S. Piscitelli, E. Rizzo, T. Ricci, A. Crespy, B. Angeletti, M. Balasco, S. Barde Cabusson, L. Bennati, A. Bole`ve, S. Byrdina, N. Carzaniga, F. Di Gangi, J. Morin,  A. Perrone,  M. Rossi, E. Roulleau, and B. Suski.
Received 23 September 2007; revised 8 March 2008; accepted 2 April 2008; published 24 July 2008.
Abstract. La Fossa cone is an active stratovolcano located on Vulcano Island in the Aeolian Archipelago (southern Italy). Its activity is characterized by explosive phreatic and phreatomagmatic eruptions producing wet and dry pyroclastic surges, pumice fall deposits, and highly viscous lava flows. Nine 2-D electrical resistivity tomograms (ERTs; electrode spacing 20 m, with a depth of investigation >200 m) were obtained to image the edifice. In addition, we also measured the self-potential, the CO2 flux from the soil, and the temperature along these profiles at the same locations. These data provide complementary information to interpret the ERT profiles. The ERT profiles allow us to identify the main structural boundaries (and their associated fluid circulations) defining the shallow architecture of the Fossa cone. The hydrothermal system is identified by very low values of the electrical resistivity (<20 W m). Its lateral extension is clearly limited by the crater boundaries, which are relatively resistive (>400 W m). Inside the crater it is possible to follow the plumbing system of the main fumarolic areas. On the flank of the edifice a thick layer of tuff is also marked by very low resistivity values (in the range 1 – 20 W m) because of its composition in clays and zeolites. The ashes and pyroclastic materials ejected during the nineteenth-century eruptions and partially covering the flank of the volcano correspond to relatively resistive materials (several hundreds to several thousands W m). We carried out laboratory measurements of the electrical resistivity and the streaming potential coupling coefficient of the main materials forming the volcanic edifice. A 2-D simulation of the groundwater flow is performed over the edifice using a commercial finite element code. Input parameters are the topography, the ERT cross section, and the value of the measured streaming current coupling coefficient. From this simulation we computed the self-potential field, and we found good agreement with the measured self-potential data by adjusting the boundary conditions for the flux of water. Inverse modeling shows that self-potential data can be used to determine the pattern of groundwater flow and potentially to assess water budget at the scale of the volcanic edifice.

Comment  on ‘‘Rapid fluid disruption:  A source for self-potential anomalies on volcanoes’’

by M. J. S. Johnston,  J. D. Byerlee, and D. Lockner

Department of Hydrogeophysics and Porous Media, European Center for Research in Environmental Geosciences, CNRS-CEREGE, Aix-en-Provence, France
Received 9 July 2001; revised 8 February 2002; accepted 13 February 2002; published 7 August 2002.
INDEX TERMS: 3914 Mineral Physics: Electrical properties; 5114 Physical Properties of Rocks: Permeability and porosity; 5139 Physical Properties of Rocks: Transport properties; 5109 Physical Properties of
Rocks: Magnetic and electrical properties; 1832 Hydrology: Groundwater transport; KEYWORDS: self-potential, electrokinetic, volcano, electric properties, fluid disruption, zeta potential
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B8, 10.1029/2001JB000788, 2002A. Revil

The volcano-electric effect

A. Revil and G. Saracco, P. Labazuy
Received 18 February 2002; revised 6 August 2002; accepted 27 February 2003; published 15 May 2003.
Abstract. The formation of a magmatic intrusion at depth is responsible for the formation of various thermohydromechanical (THM) disturbances including the upsurge of shock waves and diffusion of pressure fronts in the volcanic system. We couple electromagnetic theory (Maxwell equations) and thermoporoelasticity (Biot equations) to look at the ground surface electrical signature of these THM disturbances. The nature of this coupling is electrokinetic, i.e., associated with water flow relative to the mineral framework and the drag of the excess of charge located in the vicinity of the pore water/mineral interface (the groundwater flow disturbance being related here to the THM disturbances in drained conditions). A new set of laboratory data shows that the electrokinetic coupling is very substantial in fractured basaltic and volcaniclastic materials, and in scoria with several hundreds of millivolts of electrical potential gradient produced per megapascal of pore fluid pressure variations. Our theoretical analysis predicts the diffusion of electromagnetic disturbances and quasi-static electrical signals. These signals can be used as precursors of a volcanic eruption. Indeed, electromagnetic phenomena recorded at the ground surface of a volcanic system, once properly filtered to remove external contributions, provide a direct and quasi-instantaneous insight into the THM disturbances occurring in the heart of the volcanic structure prior and during a volcanic event. Tomography of the quasi-static electrical field is discussed and applied to self-potential profiles performed at the Piton de la Fournaise volcano during the preparation phase of the March 1998  eruption.
INDEX TERMS: 0925 Exploration Geophysics: Magnetic and electrical methods; 1832  Hydrology: Groundwater transport; 5109  Physical Properties of
Rocks: Magnetic and electrical properties; 5114 Physical Properties of Rocks: Permeability and porosity;
5139  Physical Properties of Rocks: Transport properties; KEYWORDS: self-potential, geoelectric, forecasting, volcanic activity, tomography, shock wave
Citation:    Revil, A., G. Saracco, and P. Labazuy, The volcano-electric effect, J. Geophys. Res., 108(B5), 2251, doi:10.1029/2002JB001835,   2003.

Geophysical  investigations  at Stromboli  volcano,  Italy: implications for ground water flow and paroxysmal activity

A. Revil, A. Finizola, F. Sortino and M. Ripepe
Accepted 2003 October 28. Received 2003 October 28; in original form 2003 May 22
SUMMARY. Stromboli volcano (Italy) is characterized by a permanent mild explosive activity disrupted by major and paroxysmal eruptions. These strong eruptions could be triggered by phreato- magmatic processes. With the aim of obtaining a better understanding of ground water flow in the vicinity of the active vents, we carried out a set of geophysical measurements along two profiles crossing the Fossa area (through the Pizzo, the Large and the Small Fossa craters). These measurements include electrical resistivity, induced polarization, self-potential, temper- ature and CO2 ground concentration. These methods are used in order to delineate the crater boundaries, which act as preferential fluid flow pathways for the upflow of hydrothermal fluids. The absence of fumarolic activity in the Fossa area and the ground temperature close to 100 ◦C at a depth of 30 cm indicate that the hydrothermal fluids condense close to the ground surface.
Part of this condensed water forms a shallow drainage network (<20 m) in which groundwater
flows downslope toward a perched aquifer. The piezometric surface of this aquifer is located
∼20 m below the topographic low of the Small Fossa crater and is close (<100 m) to the active vents. Electrical resistivity tomography, temperature and CO2  measurements show that this
shallow aquifer separates the underlying hydrothermal body from the ground surface. Further
studies are needed to ascertain the size of this aquifer and to check its possible implications for the major and paroxysmal events observed at the Stromboli volcano.
Key words: fluid flow, CO2 soil concentration, Self-potential, Stromboli, volcanic activity.

Hydrogeological  insights at Stromboli  volcano (Italy) from geoelectrical, temperature,   and CO2  soil degassing investigations

A. Finizola,1,2  A. Revil,3  E. Rizzo,4  S. Piscitelli,4 T. Ricci,5  J. Morin,2,6  B. Angeletti,3
L. Mocochain,3  and F. Sortino1
Received 8 May 2006; revised 15 July 2006; accepted 31 July 2006; published 7 September 2006.
[1]   Finding the geometry of aquifers in an active volcano is important for evaluating the hazards associated with phreato- magmatic  phenomena  and  incidentally  to  address  the problem  of  water  supply.  A  combination  of  electrical resistivity  tomography  (ERT),  self-potential,  C02,  and temperature  measurements  provides  insights  about  the location and  pattern of  ground  water flow at  Stromboli volcano. The measurements were conducted along a NE-SW profile across the island from Scari to Ginostra, crossing the summit (Pizzo) area. ERT data (electrode spacing 20 m, depth  of  penetration  of   200  m)  shows  the  shallow architecture through the distribution of the resistivities. The hydrothermal system is characterized by low values of the resistivity  (<50  S m)  while  the  surrounding  rocks  are resistive (>2000 S m) except on the North-East flank of the volcano where a cold aquifer is detected at a depth of
80 m (resistivity in the range 70 – 300 S m). C02  and temperature measurements corroborate the delineation of the hydrothermal body in the summit part of the volcano while a negative self-potential anomaly underlines the position of the cold aquifer. Citation:    Finizola, A., A. Revil, E. Rizzo, S. Piscitelli, T. Ricci, J. Morin, B. Angeletti, L. Mocochain, and F. Sortino (2006), Hydrogeological insights at Stromboli volcano (Italy) from geoelectrical, temperature, and CO2  soil degassing investigations,  Geophys. Res.  Lett.,  33,  L17304,  doi:10.1029/
2006GL026842.

Streaming electrical potential anomaly along faults in geothermal areas

A. Revil and P. A. Pezard / August 15, 1998

Streaming potential in porous media


A. revil, H. Schwaeger and L. M. Cathles III, P.D. Manhardt / September 10, 1999

RELACIÓN DE TESIS

“APLICACIÓN DEL MÉTODO GEOFÍSICO DEL POTENCIAL ESPONTÁNEO PARA EL ESTUDIO ESTRUCTURAL DEL VOLCÁN MISTI”

Autor: Ing. Domingo Ramos Palomino
Arequipa – Perú
2000

Resumen
El volcán Misti (16°18’, 71°24’, 5822 msnm), es un volcán andesítico activo a cuyas faldas se encuentra la ciudad de Arequipa (2333 msnm) la segunda urbe más importante del Perú. Aunque históricamente sólo se han reportado pequeñas crisis eruptivas con emisiones de cenizas y fumarolas, los estudios geológicos muestran que severos eventos eruptivos han ocurrido durante la construcción de su casi perfecto cono. La geología no puede dar detalles sobre las estructuras internas del aparato volcánico, ni sobre el estado actual del sistema hidrotermal. El propósito del presente trabajo geofísico es de aportar elementos para construir un modelo estructural del volcán Misti. El método del potencial espontáneo o PE estudia las características y establece mapas del campo eléctrico natural en superficie, a fin de definir las relaciones entre la circulación de los fluidos volcánicos, la litología de las formaciones y la presencia de estructuras.
Se ha efectuado más de 157 km de mediciones que han permitido cubrir con detalle los 400 km2 que ocupa el volcán. Los datos se diez (10) perfiles radiales equidistantes han proporcionado la información necesaria para establecer mapas y perfiles de PE. Los resultados muestran claramente una anomalía PE, denominada “A1”, negativa, de amplitud extraordinaria (3881 mV, récord mundial en valores de PE sobre volcanes) que separa dos zonas bien distintas: a) La zona inferior – “hidrogeológica”- del estrato-cono, donde la correlación entre el PE y la altitud es negativa, y que se explica por la electrofiltración originada por el agua de infiltración sobre los flancos del volcán. b) La zona superior – “hidrotermal”- del cono, de correlación PE-altitud positiva que se explica por el flujo ascendente de fluidos hidrotermales. La “Anomalía A1”, casi circular, con diámetros de 5 x 6 km, está situada entre los 4000-4600 m de altitud, y se le interpreta como relacionada a un límite estructural correspondiente a una antigua caldera.
Por otro lado, el análisis de los gradientes del PE vs altitud  (o Coeficientes de electrofiltración Ce) han permitido obtener un mapa donde se observa una distribución de gradientes formando zonas concéntricas, lo cual está muy probablemente relacionado a variaciones de porosidad en las formaciones rocosas. En el sector NE, entre 5,000 y 5,700 m de altitud, los valores Ce sugieren la existencia de una zona de “Anomalía A2” de aproximadamente 1 x 1,5 km de diámetro. Además, los análisis geoquímicos de fumarolas que surgen en las inmediaciones del límite sur de la “Anomalía A2” muestran que éstas contienen una componente magmática. Esta anomalía (A2) es interpretada como la huella de un antiguo cráter o de un pequeño colapso lateral, ahora recubiertos.
Como aplicación práctica inmediata de los resultados de este trabajo, se ha establecido sobre el flanco SO del Misti, un perfil de 52 estaciones fijas de PE a fin de vigilar la variación de la intensidad del sistema hidrotermal. Los primeros resultados que se han obtenido son excelentes y permiten pensar en la aplicación generalizada del método PE a la vigilancia de volcanes activos del sur del Perú.

“MÉTODO DEL POTENCIAL ESPONTÁNEO APLICADO EN EL VOLCÁN UBINAS Y MÉTODOS GEOQUÍMICOS APLICADOS EN VIGILANCIA VOLCÁNICA Y SÍSMICA”

Autor: Ing. Katherine Gonzales Zuñiga
Arequipa – Perú

Resumen
El estrato-volcán Ubinas (16°22’S, 70°54’W; 5672 m.s.n.m.), se encuentra ubicado en la Zona Volcánica de los Andes Centrales, y es considerado como el volcán más activo del Perú, habiéndose registrado hasta 23 pequeñas erupciones desde 1550, entre crisis fumarólicas y emisiones de ceniza.  Actualmente, su actividad visible se limita a emisiones fumarólicas que surgen únicamente en el fondo del actual cráter.  Es considerado como potencialmente peligroso debido a su cercanía a poblados ubicados en el valle adyacente, siendo el distrito de Ubinas el más importante y localizado a sólo 6 Km al SE del cráter.  Una eventual erupción volcánica tendría un efecto catastrófico para los 3500 habitantes que habitan en este valle.
La aplicación combinada de métodos geofísicos como el de Potencial Espontáneo (P.E.), Sísmica y métodos geoquímicos como el análisis de la concentración del CO2  en los gases del suelo, y geoquímica de los fluidos, han permitido lograr un mejor conocimiento sobre la estructura interna y la circulación de los fluidos componentes del sistema hidrotermal del volcán Ubinas.
Estructuralmente, se ha puesto en evidencia: 1) la presencia de una antigua gran caldera de forma casi circular asociada al límite entre la destrucción del Ubinas antiguo y el emplazamiento del Ubinas Moderno, 2) la presencia de un control estructural en profundidad asociado al sistema de fallamiento local de dirección NW-SE y la influencia de este sobre el comportamiento del campo de potencial espontáneo en el volcán Ubinas, y 3) la prolongación del lineamiento de dirección N130º que cruza parte de la caldera del volcán Ubinas.
Asimismo, se ha puesto de manifiesto la presencia de un importante sistema hidrotermal en el cual, la dinámica de los fluidos se ven limitados por la presencia de dos controles estructurales.  El primero de estos dos principales controles estructurales se relaciona a la presencia de la antigua caldera que ha sido puesta en evidencia por los datos de P.E. y que corresponde al límite entre dos unidades geológicas, el Ubinas antiguo y el Ubinas moderno.  Las diferencias litológicas correspondientes a este límite permitirían la circulación de los fluidos en forma de células convectivas al interior del volcán.  El segundo control estructural se refiere a la falla de dirección N130° que cruza parte de la caldera llegando al cráter y mediante el cual se permite la emisión de las fumarolas en el interior del cráter.  La prolongación de esta falla ha sido evidenciada por el gas del suelo (CO2).
La geoquímica de los fluidos del volcán Ubinas, sugiere la mezcla de tres miembros extremos.  Los fluidos volcánicos (VF), se mezclan con aguas frescas (FWR) probablemente asociadas a aguas meteóricas y de infiltración que alimentan acuíferos localizados a cierta distancia del volcán.  Una segunda mezcla se produce entre los fluidos volcánicos provenientes probablemente de la degasificación de un cuerpo magmático más profundo y los componentes de un reservorio clorurado (DCR).
La disposición de los productos volcánicos (especialmente de productos finos y compactos), la falta de emisiones gaseosas externas al cráter activo actual del volcán Ubinas, así como niveles no anómalos de CO2 a lo largo de los flancos, permiten considerarlo como una estructura impermeable.
La ubicación y dimensiones del sistema hidrotermal han sido develados por los resultados del monitoreo sísmico realizado en 1998, que señala que la principal actividad ocurre entre los 2000 y 4800 m.s.n.m. (600-2600 m por debajo de la caldera).  La influencia del sistema que da origen al sistema hidrotermal del volcán Ubinas, se extiende a niveles más regionales y profundos y la presencia de fallamientos en la región explicaría la componente magmática encontrada en las aguas de la Laguna Salinas, la cual está geoquímicamente también relacionada a la fuente-origen de actividad del volcán Ubinas.

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Categoría: Apuntes, Artículos, Monografías, Tesis

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