High resolution electromagnetic methods and low frequency dispersion of rock conductivity

The influence of frequency dispersion of conductivity (induced polarization) of rocks on the results of electromagnetic (EM) sounding was studied on the basis of calculation of electric field of vertical magnetic dipole above horizontally layered polarizable sections. Frequency dispersion was approximated by the Debye formula. Polarizable homogeneous halfspace, two, three and multilayered sections were analyzed in frequency and time domains. The calculations for different values of chargeability and time constants of polarization were performed. In the far zone of a source, the IP of rocks led to quasi-wave phenomena. They produced rapid fluctuations of frequency and transient sounding curves (interference phenomena, multireflections in polarizable layers). In the case of transient sounding in the near zone of a source quasistatic distortions prevailed, caused by the counter electromotive force arising in polarizable layers which may lead to strong changes in transient curves. In some cases quasiwave and quasistatic phenomena made EM sounding curves non-interpretable in the class of quasistationary curves over non-dispersive sections. On the other hand, they could increase the resolution and depth of investigation of EM sounding. This was confirmed by an experience of "high-resolution" electroprospecting in Russia. The problem of interpretation of EM sounding data in polarizable sections is nonunique. To achieve uniqueness it is probably necessary to complement them by soundings of other type

High resolution electromagnetic methods and low frequency dispersion of rock conductivity

The influence of frequency dispersion of conductivity (induced polarization) of rocks on the results of electromagnetic (EM) sounding was studied on the basis of calculation of electric field of vertical magnetic dipole above horizontally layered polarizable sections. Frequency dispersion was approximated by the Debye formula. Polarizable homogeneous halfspace, two, three and multilayered sections were analyzed in frequency and time domains. The calculations for different values of chargeability and time constants of polarization were performed. In the far zone of a source, the IP of rocks led to quasi-wave phenomena. They produced rapid fluctuations of frequency and transient sounding curves (interference phenomena, multireflections in polarizable layers). In the case of transient sounding in the near zone of a source quasistatic distortions prevailed, caused by the counter electromotive force arising in polarizable layers which may lead to strong changes in transient curves. In some cases quasiwave and quasistatic phenomena made EM sounding curves non-interpretable in the class of quasistationary curves over non-dispersive sections. On the other hand, they could increase the resolution and depth of investigation of EM sounding. This was confirmed by an experience of "high-resolution" electroprospecting in Russia. The problem of interpretation of EM sounding data in polarizable sections is nonunique. To achieve uniqueness it is probably necessary to complement them by soundings of other type

Magnetotelluric monitoring of geodynamic processes

Electromagnetic (EM) monitoring of geodynamic processes can be based on the two different seismo-electrical phenomena: a change in resistivity of some geological cross-sections and a generation of EM fields of internal (geodynamic) origin. Continuous observation of the natural magnetotelluric (MT) field simultaneously provides information on both such phenomena. The transfer functions between components of the MT field reflect geoelectrical section and the residual field includes the EM field of internal origin. Their variations in time give independent information on geodynamic processes. The transfer functions and the residual field can be determined by known deterministic methods, although it appears more convenient to apply the methods of adaptive data processing. They elicit information on both phenomena in real time. Continuous MT observations were carried out at the Bishkek geodynamic testing ground (Kirgizia) during 1993. Their results show how informative MT monitoring is

Time-Domain Modeling of Three-Dimensional Earth's and Planetary Electromagnetic Induction Effect in Ground and Satellite Observations

Electric currents induced in conductive planetary interiors by time-varying magnetospheric and ionospheric current systems have a significant effect on electromagnetic (EM) field observations. Complete characterization of EM induction effects is difficult owing to nonlinear interactions between the three-dimensional electrical structure of a planet and spatial complexity of inducing current systems. We present, a general framework for time-domain modeling of three-dimensional EM induction effects in heterogeneous conducting planets. Our approach does not assume that the magnetic field is potential, allows for an arbitrary distribution of electrical conductivity within a planet, and can deal with spatially complex time-varying current systems. The method is applicable to both data measured at stationary observation sites and satellite platforms, and enables the calculation of three-dimensional EM induction effects in near real-time settings.Applied Geophysics and Petrophysic

Magnetotelluric monitoring of geodynamic processes

Electromagnetic (EM) monitoring of geodynamic processes can be based on the two different seismo-electrical phenomena: a change in resistivity of some geological cross-sections and a generation of EM fields of internal (geodynamic) origin. Continuous observation of the natural magnetotelluric (MT) field simultaneously provides information on both such phenomena. The transfer functions between components of the MT field reflect geoelectrical section and the residual field includes the EM field of internal origin. Their variations in time give independent information on geodynamic processes. The transfer functions and the residual field can be determined by known deterministic methods, although it appears more convenient to apply the methods of adaptive data processing. They elicit information on both phenomena in real time. Continuous MT observations were carried out at the Bishkek geodynamic testing ground (Kirgizia) during 1993. Their results show how informative MT monitoring is