Drag-out effect of piezomagnetic signals due to a borehole: the Mogi source as an example

We show that using borehole measurements in tectonomagnetic experiments allows enhancement of the observed signals. New magnetic dipoles, which vary with stress changes from mechanical sources, are produced on the walls of the borehole. We evaluate such an effect quantitatively. First we formulate a general expression for the borehole effect due to any arbitrary source models. This is valid everywhere above the ground surface as well as within the cylindrical hole. A first-order approximate solution is given by a line of horizontal dipoles and vertical quadrupoles along the central axis of the borehole, which is valid above the ground surface and a slightly away (several tens of cm) from the top of the borehole. Selecting the Mogi model as an example, we numerically evaluated the borehole effect. It turned out that the vertical quadrupoles produce two orders of magnitude more intense magnetic field than the horizontal dipoles. The borehole effect is very local, i.e. detectable only within a few m from its outlet, since it is of the same order or more than the case without a borehole. However, magnetic lines of force cannot reach the ground surface from a deeper portion (>10 m) of a borehole

Electric field polarization around Ioannina VAN station, Greece, inferred from a resistivity mapping

In the summer of 1997, we made a bipole-dipole mapping survey around Ioannina station of VAN (Varotsos, Alexopoulos, and Nomicos), where detection of the pre-seismic electric signal (SES) has been repeatedly reported. Since we had found the characteristic directional properties of the electric field in the previous study, the present study was aimed to examine it by investigating the shallow electric structure around the station. The apparent resistivity tensor was derived from two sets of measured voltages at each receiver position. From a rough sketch of the resistivity tensor distribution, we found that the electric field was enhanced along the direction parallel to the trend of the basin at receivers located in the conductive basin, and perpendicular to it at receivers in the resistive mountainside. Conductance distribution models with thin plates were constructed by using the measured voltages. The results showed that the VAN station is located on the resistive portion near the contact between the conductive and the resistive part. Furthermore, we simulated the apparent resistivity tensor near the VAN station on the inferred conductance distribution model. Although the directional property similar to those of magnetotelluric (MT) and lightning electric field was not reproduced there, we found that the electric field polarization is affected by heterogeneous structure not only around receivers but also around the source. (C) 2000 Elsevier Science B.V. All rights reserved

Evaluation of electric and magnetic field monitoring of Miyake-jima volcano monitoring of Miyake-jima volcanomonitoring of Miyake-jima volcano (Central Japan): 1995-1999

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Drag-out effect of piezomagnetic signals due to a borehole: the Mogi source as an example

We show that using borehole measurements in tectonomagnetic experiments allows enhancement of the observed signals. New magnetic dipoles, which vary with stress changes from mechanical sources, are produced on the walls of the borehole. We evaluate such an effect quantitatively. First we formulate a general expression for the borehole effect due to any arbitrary source models. This is valid everywhere above the ground surface as well as within the cylindrical hole. A first-order approximate solution is given by a line of horizontal dipoles and vertical quadrupoles along the central axis of the borehole, which is valid above the ground surface and a slightly away (several tens of cm) from the top of the borehole. Selecting the Mogi model as an example, we numerically evaluated the borehole effect. It turned out that the vertical quadrupoles produce two orders of magnitude more intense magnetic field than the horizontal dipoles. The borehole effect is very local, i.e. detectable only within a few m from its outlet, since it is of the same order or more than the case without a borehole. However, magnetic lines of force cannot reach the ground surface from a deeper portion (>10 m) of a borehole