An automatic system for data logging and verification of multi-channel, multi-site data.

摘要

William Gilbert announced in 1600 that the Earth is a great magnet. Since that time, numerous methods and techniques have been developed to utilise the many properties of the Earth’s natural magnetic field. One such geophysical technique utilises the time changes in the natural geomagnetic field, which occur with frequencies from kilohertz to periods of the order of one day, to investigate the electrical properties of the Earth. When time changesudwith periods of the order of several seconds to one day areudused, the technique is referred to as the magnetotelluric (MT) method.udThe MT method is a way of determining the distribution of the Earth’s electrical conductivity structure from measurements of the naturally occurring time changes in the electric and magnetic fields on the ground surface. For a complete description of the geo-electromagneticudfield, five orthogonal field components must be recorded. The five data components are referred to as X, Y, H, D and Z, all being functions of time. The first two data components are the potential differences between two points on the Earth’s surface. X is a measure of the induced electric field in the north-south direction, Y is a measure of the electric field in the east-west direction. H, D and Z represent the three orthogonal components of the magnetic field. H is a measure of the magnetic field in the north-south direction, D is a measure in the east-west direction and Z is the vertical component. In practice, a sixth data channel is also recorded. This channel is temperature which is used to correct the instruments response to thermal effects.udThe six-channel MT instrumentation produces analogue output of the field values. These voltages are sampled at a discrete rate that defines the sampling interval simultaneously on all six channels. A discrete six-channel time series, suitable for computer analysis, is thus formed.udThe goal of each field MT system is to obtain such data over a long continuous period of time (say six months) to act as a data base for subsequent analysis. The data base will consist of time variations of the Earth’s electric and magnetic fields recorded every 5 s. Five such field systems are arranged in a spatial array, giving 30 data channels of simultaneous magnetotelluric data. Such an MT-based field monitoring system is to be set up in western Turkey, a region which is seismically active and earthquake prone. The purpose of the experiment is to use the recorded data base to investigate stress-induced changes in the crust of the region which may be detected by the fields recorded.