Magnetotelluric sounding, magnetotelluric sounding instruments and field work

When selecting and designing instruments and equipment suitable for field measurement of magnetotelluric field, it is necessary to consider some characteristics of the magnetotelluric field, such as the randomness of the field and the weak signal of the field in most cases, the frequency bandwidth, the large dynamic range and the need for a long observation time. The instrument is required to have sufficient accuracy, reliable stability, strong anti-interference ability, and to be strong, lightweight, less energy-consuming, suitable for unsupervised continuous work of the wax stool.

a) Instruments. The instruments used to determine natural electromagnetic fields generally include three parts: receiver, amplification and filter, and recorder. In addition, it is equipped with a power supply and a calibration system.

The receiver includes: a magnetic signal receiver and an electrical signal receiver. The key to this is a magnetic receiver capable of micromagnetic measurements.

Usually the use of inductive magnetic receiver (also known as magnetic probe) or fluxgate magnetic probe, there are also optical pumps or superconducting magnetic probes, of which the more practical is inductive magnetic probes. Stable lead electrodes (for long-term observation) and various types of non-polarized electrodes are commonly used in electrical receivers.

Analog amplifiers and filters are an important part of the magnetotelluric field signal detection system, and the electrical signal is amplified and filtered and then recorded in analog or digital form. In recent years, various instruments have worked the pre-amplification and filtering part to reduce the noise and temperature drift of the amplifier to improve the resolution of the whole machine.

Recorders have developed rapidly in recent years, from the early optical spot and analog tape recording to notebook microcomputer digital tape recording.

b) Field work.

The layout of the network for magnetotelluric bathymetry is similar to other geophysical surveys, and is mainly determined by the specific requirements of the work task. Bathymetric points are best placed on a flatter area. The surface resistivity of rivers, lakes and swamps is uneven, and it is easy to produce distortion of the electric field. Power stations, radio stations, and larger electrical areas can all generate strong drifting currents underground, and the measurement points should be arranged to avoid them as much as possible.

ExeyHX should be measured in the field

Hyhz five components. In principle, the x and y coordinates can be arbitrarily selected, but for convenience, the magnetic north-south and magnetic east-west are mostly used as the direction of the measurement coordinates. The magnetic probe is located at the measuring point.

The measuring electrodes can be arranged in a cross-shaped, T-shaped, and L-shaped. The distance between the electrodes depends on the strength of the electric field signal, the magnitude of the interference and the sensitivity of the instrument, generally between 100 and 200m, and the maximum can reach 500m. In addition, the magnetic probe should be surrounded by cars, horses and pedestrians as much as possible to avoid interference, and the distance between the instrument vehicle and the instrument vehicle should be more than 50m.

The instrument should be calibrated to determine the actual electric and magnetic field strength values corresponding to the obtained electric and magnetic field signals.

Like terrestrial magnetotelluric bathymetry, marine magnetotelluric bathymetry (ocean MT) also makes use of natural electromagnetic fields (passive sources). Marine MT usually places magnetic signal receivers (magnetic probes) and electrical signal receivers (electrodes) on the seabed. There is no big difference between a magnetic receiver and a land.

The placement of electrodes, due to the difficulty of laying large pole distance electrodes on the seabed, is now gradually using a device called chopper type salt water bridge, which has an electrode distance of only a few meters and can be connected with the instrument box.

Because seawater has good conductivity, when the electromagnetic burial wave is transmitted from the surface of the sea to the seabed, most of its energy is absorbed by the seawater, and its intensity is greatly attenuated, especially the high-frequency part attenuates faster. In Figure 4-4-1, the seafloor electromagnetic field (ESF

and BSF and seawater surface electromagnetic fields (ESS

and the ratio of BSS to the period. where the conductivity of seawater = s m, the conductivity of the seabed rocks bottom.

Take s m (dashed line) and s m (solid line) respectively.

Figure 4-4-1 Attenuation of the horizontal electromagnetic field in seawater.

Similarly, it can be seen from the spectrum of the electric field signal measured on the seafloor (Figure 4-4-2) that the high-frequency portion of the electromagnetic wave has been greatly attenuated due to the absorption of seawater.

Since the movement of conductive seawater in the earth's magnetic field can produce a liquid hail electromagnetic field, this electromagnetic field is a kind of interference relative to the electromagnetic waves that are perpendicular to the earth's surface as the MT field source, which can also be called "pollution" to the MT field source. The frequency band and degree of this "pollution" are related to the spatial wavelength and time period of seawater movement, and the geomagnetic field strength of the geographical location.

As a result, the frequency bands that can be utilized by marine MT are greatly limited compared to terrestrial. At present, marine MT work is mainly used to study the lithosphere and asthenosphere at depths of several hundred kilometers. It is the only method to date to study the electrical structure of the seabed below 30 km.

The results show that there is an electrical abrupt surface at a depth of 50 100 km below the seafloor, with a low conductivity layer above it and a high conductivity layer below it, and the conductivity changes by s m. Due to its high lateral resolution, MT is also a useful tool for the study of mid-ocean ridges with uneven electrical levels due to hydrothermal changes.

Figure 4-4-2 Spectrum of electric field signals measured on the seabed.