RMT and CSRMT equipment
In this section:
In the late 80s - early 90s, a two-channel recorder was created at the hydrogeology center of the University of Neuchâtel to measure the surface impedance (apparent resistivity and impedance phase) of signals of radio transmitters. This recorder is focused on profiling at 3-4 pre-selected frequencies. The overview of the parting panel is shown in Figure 1. All the photos shown here were kindly provided by our colleague from the University of Cologne, Marcus Gurk. There are special digital panels on the recorder, with the help of which the frequencies of target radio transmitter are set with an accuracy of hundreds of Hz. Directly at the point of measurement, a special toggle switch switches between pre-set frequencies. The measurement results in the form of values of apparent resistivity and phase impedance are displayed on the LCD display and recorded in a notebook.
Figure 1. Different versions of the RMT recorder, developed at the Hydrogeology Center of the University of Neuchâtel, Switzerland. The digital panels set the frequency at which the measurement will be made. A switch with the inscription "1, 2, 3, 4" selects the desired set frequency. LCD displays display measurement results.
In the first modification, grounded lines with a length of 1–5 m were used as electric antennas, and the magnetic field was measured with only one small vertical loop (Figure 2, 3). The frequency range of the recorder 10-240 kHz.
Figure 2. CHYN recorder in the field.
Figure 3. This is about finding transmitters and their directions before we start a survey. Kostas Chalikakis is sitting under the tree in Greece rotating the air loop to find (hear) the direction of minimum noise of a radiotransmitter at a given frequency. The ear is more sensitive for finding a minimum rather than a maximum in noise. Frank Bosch is writing down a table with available transmitter frequencies, transmitter directions and signal strength.
In the second modification, measurements of the electric field with the capacitive electrodes were implemented, and the magnetic field was measured both in the horizontal direction and in the vertical direction (Figure 4).
Figure 4. Measurements along the profile by an improved Swiss CHYN PMT recorder with two-component magnetic sensors.
It was this group of researchers that the now generally accepted term "radio magnetotelluric" was first introduced. Although, in essence, this technology and measurement technique did not differ from the VLF-R methodology (Very Low Frequency - Resistivity).
The next important step forward was made by geophysicists from the University of Uppsala, Sweden, Laus Pedersen and Mehrdad Bastani together with Metronix. At the end of the nineties and the beginning of the two thousandths, a five-channel recorder was created for conducting tensor measurements using the PMT EnviroMT method (Figure 5).
Figure 5. Overview of the Swedish recorder EnviroMT.
The EnviroMT recorder was the first recorder for RMT soundings, fully inheriting the measurement ideology of magnetotellurics. The measurements were carried out in the time domain in a wide frequency band. EnviroMT recorder has two frequency ranges: 1-25 kHz with a sampling frequency of 200 kHz and 10-250 kHz with a sampling frequency of 2 MHz. The first low-frequency range is intended for recording signals from a controlled source, also used for the first time in the RMT method by Swedish geophysicists. As a source of EM fields, two orthogonal horizontal magnetic dipoles or vertical frames were used (Figure 6).
Figure 6. Antennas of the controlled source in the form of a horizontal magnetic dipole for the EnviroMT recorder.
The electric field was measured by grounded antennas with a length of 5-10 m. Compact inductive sensors 30 cm long and 7.5 cm in diameter were used to measure the magnetic field (Figure 7).
Figure 7. Magnetic coils for EnviroMT receiver [Bastani, 2001].
One of the significant advantages of the EnviroMT equipment was the creation of a tensor data processing algorithm and the implementation of data processing directly in the field at the measuring point using a personal computer. A photo of the working display of the EnviroMT recorder with calculated sensing curves is shown in Figure 8.
Figure 8. Screen of the EnviroMT recorder.
In the mid-2000s, the first mass-produced recorder for radiomagnetictelluric soundings RMT-F1 was developed in St. Petersburg. Initially, the recorder was four-channel and allowed to register two components of the electric field and two components of the magnetic field to measure the surface impedance in the frequency range 10-1000 kHz with an ADC bit width of 16 bits. The photo of this recorder is shown in Figure 9. The first publication of the results of work with this recorder was in the Near Surface Geopgysics journal (EAGE) in 2008 [Tezkan, Sraev, 2008].
Figure 9. RMT-F1 recorder in the field.
Due to the hardware limitations of that time, the registration of four time series with a sampling frequency of 2.5 MHz was carried out discretely in segments of 16384 samples.
In the late 2000s, the RMT-F1 was upgraded. It received a more convenient waterproof housing, the fifth data acquisition channel for a vertical magnetic component, an extended frequency range of downto 1 kHz and an increased length of a continuous recording segment of up to 65536 samples. Figure 10 shows a photo of the updated RMT-F recorder.
Figure 10. Undated recorder RMT-F.
Figure 11 shows an interesting upgrade of RMT-F recorder by our colleagues from Potsdam.
Figure 11. Photo of improved "mobile" version of RMT-F recorder. Photo provided by Ute Weckmann.
Below you can watch a short video on how the RMT-F recorder works, which I recorded for my colleagues from the Indian Institute of Technology, Kharagpur.
For the modification with a controlled source in St. Petersburg, the GTS-1 transmitter was also developed, which allows you to create a rectangular square wave with a frequency from 0.1 Hz up to 1 MHz, having an output power of 1 kW, a maximum output voltage of 360 V, a maximum output current of 7.5 A. Photo of the GTS-1 transmitter is shown in Figure 12.
Figure 12. Photo of GTS-1 transmitter.
In the period from 2016 to 2019, a recorder of new generation ARMT-5 was developed in St. Petersburg (Figure 12). As the name implies, this recorder is designed to measure the electromagnetic field in both audio and radio frequency ranges, from 0.1 Hz to 1 MHz. The recorder has two ADC fives: 16 bits, purposed for measuring EM fields at 10-1000 kHz with a sampling frequency of 4000 and 400 kHz, and 24 bits, for measuring EM fields at 0.1-10000 Hz, with a sampling frequency of 32 and 4 kHz. The key advantages of the recorder are continuous recording of time series even at 4 MHz without limiting the length of the time series, online visualization of the recorded data in both the time and frequency domains, and built-in robust data processing with plotting transfer functions for data quality control (Figure 13).
Figure 13. An ARMT-5 recorder with an amplifier of electrical channels (bottom left) and a case with two triples of magnetic sensors (right).
Figure 14. Visualization of power spectra (on the left) and transfer functions - impedance and tipper (on the right) in the ARMT-5 recorder.
The ARMT-5 recorder allows to connect two triples of magnetic sensors at the same time to cover a wide frequency range of the recorded magnetic field. Each channel is equipped with a programmable amplifier that allows to amplify the input signal up to one hundred times.