1.1 Selection of sites and profiles
In the AMTS method observations are usually carried out on pickets along profiles, which are prepared before the beginning of work. The profiles are settled down across of geological structures. Distance between profiles and pickets are depended on prospective sizes of geological structures and task of the work. It is more preferable to use distance between pickets smaller, than between profiles. At realization of detailed work it is possible to recommend using of distance between pickets 50-100 m, and between profiles – 100-200 m and more.
Before measurements it is necessary to be convinced that there is enough space for the installation of electrical and magnetic antennae. It is preferable to execute installation on the comparatively dry and high sites, at the big distance from trees, bushes and marshes. For the operation of GPS receiver the clear view of the sky should be provided.
At realization of work on monitoring for a choice of pickets of long-term observations the complex of works aimed at searching of the most favorable conditions (the minimal influence of industrial noise, optimum position of pickets in relation to faults, optimum thickness of overlaying soil etc.) should be carried out.
1.2 Noise sources and methods of reducing of its influence
For correct measurements the site of work should not be settled down far to sources of industrial noise (electro transfer lines, transport highways, pipelines and pump stations, agricultural and industrial enterprises, radio transmitters etc.).
The noise level of industrial frequency 50 Hz should not exceed 4 millions of ADC codes. Power spectral density (PSD) of signals of this noise should not exceed level of natural fields more than on 60 dB. This excess can be checked on PSD at frequency resolution not worse then 8192 samples on a segment.
Pulse noise (natural or industrial), exceeding 8 millions ADC codes are considered as overloads, and the segments with such pulses are not accepted in spectra calculations.
Noises for the AMTS method are the micro seismic fluctuations stimulating in output of a magnetic coil sensor an additional signal, not having correlations with a signal from an electrical antenna. For reduction of influence of micro vibrations it is impossible to install equipment near to trees. The fluctuation of the roots caused by the fluctuation of the top of the tree is the cause of the noise. Marshland is also highly fluctuated site under the influence of the wind.
If necessary of performance of measurements in a coastal part of seas it is necessary to choose time of day with the minimal excitement of water surface. It should be taken into account that storm blows of the oceanic waves cause micro seismic fluctuations hundred kilometers deep into the continents.
Sounding points may be placed not closer, than in 200 m from highways with intensive movement.
Magnetic coil sensor should be placed in trench by depth of 10-15 cm or, in absence of such possibility, to install on additional support. The best ground for installation of magnetic coil sensors - sand. It is necessary to avoid installation of magnetic coil sensors on soft or swampy ground. In case of lack of necessary ground for installation, magnetic coil sensor should be installed on support providing its stability. Cable connected to the magnetic coil sensor must be poured into the ground to exclude their vibration from a wind.
Noises of groundings electrodes of the electrical antenna can be of two types. The noise can be from resistance contact electrode - ground, which has thermal noise. At resistance less than 4 kOhm noise of electrodes is insignificant.
The second source of noises of electrodes is their polarization. The polarization of electrodes has an effect for frequencies below 10 Hz. In the frequency range of 10 up to 300 Hz using of brass electrodes is possible. To get spectrograms with coherency more than 0,8 for frequencies below 10 Hz special no polarized or leaden electrodes should be used. The type of grounding electrodes is determined in accordance with the results of testing work at the site.
Performance of measurements during a rain is undesirable. At work during rain it is necessary to keep up waterproofing of magnetic coil sensors connectors, to hold a recorder in a dry place. A special attention should be given to absence of moisture on the case preamplifier of the electrical antenna. During a close thunderstorm measurements should be stopped, having disconnected an electrical antenna from preamplifier. It is possible to carry out measurements at distance of 30 km and more up to thunderstorm lightings.
1.3 Installation of the equipment
Before the installation of the equipment it is necessary at each sounding picket to fix the chosen center of the installation. The directions of the installation of electrical and magnetic antennae are selected in advance, usually along the profile or across it. At the installation of the antennae their azimuths are controlled. Magnetic coil sensors should be installed in all the pickets of the profile and the site similarly, without changing azimuths of the flanks of the antennae with a cable and without a cable. The measurements of the azimuth of the magnetic coil sensor are carried out from the flank of the antenna where the cable is connected.
At each sounding point the equipment is installed in the following order:
- at the center of site to arrange a grounding electrode and connect it by a piece of wire to the central plug ("ground") of preamplifier of electrical line;
- to lay wires of electrical line in the chosen direction and connect ends of the line to electrodes;
- to install a magnetic coil sensor in a trench horizontally and across to the electrical line. Distance from the magnetic coil sensor up to the recorder should be not less than 5 m.
- to connect the preamplifier of electrical antenna and the magnetic coil sensor to the recorder. The scheme of the connection of electrical antennas is shown on the top of the preamplifier.
1.4 Grounding of electrical lines
Each arm of a symmetrical electrical line must have its own grounding electrode. Lines, installed along the profile, with the grounding electrodes form receiving dipole Е1. Lines, installed across the profile, with the grounding electrodes form receiving dipole Е2.
Non-polarized or lead electrode should be buried into the hole of 10 – 30 cm depth (below the level of grass roots). The hole should be cleared from stones and the bottom should be covered with hover ground. If the ground is dry, the hole is filled with salt water to get intimate mud mass. The filling with salt water is recommended also for damp ground, but higher requirement to the quality of measurements. After that the electrode is put into mud, get fixed in a stable position and poured in the ground which does not let the mud dry. The connection of the electrical line with the cable of the electrode must be placed out of the hole of the grounding.
The resistance of each arm of an electrical line is measured between the plug of the preamplifier grounding and each plug of the connected electrical line. The resistance should not exceed the value of the first kOhm. The less the resistance of the grounding, the less the level of noise at the connector of the recorder.
1.5 Turning-on / off the recorder
Turning-on of the recorder is made by single long pressing of button POWER up to lighting of all light-emitting diodes. While moving from one picket to another the recorder can be not turned-off. Autonomy operation with the built-in accumulator is not less than two working days. For recharging of the built-in accumulator it is enough to use network power supply during night time. It is reasonable to turn-off the recorder, if the recharging of the built-in accumulator is carried out from the external accumulator 12 V.
Turning-on the built-in screen is made by pressing any button of a keyboard.
After turning-on the recorder and built-in screen it is necessary to pay attention to the information in the status bar at the bottom part of the screen: free space in the store, the state of the accumulator.
For turning-off the recorder it is necessary to press quickly three times button POWER during 5 sec. Unitary short-term pressing of button POWER during data acquisition will lead to stop of acquisition.
1.6 Connection of the GPS receiver
For reliable operation the GPS receiver should be installed in the place with the clear view of the sky. The start of GPS is carried out by the command of the operator – by buttons 5 – 3 – 1 – ENT (mode MANUAL) in the main menu. In case if coordinates are determined, and the verification of time is completed, on LED GPS flash green light is on.
The determination of the coordinates with help of GPS of the equipment АCF-4М is executed at the small-scale reconnaissance field work, which does not require the exact coordinates of points. Large-scale reconnaissance work should be executed at pickets with precise GPS.
1.7 Start of measurements
The data acquisition is carried out according to a schedule. For creation of the schedule in the main menu press button 3, then make switching to the First window of the schedule and set necessary parameters of the data acquisition: duration of the data acquisition, number of frequency range (D1, D2, D3), using of the notch filter, amplification factor of analog part.
In the Second window of the schedule additional parameters of the data acquisition are set: type of the start (by COMMAND or by TIME), delay of the data acquisition, cyclic acquisition for monitoring, period of the acquisition in cyclic mode, date and time of the acquisition at start by TIME. Switching to this window is carried out on button è menu in Figure 6.6 or by pressing consistently buttons 3 - → in the main menu.
In the Third window the selection or creation of the file of the schedule is carried out and the choice of the event for editing is made. Switching to this window is carried out on button è menu or by pressing consistently buttons 3 - → - → in the main menu
At the first picket of the profile it is necessary to be convinced that the mode of the start COMMAND is set. For this purpose in the Second window of the schedule this mode should be set by pressing button 1 (by pressing button 3 to turn-off CYCLIC acquisition).
At the first picket of the profile the numbers of magnetic coil sensor should be set for each channel (the magnetic coil sensor is always connected to the first channel), the number of the preamplifier of the Е-channel, the length of the electric line (10000 сm, if the line from the set of the equipment АCF-4М is used), azimuths of the antennae from the North direction. These parameters during the work at the profile are not to be changed.
Switching to the menu of setting of channel parameters is carried out on button 1 in the main menu. The editing of the parameters of the chosen channel is carried out in the corresponding window. Switching to this window is carried out on buttons 1 … 4 of the menu.
Creation of the name of a data file is carried in the menu. Switching to this menu is carried out on button ç of the main menu. In the name of the data file it is reasonable to include numbers of the profile and picket. The name of the file is displayed in the bottom line of the screen. The figures following the file name indicate the expected size of the data file and free space in the store in MBytes.
The start of the data acquisition is carried out on button ENT (to start the data acquisition). The confirmation of the start of the data acquisition is indicated by turning-off the screen and turning-on of the LED Mode. Within the first minute after the start the recorder carries out its internal technical settings.
On completing of the data collection the screen is automatically turned-on, the graph of coherence between the first and second channel is displayed.
If the data collection stops earlier, it is necessary to check in the Log File the cause of the stoppage. Switching to this window is carried out by button 6 - → in the main menu.
1.8 Data viewing and estimation of the quality
The basic operation mode of the ACF-4M system is registration of time series (TS). Simultaneously with time series the processor of the recorder calculate mutual coherencies, auto and cross spectra on each channel.
For display in the screen curves of apparent resistivity and impedance phase spectrograms are represented in physical values using calibration files of the recorder, preamplifier and magnetic coil sensors. The calibration files of are stored in the Flash-memory of the recorder. In case of absence of calibration files the use of calibration must turned-off by pressing buttons 5 - 1 in the main menu and in the line Use calibration to set NO.
Data viewing is made in the graphic kind automatically right after finish of data acquisition. The graph of coherence between channels E1 and H1 is always shown. The note in the screen NO DATA appears, if non-sufficient number of realization has been stored.
With help of coherence graphs (Е1, Н1 и Е2, Н2) operator can estimate the quality of the measurement. If the quality is good – coherence coefficient within the frequency range 8-200 Hz should exceed the value of 0.95. The lower values of the coherency within the frequency range lower than 8 Hz and higher than 200 Hz may be the result of the simplified algorithm of calculation of the spectra by the built-in processor of the recorder. The processing of TS in the program SM27.exe in the external PC significantly improves the quality of the spectra calculation.
At the auto spectra (button 2 in the window of coherence viewing) Shuman resonances at the frequencies 8, 14, 20, 26 Hz must appear to prove good quality data. The absence of resonance maxima at the autospectrum indicates the influence of noise.
2 MANAGEMENT OF THE RECORDER WITH HOST PC
2.1 General information
Management of the recorder can be realized on-line on PC through Ethernet cable. In field conditions a Notebook computer can be used.
The PC must have the following specifications and software tools:
- PC computer Intel Pentium II 400 MHz or more;
- RAM 128 МB or more;
- OS Microsoft Windows 2000 or higher;
- hard disk of sufficient volume;
- manipulator Mouse;
- parallel port (for data transferring from the recorder);
- parallel cable (Ethernet connector);
- display resolution not less then 1024x768 pixels;
- network card Ethernet – 10 Mbits (10/100 Mbits)
The recorder’s software provides the setting of parameters of the data acquisition, allows executing initial testing and control the work of the recorder, the exchange of the data with host PC, allow the operator to see results of the data acquisition in a graphic form.
2.2 Preparation of the recorder for work with host PC
The preparation includes the next steps:
- installation of the software on host PC (is carried out by loading of the exe program) in accordance with the instruction, stored in the file of format *.txt;
- connection of the recorder to host PC;
- connection of the GPS receiver;
- connection of antennae.
2.3 Connection of the recorder to host PC
Connection of the recorder to host PC is carried out using the Ethernet cable. The connection with host PC is indicated by LED Ethernet of the recorder.
2.4 Checking of the network settings of host PC
Specify the network address in the form of 192.168.0.ХХХ, where ХХХ – any number from 1 to 200, mask of sub network 255.255.255.0.
The order of network setting for Windows XP: Start – Settings – Control board – Network connections – Local network connections – Parameters – to choose Protocol Int. (ТСР/IP) – in the window IP address enter the network address 192.168.0.ХХХ – in the window Mask of sub network specify the mask of sub network 255.255.255.0.
It is necessary to specify the operating mode 10 Мbytes/sec - half-duplex. The order of the setting: Start – Settings – Control board – Network connections – Local network connections – Parameters – button Settings – in the window Parameters choose Line Speed in the list – in the window Meaning choose 10 Мbytes/sec - half-duplex.
