1. sampling interval δ t and high cutoff frequency fN
The sampling interval Δ t affects the frequency spectrum and resolution of recorded seismic waves. By sampling theorem
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The smaller δt is, the more seismic wave samples are collected per unit time, which is more beneficial to high frequency information. Of course, the smaller δt is, the more accurate the waveform can be, but it is often limited by the working speed of A/D in digital instruments. For a seismograph, the range of Δ t is limited, and there can be multiple gears to select Δ t. Determine δt size to ensure data processing.
The principle is that the highest frequency component of the required wave can be recovered. FN is the cutoff frequency of the anti-aliasing filter, which filters out high-frequency interference signals, and the size of FN is determined by the following formula:
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It is not difficult to see that fN and sampling interval Δ t are interrelated.
2. Preamplifier gain
It is a fixed gain and has two gears to choose from. The maximum allowable input amplitude of the instrument is determined by the range of A/D converter and the minimum total gain of the instrument. For example, the range of A/D converter is 10V, and the effective values of the maximum allowable input amplitude are 55mV and 220mV when the fixed gains are 27 and 25. When the input signal exceeds this allowable value, the A/D converter will overflow, limiting the output of the acquisition circuit and causing signal distortion. When the seismic signal is strong, a small fixed gain must be adopted. The higher the fixed gain of the digital seismograph preamplifier, the smaller the input noise of the instrument (referring to the noise generated by the instrument itself), and the stronger the ability of the instrument to receive deep weak signals. Generally speaking, if the target layer of exploration is deep, the fixed gain of preamplifier should be high.
3. Preamplifier filter module
Is the choice of low cut-off filtering frequency, there are several blocks to choose from. The frequency range setting of each block depends on the specific situation. Generally, the selection should be based on the spectral characteristics of effective waves and interference waves.
4. Record length
The recording length has multiple tracks, namely 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,1,12, …, +∞. The determination of recording length is based on the depth of exploration target layer. The deeper the exploration, the longer the recording length, depending on the specific situation.
(B) broadcast factors
Playback factors refer to the factors needed to display monitoring records on the spot.
(1) playback gain path selection. Is to choose the gain curve.
(2) Selection of initial gain. Is to select the initial gain of digital automatic gain control. There are many gears to choose from.
(3) Slope. Refers to the release time of adjacent tracks. The selection of this item should conform to the observation system and be determined according to the time difference between tracks.
(4) The starting time of release. It is based on the arrival time of the first break wave near the shot point and is used in combination with the slope.
(5) playback filtering. There are Qualcomm filtering and low-pass filtering. Select the low cut-off frequency of Qualcomm filtering and the high cut-off frequency of low-pass filtering. You can also choose pure wave playback.
(6) Play mode. Includes "AGC on", "AGC off" and "test" to play back the recording.
In addition, there are output adjustment (used to adjust the amplitude of display signal at the same time), compression delay time and recovery delay time, threshold and other factors.
The setting of instrument coefficient should be based on the specific situation of the instrument type used by the production unit.