2. School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China;
3. School of Electronic Information, Yangtze University, Jingzhou 434100, Hubei, China;
4. Yunnan Earthquabe Agency, Kunming 650224, China;
5. Institute of Geophysics and Geodynamics, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210046, China
The airgun is the leading seismic source in the exploration of marine resources, especially for the exploration of offshore oil and gas resources. The airgun source was invented in 1964 by Stephen Chelminski from the Bolt Company in the United States (Chen Haolin et al., 2008). The airgun source releases high-pressure air fleetly in the water to generate seismic waves. During this process, the instantaneous impact on the water body will generate a set of high-frequency pressure pulse waves. When the high-pressure air forms bubbles, they will reciprocatingly shrink and expand and rise, which generates a set of low-frequency bubble pulses (Wang Yunfeng, 1996; Wang Baoshan et al., 2018). Compared with other sources, the airgun source is environmentally friendly and has high repeatability. These advantages have made it the dominant source of marine oil and gas exploration (Liu Xueqin et al., 2017).
The oil and gas industry requires high resolution seismic exploration, which requires an artificial source with high energy and broad frequency band. However, the bubble pulse from the airgun source is a low frequency oscillation and has a long duration (Wang Baoshan et al., 2018), which is detrimental to high resolution exploration. In order to study the signal characteristics of bubble pulses, Ziolkowski A., (1970) and Schulze-Gattermann R., (1972) proposed the theory of bubble attenuation, oscillation period and oscillation model, which described the oscillation process of bubbles in an infinite water body. Based on a good understanding of the bubble characteristics, PGS-Nucleus established the theoretical model of the airgun array and developed the airgun array wavelet simulation software (Li Xuxuan et al., 2012). Most of the airgun source systems for oil exploration are composed of dozens of small airguns with different capacities. The simultaneous excitation of these airguns can effectively suppress the oscillation of the bubble, improve the energy output of the pressure pulse wave, and highlight the characteristics of the seismic wavelet (Lutter W. J. et al., 1999; Fuis G. S. et al., 2003; Chen Haolin et al., 2008; Li Xuxuan et al., 2012).
Besides high-resolution oil exploration, airgun source can also be used for deep structure imaging (Okaya et al., 2002; Qiu Xuelin et al., 2007). Avedik F. et al., (1993) proposed using asynchronous excitation to enhance bubble waves and suppress pressure pulses, in order to obtain sufficient low-frequency energy to detect the oceanic crust structure, which provides a new technical route for the sea-area and sea-land combined detection of crustal structures (Calvert A. J. et al., 2004; McIntosh K. et al., 2005; Melhuish A. et al., 2005). Recently, large volume airgun arrays have also been gradually applied to deep structure detection and medium change monitoring (Zhao Minghui et al., 2004; Chen Yong et al., 2007, 2008, 2017; Wang Baoshan et al., 2018).
Nowadays, large volume airgun arrays are usually composed of 4-6 airguns with the volume of 2000 in3 (Wei Bin et al., 2016; Chen Yong et al., 2017; Jin Zhen et al., 2018). In these arrays, all airguns are excited synchronously. The seismic signals generated are equivalent to a simple superposition of multiple airgun signals, and the signal characteristics mainly reflect the source characteristics of a single airgun (Li Xiaobin et al., 2016). Changes in the excitation mode may change the signal characteristics of the airgun array and broaden the application range of the signal (Avedik F. et al., 1993). Recently, we have developed an airgun array firing system that can excite each airgun with different time delay (Yang Wei et al., 2018). An experiment is performed to verify the signal characteristics of the large volume un-tuned airgun source with the newly developed firing system. This article describes the experimental process and preliminary results.1 EXPERIMENT
The China Earthquake Administration established the world's largest fixed transmitting seismic station in 2010, and generated seismic waves by airgun source excited in the Dayindian Reservoir (25°48′N, 100°30′E), Binchuan County, Dali Bai Autonomous Prefecture, Yunnan Province (Wang Baoshan et al., 2012; Wang Bin et al., 2016). The total storage capacity of the Dayindian Reservoir is 40.852 million m3. The airgun source consists of four 1500LL airguns produced by the Bolt Company in the United States with a volume of 2000 in3. The volume of a single airgun is currently the largest in the international industry, and the working pressure is 15MPa. Fig. 1(a) shows the location of the Binchuan Fixed Airgun Signal Transmission station. During the experiment, 2 sets of three-component short-period temporary seismographs were set up to record the airgun signal. One is located at the permanent seismic station (CFT, Fig. 1(a)) and about 4.6km from the airgun source excitation site, the other is located at the reservoir bank and about 50m from the airgun source excitation point (CKT, Fig. 1(b)). The seismograph used in the experiment consists of a Guralp-40T short-period seismometer (flat frequency response range 0.5-100.0Hz) and a Reftek-130B seismic data digitizer, with the sampling rate is 100 sps.
There are three main types of airgun sources, but the working principle of the excitation is basically the same (Yang Wei et al., 2013). The airgun source has two chambers: a control chamber and a detonating chamber. The high-voltage electric signal is generated by the airgun excitation controller for open the solenoid valve, and then the high-pressure gas in the control air chamber passes through the control air path to make the shuttle lose balance. The shuttle rapidly opens the air outlet under the push of high-pressure air, and the high-pressure air in the detonating chamber is instantaneously released into the water. It can be seen in this process that from the activation of electrical signal to the release of high pressure air, there is an opening action of solenoid valve and a mechanical movement of the shuttle, which can cause mechanical delays. The mechanical delay time of each airgun can be measured by a built-in pressure sensor, which is typically on the order of milliseconds. The mechanical delay is related to many factors (such as temperature, pressure, and maintenance status of the airgun), so the mechanical delay of each airgun may change among different shots. Most airgun array controllers compensate the delay time of each airgun in the next excitation to realize the synchronous excitation of all airguns (Fig. 2(a)). This excitation mode is also used in most large volume airgun array.
In order to achieve the asynchronous excitation of the airgun array, and thus improve the detection and measurement capability for the effective signals after propagate long-distance (Barbier M.G., 1982; Chen Yong et al., 2006; Ge Hongkui et al., 2006), we specially designed and developed a new airgun array firing system (Open Patent No.CN108777582A). Under normal working conditions, the mechanical delay of each airgun varies little. The excitation time is compensated according to the delay of each airgun in the previous excitation, and finally the excitation is performed according to the pre-set time series (Fig. 2(b)). Compared with other controllers, the newly developed firing system can realize any delay excitation within 64s for 4 airguns, and can be extended to the excitation control of arrays with 8, 12 or 16 airguns. In order to verify the accuracy of the developed airgun array firing system and analyze the effects of asynchronous excitation on the airgun array signal, we conducted two phases of experiments. The first phase was on May 22, 2017, the main purpose was to test if the developed firing system can control and excite the airgun array. A comparative analysis of synchronized excitation was also deployed against the internationally renowned Real Time Systems (Hotshot 4.0) for airgun excitation (Fig. 3). The second phase was on August 26, 2017, the main purpose was to test the asynchronous excitation and the impact on airgun array signals. Time series excitation experiments were designed for time intervals of 0ms, 10ms, 100ms and 1000ms (Table 1), respectively. This paper focuses on the analysis and discussion with the experimental data of the second phase.
We obtained almost identical waveforms from two different airgun controllers (Fig. 3). This indicates that the newly developed airgun array firing system can correct the mechanical delay well (Fig. 2). Fig. 4(a) shows the near-field vertical waveform and its time-frequency analysis generated at the time series excitation mode 1. The results are compared with the waveform excited synchronously and its time-frequency characteristics (Fig. 4(b)). It can be seen that in the time series excitation mode 1, the oscillating wave energy of the generated bubbles is stronger than the pressure pulse wave, which is opposite to the result of synchronous excitation. This indicates that the time series excitation effectively reduces the energy of the high frequency signal of the pressure pulse wave.
Figs. 5 and 6 show the vertical component recordings of the simultaneous excitation and the time series excitation at stations CKT and CFT. It can be concluded that: (1) the airgun signals are clearly recorded in the waveforms from both near-field (CKT) and far-field (CFT) recordings. The amplitude of the signal decreases with the increase of the excitation interval. The largest instantaneous amplitude is generated by the airgun synchronous excitation, followed by the time series excitation mode 1, and the smallest is the time series mode 3. (2) In the near-field CKT recording, the maximum amplitude of the synchronous excitation signal is about twice that of the time series modes 1 and 2, and in the far-field CFT recording, the maximum amplitude of the synchronous excitation signal is approximately 1.3 times that of the time series modes 1 and 2.
It can be concluded from Fig. 7 that the dominant frequency of the airgun signal at CFT is 3-5Hz compared with the signal spectrum of the synchronous excitation, time series mode 1 reduces the energy of the high frequency signal (6-18Hz) (Fig. 7(c)), while the energy in the low frequency band (3-5Hz) also decreases slightly (Fig. 7(b)). Time series modes 2 and 3 reduce the energy of the high frequency signal while increasing the energy of the low frequency signal, but the low frequency dominant frequency band becomes narrower and the main frequency is slightly raised.
In order to analyze the effective signal detecting ability of synchronous excitation and different time series excitation modes, a deconvolution analysis is conducted according to the waveform recorded at station CFT about 4.6km from the airgun source excitation point and the near-field CKT recordings (Wang Baoshan et al., 2018; Yang Wei et al., 2018). The formation medium response function remains consistent when excited in different ways. In all deconvolved waveforms (Fig. 8), we can identify the main phases (P-wave and S-wave) and the signal-to-noise ratio of the signal is comparable. This shows that the signal is a little weaker in asynchronous excitation than in synchronous excitation, but the effective signal detection ability is equivalent.
In this paper, we proposed the asynchronous mode with time series for airgun array excitation. Compared with the signal produced by the synchronous excitation mode, the frequency of the pressure pulse wave is reduced in the time series excitation mode. However, the results of the airgun array excitation in time series will be affected by factors such as the repeatability of the source system and the control accuracy of the airgun array excitation.
Since the position of the airgun source and the recording station are fixed, the repeatability of the source system can be obtained by waveform analysis of near-field recording. Fig. 9 shows the near-field waveform of time series excitation mode 1 (Table 1) recorded at station CKT with a distance of about 50m from the airgun site. The waveforms of the three excitations are basically the same, and one of the waveforms is selected as the reference signal (Fig. 9). The correlation between waveforms are calculated by using the waveform correlation method, and the correlation coefficient is above 0.95, which indicates that the waveform generated by time series excitation has high repeatability, and the newly developed airgun array firing system is stable.
The control accuracy of the airgun array firing system can be analyzed by the excitation error (the difference between the pre-set excitation time and the real excitation time) collected from airgun pressure sensor, and the waveform of the near-field CKT recording. Firstly, cross-correlation analysis is performed on the near-field recording waveform excited by time series mode 3. The correlation coefficient of near-field waveforms in multiple excitations are all above 0.95. Pressure sensors are equipped for each airgun, and the pressure variation in the airgun control chamber is collected at a time interval of 0.1ms to modulate the circuit delay for the next excitation. The feedback excitation error is not affected by the position of the airgun array, thus is more accurate. The feedback excitation errors of the four airguns in one of the time series excitations are 0ms, -1.7ms, 0.2ms and 0.5ms, respectively.
The errors between the real excitation time and pre-set excitation time of different airguns were obtained by using the following methods: analyze the excitation accuracy error from the near-field temporary station records, interpolate the recorded data sampling rate to 10000sps, select No.1 airgun excitation waveform of the time series excitation waveform as the reference signal, and conduct cross-correlation analysis with the time series excitation waveform. The calculated excitation errors of the four airguns were 0ms, -5.1ms, 1.5ms and 2.1ms (Table 2), respectively. Since the four airguns are a few meters apart from each other (Fig. 1(b)), the delay error obtained by the near-field CKT recording analysis needs correction. The propagation speed of the seismic wave in the water is 1500m/s, and the delay errors after correction are 0ms, -1.8ms, 0.3ms and 0.5ms (Table 2), respectively. It is basically consistent with the excitation error feedback by the airgun pressure sensor, further illustrating that the developed airgun array firing system has high precision.
There are two possible impacts due to the delay of the excitation time: one directly impacts on the signal, and the other is on the bubble. Although the airgun array can be simplified as single airgun according to the distance between them (Chen Meng et al., 2014), the bubbles may converge to form a larger equivalent bubble during the process of rising, and further strengthen the oscillation period of the bubbles. The dominant frequency of the bubble pulse increases gradually with the excitation interval from Fig. 7(b). This may indicate that the interaction between the bubbles decreases with the increases of the excitation interval, so the main frequency increases slightly.
The experimental results reveal the feasibility of using the new airgun array firing system to achieve asynchronous excitation. However, the airgun array we used in this experiment only consisted of 4 airguns, which limited our ability to modulate the signal to some extent. In the next step, we will carry out more experiments to explore the optimal excitation modes by analyzing more station data and testing more excitation modes.4 CONCLUSION
In view of the waveform characteristics generated by large volume airgun source excited in the onland water body, and considering the demand of long-distance detection for the underground medium change, this paper proposed a working mode of modulating the airgun signal by asynchronous excitation with time series, and carried out an experimental study on the source characteristics. The results are as follows:
(1) A working mode based on the asynchronous excitation of large volume airgun arrays in the onland water body is proposed and implemented. The newly designed and developed airgun array firing system has high precision for time control. The new firing system has stable performance, and can achieve asynchronous excitation on the order of seconds. The time control accuracy is ~0.1 ms.
(2) The asynchronous excitation mode can reduce the high-frequency energy of the pressure pulse. At the same time, it strengthens the oscillation process of the bubble, and correspondingly increases the energy of the low-frequency signal.
(3) The new mode with time series excitation is equivalent to synchronous excitation mode on the detection capability of effective signals. However, further analysis and discussion of experimental data is needed for remote detection.
The results of this paper show that the amplitude and dominant frequency of large volume airgun array can be modulated by excitation with time delay. Next, we can also use numerical simulation and other methods to determine the optional time series excitation mode, which is possible to further modulate the signals to meet different detection needs, and broaden the application range of large volume airgun array.ACKNOWLEDGEMENT
Many colleagues from the Western Yunnan Earthquake Prediction Experimental Field of the China Earthquake Administration, School of Electronic Information of Yangtze University, and the Earthquake Agency of Binchuan County, Yunnan Province participated in the field experiment. We would like to express our heartfelt thanks to them. We also appreciate the valuable suggestions from the review experts and journal editors for the article.
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