Influencing factors of seismic signals generated by un-tuned large volume airgun array in a land reservoir

Un-tuned large volume airgun array in a water reservoir is recently proposed as a new way to generate seismic waves on land. It can be used to explore the earth velocity structure and its temporal variations as well. However, the characteristics of seismic signals (especially far-field signals) from an airgun array in a reservoir and its affecting factors (firing pressure, airgun towing depth, water level of the reservoir, etc.) has not been adequately studied. We analyzed the seismic data collected from field experiments at Binchuan Transmitting Seismic Station in 2011 and 2013 and found that (1) The similarity of seismic signals decrease with distance, which is most likely induced by the decay of signal amplitude and signal to noise ratio (SNR); (2) The amplitudes of far-field airgun signals are almost linearly proportional to the firing pressure; (3) The towing depth of airgun has less effects on the far-field signals; (4) The amplitudes of far-field airgun signals are proportional to the water level of the reservoir.     

Regional Scale Seismological Investigation on the Continent Crust Using Airgun Sources——A Perspective Review

The investigations on the structure and temporal variations of the continent crust have been the long-lasting topics of seismology. The artificial sources play the most important role in these studies for their well-known locations and occur times among all the sources producing elastic waves, so that precise results are obtained. In past decades, we have successfully excited the airgun sources in on-land water bodies such as reservoirs, expanding their applications in marine seismology. Large volume airguns have been used to produce low frequency signals which can travel hundreds of kilometers after stacking, providing a source connecting the exploration seismology and traditional seismology at regional scale. In this review, we introduce the changes through which we turn the airgun into scientific instruments by experiments. We also present the advances on imaging the continental crust structure and monitoring the velocity variations using the highly repeatable signals emitted by airgun sources. The potential applications such as imaging the subsurface of urban areas and high resolution illuminating of mines are proposed and the challenges for further investigations are also discussed.     

祁连山气枪主动震源浮台漂移钢索悬吊控制技术

针对地下介质动态变化监测研究中的信噪比、震源可重复性和波速精确测量等关键问题,利用大容量气枪作为主动震源,构建了一套高性能的主动源探测技术系统,该系统主要由气枪震源和信号接收部分组成。为了解决浮台偏离设定位置的问题 ,提出了浮台漂移控制技术。实验结果表明,该技术可以有效地解决气枪激发后浮台受到水流冲击作用而偏离设定位置的问题。本研究结果可为一定激发场地情况下的气枪主动震源重复探测工作所借鉴。     

利用固定台站分析长江激发气枪信号特征

“地学长江计划”安徽实验是以气枪震源为核心的大型主动源探测实验。通过在长江安徽段20个固定点定点激发气枪震源,结合109个固定台站、11条流动测线组成的观测网络,首次利用主动源实现了对长江流域安徽段约6万km2面积的三维地下结构探测。本文利用固定台站对长江激发气枪信号进行了分析,结果表明,长江中气枪信号激发效果良好,固定台记录中气枪信号可识别的最远距离达300km。对气枪信号绝对振幅的研究结果表明：① 50km处的气枪信号约为10nm量级,200km处的气枪信号小于1 nm;② 气枪信号强度的空间分布存在一定的方位各向异性,可能与长江的几何形状有关;③ 台站背景噪声对于提取气枪信号至关重要,高质量的固定台网为识别nm量级气枪信号提供了可能。     

气枪震源资料反褶积方法及处理流程研究

气枪震源具有极高的可重复性,可用于地下介质变化的监测。但不同工作条件下气枪震源产生的信号会存在细微差异,反褶积方法能在一定程度上消除由震源变化引起的记录信号变化。为了去除气枪震源子波信号,获取气枪源到台站之间的格林函数,通常需要选取一种恰当的方法对地震波形数据进行反褶积处理。频率域水准反褶积和时间域迭代反褶积是在接收函数等领域已被广泛使用的2种反褶积方法。本文以云南宾川主动源资料为例,对比了利用这2种方法处理气枪震源资料的效果,结果表明：在计算效率方面,频率域水准反褶积方法更具优势;在处理结果的信噪比方面,时间域迭代反褶积方法表现更好,P波初至也更清晰。此外,进一步讨论了在多炮资料的处理过程中反褶积和叠加等操作的顺序问题,最后提出了从气枪震源资料中提取气枪源到台站之间的格林函数的一般流程。     

Using an airgun array in a land reservoir as the seismic source for seismotectonic studies in northern China: experiments and preliminary results

This paper reports the field setup and preliminary results of experiments utilizing an airgun array in a reservoir in north China for a seismotectonic study. Commonly used in offshore petroleum resource exploration, the airgun source was found to be more useful than a traditional explosive source for large‐scale and long offset land seismic surveys. The airgun array, formed by four 1,500 in³ airguns (a total of 6,000 in³ in volume) was placed at a depth of 6–9 m into the reservoir to generate the pressure impulse. No direct evidence was found that the airgun source adversely affected the fish in the reservoir. The peak ground acceleration recorded on the top of the reservoir dam 100 m away was 17.8 gal in the horizontal direction; this is much less than the designed earthquake‐resistance threshold of 125 gal for this dam. The energy for one shot of this airgun array is about 6.68 MJ, equivalent to firing a 1.7 kg explosive. The seismic waves generated by the airgun source were recorded by receivers of the regional seismic networks and a temporary wide‐angle reflection and refraction profile formed by 100 short‐period seismometers with the maximum source‐receiver offset of 206 km. The seismic wave signature at these long‐offset stations is equivalent to that generated by a traditional blast source in a borehole with a 1,000–2,000 kg explosive. Preliminary results showed clear seismic phases from refractions from the multi‐layer crustal structures in the north China region. Forward modelling using numerical simulation confirms that the seismic arrivals are indeed from lower crustal interfaces. The airgun source is efficient, economical, environmentally friendly and suitable for being used in urbanized areas. It has many advantages over an explosive source for seismotectonic studies such as the high repeatability that is supreme for stacking to improve signal qualities. The disadvantage is that the source is limited to existing lakes or reservoirs, which may restrict experimental geometry.     

小当量激发的远距离信号检测研究

为了研究利用小当量震源进行远距离探测的可能性,通过现场试验进行了研究.野外实验采用1次大当量激发和16次小当量激发,研究结果表明：①有可能利用数十公斤当量的炸药实现200km距离的探测;②在激发条件相同（或相近）时,不同当量的激发波形滤波后存在一定的相似性;③依据人工震源可重复性采用以下的激发组合方式和处理方法能够有效提高信噪比：针对大当量激发与小当量激发波形的相似性,利用大当量激发的波形作模板与小当量激发的波形作相关,可判断台站是否接受到小当量激发的信号并检测其震相到时信息.针对小当量激发波形的相似性,采用小当量多次激发的方式,通过N次根加权叠加方法,能有效提高记录的信噪比和震相识别精度.     

可控震源匹配扫描方法研究

常规的可控震源Chirp扫描信号的地震响应存在着相关信号旁瓣大、分辨率低的缺点;用二元伪随机编码信号作为可控震源的驱动信号,地震响应剖面中存在严重的相关噪声,降低了地震记录的信噪比.本文提出的可控震源匹配扫描方法,利用匹配伪随机序列偶调制正弦载波信号作为可控震源驱动信号进行扫描激发,得到两个匹配的原始地震记录,对两个匹配的原始地震记录分别解码后再进行叠加形成最终的综合解码地震剖面.地震数值模拟的结果表明,该方法基本不存在常规Chirp扫描情形的旁瓣效应,可有效避免m-序列编码扫描情形的相关噪声,其最后形成的综合解码地震剖面可以与脉冲震源的炮集记录相媲美.     

Testing the ability of surface arrays to monitor microseismic activity

Recently there has been much interest in the use of data from surface arrays in conjunction with migration‐based processing methods for passive seismic monitoring. In this study we use an example of this kind of data recorded whilst 18 perforation shots, with a variety of positions and propellant amounts, were detonated in the subsurface. As the perforation shots provide signals with known source positions and origin times, the analysis of these data is an invaluable opportunity to test the accuracy and ability of surface arrays to detect and locate seismic sources in the subsurface. In all but one case the signals from the perforation shots are not visible in the raw or preprocessed data. However, clear source images are produced for 12 of the perforation shots showing that arrays of surface sensors are capable of imaging microseismic events, even when the signals are not visible in individual traces. We find that point source locations are within typically 45 m (laterally) of the true shot location, however the depths are less well constrained (～150 m). We test the sensitivity of our imaging method to the signal‐to‐noise ratio in the data using signals embedded in realistic noise. We find that the position of the imaged shot location is quite insensitive to the level of added noise, the primary effect of increased noise being to defocus the source image. Given the migration approach, the array geometry and the nature of coherent noise during the experiment, signals embedded in noise with ratios ≥0.1 can be used to successfully image events. Furthermore, comparison of results from data and synthetic signals embedded in noise shows that, in this case, prestack corrections of traveltimes to account for near‐surface structure will not enhance event detectability. Although, the perforation shots have a largely isotropic radiation pattern the results presented here show the potential for the use of surface sensors in microseismic monitoring as a viable alternative to classical downhole methods.     

A comparison between airguns and explosives as wide-angle seismic sources

The relative merits of a 48-gun, 9324 cu. in. (153 litre) airgun array and a 200 kg explosive source are considered for the purposes of long-range (0–400 km) refraction seismic work, with particular reference to traveltime modelling. Theoretical source calculations indicate that in the frequency range 2.5–12.0 Hz, the airgun source will produce an RMS pressure ∼ 8% of that produced by the explosive source and an initial burst pressure ∼17% of that produced by the explosive source. Observed data support these calculations at short ranges and illustrate the greater attenuation of the airgun signal with range due to its lack of very low frequency (< 5 Hz) content. At short offsets, the airgun array provides a preferable seismic source to the explosives, due to densely spaced shots and a consistent waveform resulting in excellent trace-to-trace coherence. With increasing offsets, it may be necessary to stack the airgun data to enhance its signal-to-noise ratio: here we use a 4-fold stack. Large explosive shots, although more powerful, produce a less consistent waveform and are more widely spaced due to operational constraints. The offset at which airguns provide a preferable source is dependent on the ambient noise. This practical comparison of real sources demonstrates that, even without advanced processing, a well-tuned airgun array may provide a preferable source to explosives at offsets up to 160 km, under favourable experimental conditions.     

Study on the Influence of Airgun Excitation Conditions on Airgun Signals and Travel Time Variation Measurements

Because of the different excitation conditions of the airgun source, there will be subtle differences in airgun signals. Travel time variation of airgun signals often mix into source information which can''t fully reflect the evolution of the medium. This article uses the airgun signals from the Binchuan Transmitting Seismic Station to analyze the airgun signal''s characteristics of phase and correlation. We conducted a comparative analysis of the effects of the pair difference method and the deconvolution method on eliminating the influence of the excitation conditions in travel time variation. The results show that:(1) The pressure pulse and its subsequent wave of airgun source wavelet are less affected by excitation conditions that we can use it to obtain high-precision excitation moments. (2) Deconvolution can improve the correlation of the airgun signal. (3) The pair difference method can''t eliminate the influence of excitation conditions in travel time variation. Deconvolution can reduce excitation condition interference and the influence of the excitation condition in the travel time variation after deconvolution of the vertical signal is significantly reduced.     

Seismic airgun exploration of continental crust structures

The active sources generate seismic waves transmitting appropriate through the deep is underground key and can be used to image Abstract high-resolution subsurface structures.Therefore,an seismic source the factor to active source exploration.In order to study the structure of continental crust and its temporal variations,we selected an artificial seismic source generated from releasing air bubbles in water(airgun source hereinafter)out of a variety and of artificial sources like the is explosion,new electronic sparkers,source hammering,eccentric proven vibration,be heavy-duty train vibration,vibroseis etc.Airgun Three source Fixed a type of artificial that have been to environmentally friendly,safe,and highly efficient.Airgun western Signal China Transmission and Stations(FASTS)have been for built a few years ago in Yunnan,Xinjiang,and Gansu provinces in have been continuously them running several years.Seismic waves generated away by the the airgun sources are highly seismic reproducible waves and stacking in of can produce can good seismograms on 1300 stations km far from source,for instance,an produced Xinjiang FASTS be well about recorded 60 nearly away after 5000 stacking,China covering area of 6 million km2 and penetrating down to of a depth of km.Establishing about 10 FASTSs in would enable long-term illuminate continuous subsurface underground structures,monitoring can all 9.6 million km2 of land area.Treating from airgun sky active sources as lanterns to we achieve the situation with"Beidou surveys the and lantern illuminates underground".     

A Comparative Study on the Excitation of Large Volume Airgun Source with Different Combinations in Hutubi, Xinjiang, China

In order to study the excitation of large-volume airgun source with different combinations in Hutubi, Xinjiang,China,we conducted a targeted experiment. The method of time-frequency analysis is used to study the signals recorded by a seismometer on the shore of the excited pool, and it is concluded that different gun combinations will lead to different frequency of bubble pulse signals. Besides, linear combination method is used to analyze the signal-to-noise ratios of signals excited by different gun combinations which was recorded by seismic stations around the airgun source. In order to improve the signal-to-noise ratios, it is more effective to increase the activation energy (the number of excited guns at the same time) than to stack the excited signals with smaller energy repeatedly.     

Characteristics of the Seismic Waves from a New Active Source Based on Methane Gaseous Detonation

Active seismic sources are critical for obtaining high resolution images of the subsurface. For active imaging in urban areas, environment friendly and green seismic sources are required. In present work, we introduce a new type of green active source based on the gaseous detonation of methane and oxygen. When fired in a closed container, the chemical reaction, i.e. gaseous detonation, will produce high pressure air over 150MPa. Seismic waves are produced when high pressure air is quickly released to impact the surroundings. The first field experiment of this active source was carried out in December, 2017 in Jingdezhen, Jiangxi Province, where a series of active sources were excited to explore their potential in mine exploration. In current work, we analyzed the seismic waves recorded by near-field accelerators and a dense short-period seismic array and compared them with those from a mobile airgun source, another kind of active source by releasing high pressure air into water. The results demonstrate that it can be used for high resolution near surface imaging. Firstly, the gaseous detonation productions are harmless CO₂ and water, making it a green explosive source. Secondly, the dominant seismic frequencies are 10-80Hz and a single shot can be recorded up to 15km, making it suitable for local structure investigations. Thirdly, it can be excited in vertical wells, similar to traditional powder explosive sources. It can also act as an additional on-land active source to airgun sources, which requires a suitable water body as intermediate media to generate repeating signals. Moreover, the short duration and high frequency signature of the source signals make it safe with no damage to nearby buildings. These make it convenient to excite in urban areas. As a new explosive source, the excitation equipment and conditions, such as gas ratio, sink depth and air-releasing directions, need further investigation to improve seismic wave generation efficiency.     

Some Thoughts on the Earthquake Science Experimental Site——The Underground Cloud Map

The Western Yunnan Earthquake Predication Test Site set up jointly by the China Earthquake Administration, the National Science Foundation Commission of America, and United States Geological Survey has played an important role in development of early earthquake research work in China. Due to various objective reasons, most of the predicted targets in the earthquake prediction test site have not been achieved, and the development has been hindered. In recent years, the experiment site has been reconsidered, and renamed the “Earthquake Science Experimental Site”. Combined with the current development of seismology and the practical needs of disaster prevention and mitigation, we propose adding the “Underground Cloud Map” as the new direction of the experimental site. Using highly repeatable, environmentally friendly and safe airgun sources, we could send constant seismic signals, which realizes continuous monitoring of subsurface velocity changes. Utilizing the high-resolution 3-D crustal structure from ambient noise tomography, we could obtain 4-D (3-D space + 1-D time) images of subsurface structures, which we termed the “Underground Cloud Map”. The “Underground Cloud Map” can reflect underground velocity and stress changes, providing new means for the earthquake monitoring forecast nationwide, which promotes the conversion of experience-based earthquake prediction to physics-based prediction.     

新疆地震台网对新疆呼图壁大容量气枪震源信号的接收能力及其影响因素分析

利用新疆地震台网资料和功率谱密度法,以台站噪声的功率谱密度中值曲线作为评估台站噪声水平的依据,对6个台站的噪声水平进行了评估。对比不同台站的功率谱密度和气枪震源信号的识别情况发现,通过2000次叠加,可以识别气枪震源信号的台站的噪声水平均低于无法识别气枪震源的台站,噪声功率谱密度最大差值为40dB,最小差值15dB。最后通过对噪声水平的评估,判断部分台站无法识别气枪震源信号的主要原因是台站噪声水平较高造成的。     

气枪震源激发模式及应用

气枪震源是一种重要的人工地震震源。气枪阵列理论的提出,使得气枪阵列设计技术日趋成熟,并能够在石油勘探和地球物理探测中得到更加广泛的运用。气枪震源在不同领域中应用时,需要不同的组合和激发模式,以适应不同的探测要求。加强主脉冲和加强气泡脉冲,是目前两种主要的气枪激发模式。通过比较研究两种气枪激发模式,讨论各种激发模式在激发时间、气枪间距、频率、分辨率等方面的差异,为气枪震源的广泛运用提供依据。     

Weak Seismic Signal Extraction Based on the Curvelet Transform

Seismic signal denoising is a key step in seismic data processing. Airgun signals are easy to be interfered with by noise when it travels a long distance due to the weak energy of active source signal of the airgun. Aiming to solve this problem, and considering that the conventional Curvelet transform threshold processing method does not use the seismic spectrum information, we independently process the Curvelet scale layer corresponding to valid data based on the characteristics of the Curvelet transform of multi-scale, multi-direction and capable of expressing the sparse seismic signals in order to fully excavate the information features. Combined with the Curvelet adaptive threshold denoising the algorithm, we apply the Curvelet transform to denoising seismic signals while retaining the weak information in the signal as much as possible. The simulation experiments show that the improved threshold denoising method based on Curvelet transform is superior to the frequency domain filtering, wavelet denoising and traditional Curvelet denoising method in detailed information extraction and signal denoising of low SNR signals. The calculation accuracy of the relative wave velocity variation of underground medium is improved.     

SEISMIC SOURCES FOR SHALLOW REFLECTION SURVEYING*

A standard seismic reflection profile was shot along a disused railway track at Onley, near Rugby, U. K. Four different seismic sources including explosives, the propane/oxygen gas-gun, the Bolt airgun, and the borehole sparker were used and compared with each other in terms of output energy, penetration and resolution. The results indicated that the resolution of the borehole airgun and the gas-gun was slightly higher than that possible with gelignite. Both these sources had an output energy which was equivalent to 30 g of gelignite. The borehole sparker was only useful for obtaining seismic information on the nearsurface weathered layer, since its output power (1 kJ) was very limited. However, McCann and McCann (1982) used a high-power sparker source (14 kJ) on the nearby Grand Union Canal for a wide angle seismic reflection survey and achieved a maximum penetration of 250 m, which is comparable with the results obtained on land with the seismic sources mentioned above. The seismic reflection profile, which was interpreted in the light of borehole information in the area and the results of McCann and McCann (1982), successfully identified the surface of the Palaeozoic rocks. The problem of detecting the presence of thin, high-velocity layers in a seismic refraction survey without the availability of other information to calibrate the seismic section is also highlighted.     

格尔木地震台阵勘址数据分析与台阵布局设计

为了增强西部地区的地震监测能力, 中国拟在格尔木地区建立一个小孔径地震台阵。 本文对台阵勘址数据进行了噪声与信号的相关性分析, 得到地震监测的最佳台间距。 结果表明, 对于近震和区域震的监测, 该台阵子台间距最好小于500 m, 对于远震的监测, 台站组合间距应为1500～2000 m。 最后将勘址布设的台阵作为初选台阵进行了台阵响应计算, 计算显示, 台阵响应的主瓣在NW-ES方向较窄, 表明对来自该方向事件的慢度分辨率较高; 由于呈“L”型分布, 该初选台阵确定某些方向地震的方位角较好, 但检测其他方向事件的方位角精度不高, 这可以通过台阵校正进行改善; 台阵响应中出现的多处侧瓣是由于子台间距较大造成的。