JOURNAL OF COAL SCIENCE & ENGINEERING (CHINA)

ISSN 1006-9097

pp 648–650

Vol.14 No.4

Dec. 2008

Study on the genesis of karstic collapse column and characteristics of high resolution seismic data in one coal field∗ ZHANG Shao-hong(张绍红)1, LIN Chang-rong(林昌荣)2 ( 1. College of Resource and Environment Engineering, Liaoning Technical University, Fuxin 123000, China; 2. Faculty of Resource and Information Technology, China University of Petroleum(Beijing), Beijing 100083, China )

Abstract On the standpoint of the disaster prevention from water inrush, discussed the genesis and geologic condition of karstic collapse column in one coal field, analyzed the geophysical characteristics of karstic collapse column by using high resolution 3D seismic data. It shows the effective result of the technology of high resolution 3D seismic prospecting in the exploration of the karstic collapse column, and presents some prediction methods and prevention measures. Keywords karst collapse column, high resolution seismic, water inrush, disaster prevention, structure property

Introduction

1

In many coal fields in North China there are a lot of collapse columns because the Ordovician limestones are weather-worn and collapsed below the coal base. The collapse columns destroy the minable coal seams and make the surrounding beds of the mine work-planes unstable, then form the permeable passages, induce the water inrush on the top and base of the coal seams and make great economic losses. Thus it is important to recognize the collapse columns using seismic exploration and to make the prediction and prevention for the disasters caused by collapse columns. The ground geophysical prospecting, especially 3D seismic exploration, has high accuracy and dense control grid, therefore it is the most effective technique to explore the collapse column till present[1 2]. In the last several years many collapse columns were identified by using the 3D seismic data in many coal mines and the data provided the reliable evidence for coal production. The genesis of one collapse column in one coal field of North China is analyzed, and the features on the high resolution seismic data is discussed, more over the prevention is proposed.

The collapse column is referred as to the dissoluble rocks deposited under the coal seams. In the rocks there are karstic cavities produced by the physical and chemical actions, and because of the gravitational action of the above depositions and coalmines the rocks collapse. Because the fallen rock presents like a cylindrical cavity, we name the geological deposition as the “collapse column”. Through a long geologic period the collapse column forms based on the karst development and is controlled by the tectonic structure[2 4]. The survey area is located in the center of one Neopaleozoic coal-forming basin in North China. The area is featured as a fully covered coal field and deposits Neopaleozoic Carboniferous and Permian coalrocks. The two main target coal seams of the area are the coalbed-2 (Permian Shanxi sandstone) and coalbed-8 (Carboniferous Taiyuan sandstone). The buried depths of the two coal seams are less than 700 m deep. The survey area possesses the production condition of the collapse column, that is, it has dissoluble rocks and erosional water, the developed geologic structures and good alternating water-circulation.

∗ Supported by the National Natural Science Foundation of China (2007CB209600) Tel: 86-418-3391682, E-mail: [email protected]

Geologic genesis of the collapse column in the survey area

ZHANG Shaohong, et al. Study on the genesis of karstic collapse

In the earlier period due to the Caledonian Movement the survey area lacks of the depositions from the upper Ordovician through the lower Carboniferous, the sediments including the upper and middle and Carboniferous, Permian, Triassic, and Quaternary are widely deposited on the denudation surface of the middle Ordovician. The lower Ordovician are mainly the dolomite rock and dolomitic limestone, 190 m thick, karst fissure developed, and often seen large karst cavity; the middle Ordovician are mainly the thick limestone and granophyric limestone, 470 m thick, the karst fissure developed, mainly dissolved pore and dissolution, both of which are strong karst aquifers. In the survey area the Ordovician limestone below the Carboniferous Shanxi sandstone (coalbed-8) is a watery fracture and is a principle corrosive water source for the collapse columns both in the coalbed-8 and in the coalbed-2. The later Yanshan Movement and Himalayan Movement further form multiple fractures and faults that act as the water transmitting passage, and strengthen the circulation and drainage of underground water. The rocks with corrosive water are the uppermiddle Cambrian and Ordovician limestone, dolomitic limestone and brecciated limestone, and the Permian sandstone. The karst fractures of Ordovician limestone are the principle watery sediments for the two target coal seams. Because in the survey area the tectonic structure develops and the drainage and alternation of underground water are good, the limestones are gradually corroded and form the karsts. The krasts go through later crustal movements and fall down to produce the collapse columns with the gravitational action from the overlying strata.

2

Characteristics of the collapse column on the seismic data in the survey area

For the 3D high-resolution seismic geometry, the seismic line space is 40 m and the trace space is 20 m so as to disclose the distribution characteristics of the underground seismic wave-field, The trial result shows the parameters meet with the sampling theorm and there are no spatial aliases on the seismic records. The shot depth is designed at 12 m and the dynamite power is 2 kg. The preamplifier gain of seismic recording is 48 dB, seismic record is 2 s long, sample rate is 1 ms, and the CDP interval is 5 m. The wavelet’s dominant frequency is about 70 Hz and is 150 ms long. In the seismic processing we applied full 3D data processing technology, pre-stack depth migration technology and static technique for complex zone, etc., to improve the seismic image quality. The processed seismic profiles show that both the target coalbeds

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have obvious impendence contrast from the surrounding sediments, thus their seismic reflections are strong and have good continuation, easy to be traced. It is beneficial for us to recognize the collapse column using the geophysical characteristic (such as kinetics properties) of these seismic reflections[5]. For example, one big collapse column is discovered by the seismic exploration (Fig.1).

Fig.1

Feature of one collapse column on the seismic profile

In Fig.1 the coalbed-2 reflection (480~500 ms) and coalbed-8 reflection (560~580 ms) have strong amplitude and have good continuation. But for the collapse column the interrupt points have weak amplitudes, wide phase, and low frequency—we can use these subtle differences to define the location the collapse column. Contrasting the collapse column, we can know that the strata become loose and the seismic wave has low transmitting velocity. The fractures and cavities caused by the collapse columns induce the Rayleigh surface-wave suddenly disappear or to produce scattering wave at the location. In general there is an obvious impendence between the coal seams and the surrounding depositions, which can produce the strong seismic reflections. Once the top and base of the coal seams are destroyed by the collapse column, the seismic reflections are interrupted or disappeared on the seismic sections. In the meantime, the destruction of the top of the coalbeds can produce various low-frequency distortions. On the seismic profile with high signal-noise-ratio if the geology scale is large enough to be bigger than or equal to the first Fresnel zone, we can distinguish the collapse column. The geologic model of the collapse column is illustrated on

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Journal of Coal Science & Engineering (China)

Fig.2 by the interpretation of the seismic profiles (Fig.1).

column, and inject cement material in the dam. We should keep enough coal blocks. According to the relevant theory and practical empirical formula we can calculate the height of the proven collapse column and the critical value for the scale of the coal blocks[2 3].

4

Fig.2

Geologic sketch diagram of the collapse column

Based on the analysis of the 3D seismic and geologic data in the survey area, we can know that the collapse column has the following principle properties: ① On the seismic section the inside of the collapse columns presents chaotic ghost reflection and the amplitude is weak. ② Around the collapse column there are many small faults and fractures in small size, and the strike of the fracture is parallel to the tangential direction of the column cylinder. For example there are many small faults and fractures with no obvious down-throw (less than 2 m) for the coalbed-2 deposition. ③ The strata occurrence changes, and the ambient coal seams and other rocks incline toward the center of the collapse column, normally 3~8° and about 15~40 m the affected coverage. ④ Around the collapse column the water trickling often happens，and the duration is sometime long, which is related to the local hydrological condition. ⑤ Around the collapse column the coal seams are oxidized, and the affection range is related to the size of the collapse column, the developmental phase of the fracture and the action of the underground water.

3

The collapse column is the concrete presentation of the sediment after the karst falls. The results show that it is an effective way to recognize the karst column by using 3D high-resolution seismic data in the coal field. In the future we can further utilize the obvious property of travel-time delay caused by the diffraction wave while transmitting the low-velocity zone inside the collapse column to diagnoses whether the karst column is water-filling or not[6]. We can integrate the geologic and geophysical features of the karst column, analyze its genesis and predict the spatial distribution to improve the reliability of its prevention. References [1]

[2]

[3]

Prevention of collapse column

In order to prevent the disaster of water inrush due to the collapse column, we can predict the possible anomaly zone of the collapse column, and predict its whole conformation and its size according to the genesis of the collapse column and the uncovered shape from the top coal seams. Then we can apply the pit radiowave pentration method, seismic and Georadar method as well as drilling method to explore the collapse column[3 4]. We must completely plug the proved water-filling karst column, and we must immediately stop the working on both the coal mining work-plane and the digging plane; more over we should seal off the passage and mining plane by building the water barrier (bulkhead), drill a well and inject cement material to the water-source sediments of the karst column on the ground and underground. We must also build a dam to the mining tunnel for the proved water-unfilling karst

Conclusions

[4]

[5]

[6]

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