深层碳酸盐岩岩溶风化壳溶洞型油气藏地震波场的理论研究

碳酸盐岩洞缝型油气藏是我国乃至世界的重要油气藏类型之一，属典型的复杂非均匀介质。其中，岩溶洞穴型油气藏是最容易发现的。为了解决这类储层的勘探开发及预测问题，需要从理论上对它们的地震波场进行研究。我们利用随机介质模型和非均匀介质弹性波动方程计算了不同高度和宽度的充满流体藏层的合成地震剖面。结果表明当绕射波偏移为珠子状的同相轴时即使远小于四分之一波长的溶洞在常规带宽的地震剖面上也是可分辨的，绕射波是溶洞高度和宽度的函数。我们引入一个宽度-振幅因子。可以用这个宽度-振幅因子从对无限宽溶洞计算的绕射波计算有限宽度的溶洞的绕射振幅。     

A box model of circulation and melting in ice shelf caverns

A simple box model of the circulation into and inside the ocean cavern beneath an ice shelf is used to estimate the melt rates of Antarctic glaciers and ice shelves. The model uses simplified cavern geometries and includes a coarse parameterization of the overturning circulation and vertical mixing. The melting/freezing physics at the ice shelf/ocean interface are those usually implemented in high-resolution circulation models of ice shelf caverns. The model is driven by the thermohaline inflow conditions and coupling to the heat and freshwater exchanges at the sea surface in front of the cavern. We tune the model for Pine Island Glacier and then apply it to six other major caverns. The dependence of the melting rate on thermohaline conditions at the ice shelf front is investigated for this set of caverns, including sensitivity studies, alternative parameterizations, and warming scenarios. An analytical relation between the melting rate and the inflow temperature is derived for a particular model version, showing a quadratic dependence of basal melting on small values of the temperature of the inflow, which changes to a linear dependence for larger values. The model predicts melting at all ice shelf bases in agreement with observations, ranging from below a meter per year for Ronne Ice Shelf to about 25 m/year for the Pine Island Glacier. In a warming scenario with a one-degree increase of the inflow temperature, the latter glacier responds with a 1.4-fold increase of the melting rate. Other caverns respond by more than a tenfold increase, as, e.g., Ronne Ice Shelf. The model is suitable for use as a simple fast module izn coarse large-scale ocean models.     

SEISMIC MODELLING WITH CHANNEL WAVES IN SEAM STRUCTURES INFLUENCED BY MYLONITE ZONES1

Channel waves generated in coal-seams and their reflections from discontinuities are widely used to indicate the tectonic and stratigraphic features of coal deposits, resulting in greater efficiency and safety in coal-mining. In the mining area of Ibbenbüren (F.R.G.) seam structures sometimes contain so-called mylonite zones, which are crushed coal deposits capable of binding gas. If mining hits a mylonite zone this would probably not only reduce output of the mine, but could even cause gas explosions. To investigate the influence of a mylonite zone on the propagation of channel waves, Rayleigh channel wave measurements for 2D analogue models were performed and synthetic seismograms of Love channel waves were calculated. Analogue modelling of the mylonite zone using Rayleigh seam waves within the seam was carried out using a perforation technique. Calculations were made to obtain an estimate of velocity reduction due to perforation. The results agree well with velocity values measured up to a perforation of 25% in a 2D epoxy resin model. Reflected channel wave energy was found by applying dispersion analysis in the case where the impedance reduction between the mylonite seam structure and the undisturbed seam was approximately 5%. The horizontal width of the mylonite structure was detectable from the time lag between reflected channel wave signals from both in-seam borders of the mylonite zone. Resolution of two discrete borders was possible for a width of 1.5 λ's. The influence of a vertical fault, positioned within the mylonite zone, could only poorly be resolved. Numerical model investigations of Love seam waves were concerned mainly with the variation of the horizontal width of the mylonite zone. Mylonite zones with dimensions of the order of 0.4 λ's allow definite statements about their widths from dispersion and spectral analyses. For zones with smaller widths down to 0.2 λ's, it was found that reflectivity and transmissivity analyses give a qualitative criterion for distinguishing a mylonite structure surrounding a fault from a pure fault.     

Sensing underground coal gasification by ground penetrating radar

The paper describes the results of research on the applicability of the ground penetrating radar (GPR) method for remote sensing and monitoring of the underground coal gasification (UCG) processes. The gasification of coal in a bed entails various technological problems and poses risks to the environment. Therefore, in parallel with research on coal gasification technologies, it is necessary to develop techniques for remote sensing of the process environment. One such technique may be the radar method, which allows imaging of regions of mass loss (voids, fissures) in coal during and after carrying out a gasification process in the bed. The paper describes two research experiments. The first one was carried out on a large-scale model constructed on the surface. It simulated a coal seam in natural geological conditions. A second experiment was performed in a shallow coal deposit maintained in a disused mine and kept accessible for research purposes. Tests performed in the laboratory and in situ conditions showed that the method provides valuable data for assessing and monitoring gasification surfaces in the UCG processes. The advantage of the GPR method is its high resolution and the possibility of determining the spatial shape of various zones and forms created in the coal by the gasification process.     

Numerical study of seismic scattering and waveguide excitation in faulted coal seams

Finite‐difference P‐SV simulations of seismic scattering characteristics of faulted coal‐seam models have been undertaken for near‐surface P‐ and S‐wave sources in an attempt to understand the efficiency of body‐wave to channel‐wave mode conversion and how it depends on the elastic parameters of the structure. The synthetic seismograms clearly show the groups of channel waves generated at the fault: one by the downgoing P‐wave and the other by the downgoing S‐wave. These modes travel horizontally in the seam at velocities less than the S‐wavespeed of the rock. A strong Airy phase is generated for the fundamental mode. The velocity contrast between the coal and the host rock is a more important parameter than the density contrast in controlling the amplitude of the channel waves. The optimal coupling from body‐wave energy to channel‐wave energy occurs at a velocity contrast of 1.5. Strong guided waves are produced by the incident S‐sources for source angles of 75° to 90° (close to the near‐side face of the fault). As the fault throw increases, the amplitude of the channel wave also increases. The presence of a lower‐velocity clay layer within the coal‐seam sequence affects the waveguiding characteristics. The displacement amplitude distribution is shifted more towards the lower‐wavespeed layer. The presence of a ‘washout’ zone or a brecciated zone surrounding the fault also results in greater forward scattering and channel‐wave capture by the coal seam.     

THE GRAVIMETRIC DETECTION OF MINING SUBSIDENCE*

A series of gravity measurements were taken over a period of time (c. 150 days each) above two adjacent working coal faces. Precise levelling and gravity measurements were taken along the same profile before, during, and after seam extraction. The observed change in gravity agrees well with the levelling data. The combined data sets illustrate the validity of a simple Bouguer relationship for the gravity gradient. The results of two-dimensional modelling are used to estimate the gravitational effect of the extracted material. A local feature detected only on the gravity signal may be due to an outcropping limestone layer. This controlled experiment demonstrates the possibility of using high-precision gravity measurements as a substitute for levelling, particularly in surveys of large areal extent where the cost of obtaining comprehensive coverage by levelling may be prohibitive.     

THE INFLUENCE OF AN ASYMMETRY IN THE SEQUENCE ROCK/COAL/ROCK ON THE PROPAGATION OF RAYLEIGH SEAM WAVES*

Small offsets in hard coal seams can be detected with the aid of seam (channel) waves. Transmission and reflection of seam waves depend, among other parameters, upon the symmetry properties of the sequence rock/coal/rock. Two typical unsymmetrical sequences are found in European coal deposits: (i) coal seams with roof and floor of differing acoustic impedance and (ii) coal seams interlayered with rock and soil. Two-dimensional analog models with appropriate impedance contrasts are used to study the effect of the unsymmetrical layers upon the propagation of Rayleigh seam waves. Data analysis is based upon amplitude measurements both parallel and perpendicular to the layers and dispersion curves. The effect of unsymmetrical roof (rock 1) and floor (rock 2) was studied with models containing homogeneous coal seams. Leaky mode wave groups with phase velocities (c_R) in the range between the SV-wave velocities (β_{r1, 2}) of the two rock materials, i.e. β_r1≥c_R > β_r2, form a characteristic part of the Rayleigh seam wave signal. Using Knott's energy coefficient calculations it is shown that in that range energy leakage into the surrounding rock by refracted SV-waves is restricted to only one of the two interfaces, namely coal/rock 2. At the other interface, coal/rock 1, all waves are totally reflected. Thus, the high amplitudes of these leaky mode wave groups are explained by “quasi-normal mode” features. The influence of a dirt bed on wave propagation was studied in models where the roof and the floor have the same elastic properties. The maximum thickness of the dirt bed did not exceed 20% of the total seam thickness. The effect of the bed's location within the seam was also investigated. For all recorded normal-mode wave groups either the total seam or the coal layers could be regarded as wave guides. This was shown by the fact that the phases could be associated with the phase velocity dispersion curves calculated for the symmetrical sequence rock/coal/rock. These curves are relevant under the condition that the thickness of the coal layer assumed under the calculation coincides with the thickness of the effective wave guide of the respective wave groups. Wave groups guided in the total seam are not influenced by either the thickness or the position of the dirt bed. On the other hand, for wave groups guided in the coal layers, the quotient of signal amplitudes in the coal layers is influenced by the position of the dirt bed.     

A CROSS-HOLE AND FACE-TO-BOREHOLE IN-SEAM SEISMIC EXPERIMENT AT INVINCIBLE COLLIERY,AUSTRALIA*

An experimental cross-hole and face-to-borehole in-seam seismic survey was carried out at Invincible Colliery in the Western Coalfield of New South Wales. Objectives were to determine propagation characteristics of the Lithgow seam and to establish the infrastructure for seismic mapping hole-to-hole in Australia. The seam supports leaky P-, S- and P-SV-modes. These modes propagate with group velocities (at 60 Hz) of 3.1, 1.5 and 1.2 km/s respectively. Particle motion polarization is well developed, as is dispersion of the SH-mode. Attenuation rates are high. The seam is lossy (Q of approximately 20). Two prominent structures were mapped by mode conversion. One is believed to be a fracture zone, the other a zone of intense roof thrusting. The old workings and a minor strike-slip fault, which intersected raypaths, were found to be relatively transparent to P- and S-waves at 60 Hz. Telemetry delay and shot-break timing errors of the exploder box are significant. The resulting traveltime scatter is reduced by means of a least-squares “statics” procedure. The group velocity estimation algorithm (based on Fourier transform) yields dispersion characteristics which can be matched with theoretical results for a simple model of a coal seam waveguide. The experiment demonstrates the capability to retrieve in-seam seismic data of diagnostic quality over an appreciable distance (2 km). The experience gained in both survey layout and data processing will be beneficial to future seam wave surveying of Australian coal mines.     

Tafoni in the El Chorro area,Andalucia, southern Spain

Using a combination of field, laboratory and micromorphological evidence, this study examines tafoni (singular, tafone) in the El Chorro area of Andalucia, southern Spain, and makes inferences concerning the processes responsible for their formation. Twenty-five tafoni were randomly selected for field examination. The morphology of these cavernous rock domes is characterized by a helmet-shaped outer roof and an arched-shaped cavern, often with a partially overhanging visor; measurements of height, width and depth of the caverns revealed marked variations in size. The presence or absence of lichen cover, surface varnish, overhanging visor, cavern backwall stripes, rock flaking, weathering pits and cavern floor sediments was also noted. Surface hardness values, obtained using a Schmidt hammer, are relatively low but significantly higher on the outer roof of the tafoni than on the inner cavern walls. Analysis of sediment samples collected from the cavern backwalls and floors indicates predominantly sandy textures, alkaline pH values and some base cation enrichment. Micromorphological analysis of thin sections, prepared from undisturbed blocks, reveals large quantities of pore-filling cement, consisting mainly of calcite, mineral grains affected by weathering and pseudomorphic replacement, and dark, rounded nodules with a metallic appearance. In terms of their formation, different processes appear to act on different parts of the landform. On the outer roof surfaces, case hardening, resulting from near-surface cementation and surface varnish development, is dominant. On the inner cavern surfaces, however, core softening, resulting from granular disintegration and flaking, dominates. Exfoliation weathering, running water and wind deflation also appear to play an important role in tafone formation. A phased model of tafone evolution is proposed whereby the features pass through four phases of development–initiation, enlargement, amalgamation and degradation; in the study area there are examples of tafoni in each of these phases. Much of the evidence suggests that the tafoni are actively developing under current environmental conditions. © 1997 by John Wiley & Sons, Ltd.     

大型地下洞室群地震水平加速度传播特征研究

考虑了高山峡谷基岩场地的深度衰减效应,假设地震危险性分析提供的水平峰值加速度的所在位置,选用攀枝花—会理地震波,通过基线校正和高频滤波作为实际地震动输入,利用FLAC3D软件对某水电站大型地下洞室群进行了地震水平加速度传播特性的研究。数值分析结果表明,厂房周围监测点水平加速度时程和输入地震水平加速度时程相似,但存在0.08～0.12s的滞后时差,输入地震动水平峰值加速度向上传播时具有放大效应,在洞室群附近放大两倍左右。所得结果对大型地下洞室群地震动传播特征问题研究具有一定的参考意义。     

正则化方法在无线电波层析中的应用

无线电波透视法是常用的工作面地质构造探测方法之一,目前普遍使用的SIRT方法层析分辨率不高。本文采用约束正则化方法,推导Tikhonov正则化和全变差正则化的最小化问题表达式,讨论影响层析结果的主要因素,对典型理论模型进行了层析成像实验。结果表明:正则化方法具有比SIRT方法更好的分辨率;射线条数越多、噪声水平越低,层析分辨率越高;Tikhonov正则化在正则参数增大时层析结果更光滑,减小时则更贴近异常,全变差正则化与其相反。最后对实际坑透数据进行层析,识别出的异常构造基本吻合已知疑似构造位置,从而说明正则化方法在无线电波透视应用中的可行性。      

THE INFLUENCE OF OBLIQUELY DIPPING DISCONTINUITIES ON THE USE OF RAYLEIGH CHANNEL WAVES FOR THE IN-SEAM SEISMIC REFLECTION METHOD *

The geological sequence rock-coal-rock represents a seismic low-velocity channel. Channel waves generated in a coal seam and their reflections from discontinuities can be used for proving the minability of the seam. To investigate the process of reflection, two-dimensional models of the sequence rock-coal-rock have been investigated by means of the ultrasonic transducer technique. Two-dimensional models have the advantage that the wave field can be observed at an arbitrarily chosen point of the model plate. Thus, by means of these models the direct and the reflected Rayleigh channel wave can be observed along their path of propagation. From the geophysical point of view the various types of discontinuities of a coal seam can be divided into two basic types: one is restricted to the seam, the other includes in addition a fractured zone in the adjoining rock. The investigation of the symmetrical Rayleigh channel wave reflected by a discontinuity yields the following results: For dip angles γ between approximately 90° and approximately 60° the reflectivity is virtually independent of the type of discontinuity. This does not hold for the limiting case of γ= 90° (vertical dip) for which the reflectivity increases with increasing influence of the fractured zone. For dip angles γ between approximately 60° and approximately 40° the reflectivity is still independent of the fractured zone but the shape of the reflected wave deteriorates with increasing influence of the fractured zone. For dip angles γ below approximately 40° the reflected wave deteriorates such that the application of the in-seam seismic reflection method will be difficult or even impossible. The conversion of the direct wave of the symmetrical fundamental mode into a reflected wave of the antisymmetrical fundamental mode has been observed.     

煤矿井下槽波三维数值模拟及频散分析

采用交错网格高阶有限差分法编制了地震波场三维正演模拟软件,设计了基于镜像法原理处理煤矿井下近水平和起伏巷道特殊空间的算法;模拟了煤矿井下含巷道和不含巷道情况下煤层中传播的地震波场,并分析其频散特征.结果发现:由于巷道的影响,巷道壁上产生很强的巷道振型槽波,煤层中则出现了以Love型为主的槽波,据此分析了实际槽波记录的形成机理,研究结果对今后煤矿井下巷道地震超前探测和工作面弹性波透视等具有重要的理论意义和实际价值.     

煤矿冲击地压的微地震监测研究

为了研究煤矿冲击地压与岩层在三维空间破裂之间的关系,进而探索依据岩层破裂规律预测和预报冲击地压的可能性,文中采用自行研制的防爆型微地震定位监测（MS）系统,基于定位原理,监测了山东华丰煤矿冲击地压煤层（四层煤）及其解放层（六层煤）开采过程中的岩层破裂过程和二次应力场分布变化的过程,得到了如下结论：冲击地压的发生与岩层破裂密切相关,四层煤下顺槽处于六层煤顶板破裂区的外边缘时,正处于高应力区内,在此处掘进容易引发冲击地压,必须将六层煤下顺槽位置向实体煤侧移动20 m以上,或将四层煤下顺槽位置内移20 m以上,才能消除四层煤的冲击地压;六层煤和四层煤开采时,工作面前方断层活化的距离分别为250 m和350 m左右,根据这一距离,及时对断层带进行卸压处理,可以消除由断层带引发的冲击地压;监测显示了工作面周围岩层的三维破裂形态和范围,为矿井确定防水煤柱的高度提供了可靠的依据;监测证明了厚层砾岩的破裂、断层活化、采场附近关键层的破裂是引起冲击地压的主要原因,证明了所研制的硬件和定位软件具有较高的精度和实用性,可以在煤矿和边坡、隧道等领域应用.     

LOVE-TYPE SEAM-WAVES IN WASHOUT MODELS OF COAL SEAMS*

The propagation of Love seam-waves across washouts of coal seams was studied by calculating synthetic seismograms with a finite-difference method. Seam interruption, seam end and seam thinning models were investigated. The horizontal offset, the dip of the discontinuities and the degree of erosion served as variable parameters. Maximum displacement amplitudes, relative spectral amplitudes and phase and group slowness curves were extracted from the synthetic seismograms. Both seam interruption and seam thinning reduce the maximum displacement amplitudes of the transmitted Love seam-waves. The degree of amplitude reduction depends on the horizontal offset and the degree of erosion. It is four times greater for a total seam interruption than for an equivalent seam thinning with a horizontal offset of four times the seam thickness. In a seam cut vertically, the impedance contrast between the coal and the washout filling determines the maximum displacement amplitudes of the reflected Love seam-waves. They diminish by a maximum factor of four in oblique interruption zone discontinuities with a dip of maximum 27°, and by a maximum factor of ten in a seam thinning with a degree of erosion of at least 22%. The analysis of the relative spectral amplitudes indicates a preferential transmission of those phases with frequencies below, and a preferential reflection of those phases with frequencies above the first mode Airy-phase. The relative spectral amplitudes of the reflected Love seam-waves show a distinct interference pattern of the waves reflected at both interruption zone discontinuities. The dispersion analysis reveals a flattening of the phase and group slowness curves with increasing frequencies, horizontal offset and degrees of erosion. These results imply that a detection of washouts in-mine will be possible in a frequency range including at least the first mode Airy-phase. An interference pattern and a flattening of the dispersion curve indicate a washout rather than other seam obstructions and leads to an estimate of the washout dimension.     

Comparison of global geopotential models from the champ and grace missions for regional geoid modelling in Turkey

The continuous efforts on establishment and modernization of the geodetic control in Turkey include a number of regional geoid models that have been determined since 1976. The recently released gravimetric Geoid of Turkey, TG03, is used in geodetic applications where GPS-heights need to be converted to the local vertical datum. To reach a regional geoid model with improved accuracy, the selection of the appropriate global geopotential model is of primary importance. This study assesses the performance of a number of recent satellite-only and combined global geopotential models (GGMs) derived from CHAMP and GRACE missions’ data in comparison to the older EGM96 model, which is the underlying reference model for TG03. In this respect, gravity anomalies and geoid heights from the global geopotential models were compared with terrestrial gravity data and low-pass filtered GPS/levelling data, respectively. Also, five new gravimetric geoid models, computed by the Fast Fourier Transform technique using terrestrial gravity data and the geopotential models, were validated at the GPS/levelling benchmarks. The findings were also compared with the validation results of the TG03 model. The tests showed that as it was expected any of the high-degree combined models (EIGEN-CG03C, EIGEN-GL04C, EGM96) can be employed for determining the gravity anomalies over Turkey. In the west of Turkey, EGM96 and EIGEN-CHAMP03S fit the GPS/levelling surface better. However, all the tested GGMs revealed equal performance when they were employed in gravimetric geoid modelling after de-trending the gravimetric geoid model with corrector surface fitting. The new geoid models have improved accuracy (after fit) compared to TG03.     

Multivariate statistical analysis of underground gas storage caverns on groundwater chemistry in Korea

An understanding of groundwater flow and chemistry is important to operate underground storage caverns. Groundwater flow is mainly affected by cavern operating conditions. Groundwater chemistry is modified by disinfection activities for removing possible biological clogging and by mixing with cement pore water. It is important to discern these two effects, because wells affected by the disinfection activities may have hydrological connections with water curtains used to inject the disinfectant. However, it is difficult to separate these two effects using graphical methods because of their similar chemical characteristics. Instead, multivariate statistical analysis, such as principal component analysis (PCA) and factor analysis (FA), can be used. Groundwater samples for chemical analysis were obtained from four surveys in 1999–2000. Based on the results from PCA and FA, it appears that there were temporal variations of seepage water into the propane area when the cavern operation fluctuated, but we could not observe such variation in the butane area. These changes may occur mainly at depth, where water flow is slow and water renewal in the cavern surrounding is limited. Copyright © 2008 John Wiley & Sons, Ltd.     

An application of 4D seismic monitoring technique to modern coal mining

To study the impact of modern coal mining on the overlying formation, a full‐life‐cycle four‐dimensional seismic monitoring study has been carried out. Four seismic data campaigns have been performed using flexi‐bin geometry with square bins, with total duration of 171 days. The four seismic datasets have been processed with the same processing workflow and parameters; major problems such as statics correction, signal‐to‐noise ratio, resolution, and consistency processing are addressed taking into account the geological features of the research area. This guarantees that remaining four‐dimensional differences between the time‐lapse datasets show mostly geological factors due to the coal mining and effects such as surface subsidence. Our four‐dimensional seismic monitoring of modern coal mining shows that mined and unmined areas have significant zoning characteristics; coal mining has a direct impact on the overlying formation. The mining leads to obvious event subsidence, which reflects that overlying formations undergo subsidence during the mining process. The overlying formation appears as two zones called caving zone and fractured zone. We determine the fault dip of the overlying formation at one end of the working face to be 56°or so by calculation and conversion. We also see that, during the coal mining process, over time, the overlying formation has a self‐recovery capability, which gradually strengthens from the roof siltstone upward to the Aeolian sandstone near the surface. The stability of 20‐m coal pillars between working faces displays a strengthening trend and remains safe during the mining process due to both coal seam supporting and formation compaction effects.     

Research of the electrical anisotropic characteristics of water-conducting fractured zones in coal seams

Water flooding disasters are one of the five natural coal-mining disasters that threaten the lives of coal miners. The main causes of this flooding are water-conducting fractured zones within coal seams. However, when resistivity methods are used to detect water-conducting fractured zones in coal seams, incorrect conclusions can be drawn because of electrical anisotropy within the water-conducting fractured zones. We present, in this paper, a new geo–electrical model based on the geology of water-conducting fractured zones in coal seams. Factors that influence electrical anisotropy were analyzed, including formation water resistivity, porosity, fracture density, and fracture surface roughness, pressure, and dip angle. Numerical simulation was used to evaluate the proposed electrical method. The results demonstrate a closed relationship between the shape of apparent resistivity and the strike and dip of a fracture. Hence, the findings of this paper provide a practical resistivity method for coal-mining production.     

Moment tensor inversion for two micro-earthquakes occurring inside the Háje gas storage facilities, Czech Republic

Broadband data from the P?íbram seismological network was used to investigate the source of two earthquakes, with magnitudes M _w ?=?0.2 and 0.4 respectively, occurring in the period of October–November 2009 in the Háje natural gas storage area (Czech Republic). Both events were located inside the limits of the storage area and at depths similar to those of the underground caverns where the gas is stored. We applied an inversion technique using the software ISOLA for moment tensor retrieval in order to assess the source process of both events and recognize whether a significant isotropic component existed that could be interpreted as a possible cavern collapse. We also performed an uncertainty analysis so as to confirm the reliability of the focal mechanism solutions and we controlled the consistency between the inverted focal mechanisms and those calculated using the P-waves first motions. Our results showed that the nodal plane orientation, the centroid depth, and the magnitude remained stable. Furthermore, we calculated synthetic waveforms for collapse-type ruptures and compared them with the original records. The match between the synthetic and the original data was very poor supporting the interpretation of the shear character of the events. The combination of the inversion results, which indicated significant double-couple components and of the synthetic tests, which supported the inexistence of an isotropic component at the source, led to the conclusion that the possibility of rocks falling from the ceiling of the caverns or a cavern collapse is highly unlikely.