Height of the Fractured Zone in Coal Mining under Extra-Thick Coal Seam Geological Conditions

The height of the fractured zone caused by coal mining is extremely significant for safely mining under water, water conservation, and gas treatment. At present, the common prediction methods of overburden fractured zone height are only applicable to thin and medium-thick coal seams, not suitable for thick and extra-thick coal seams. In order to determine the overburden fractured zone distribution characteristics of extra-thick seam mining, failure process analysis method of overlying strata was proposed based on key strata theory. This method was applied to 15 m coal seam of Tongxin coal mine, and fractured zone height was determined to be 174 m for 8100 panel. EH4 electromagnetic image system and borehole televiewer survey were also conducted to verify the theory results. The distribution of the electrical conductivity showed that the failure height was 150–170 m. Observation through the borehole televiewer showed that the fractured zone height was 171 m. The results of the two field test methods showed that the fractured zone height was 150–171 m, and it was consistent with the theory calculation results. Therefore, this failure process analysis method of overlying strata can be safely used for other coal mines.

Critical Height of Unfilled Zone in Underlying Gob Piles

To address problems associated with mining above gob piles in the Chinese midwest mining area, a mechanic model for a coal pillar was established based on the displacement variation method. Additionally, the effect of critical unfilled zone height in the underlying gob (UZHUG) on the coal pillar was investigated through a 3DEC numerical simulation; fieldwork was conducted with a borehole televiewer to measure the UZHUG in the Yuanbaowan Coal Mine. The results indicated that the shear stress value and distribution layout in the coal pillar were affected by the UZHUG. The larger the UZHUG, the greater the shear stress and the wider the distribution scope, moving from two sides to the middle zone. Moreover, the larger the UZHUG, the smaller the maximum bearing capacity of the coal pillar, and subsequently, the greater the horizontal deformation of the coal pillar and roof convergence. The critical UZHUG is 2 m considering coal pillar deformation and stress transfer characteristics. Field measurements have confirmed that the UZHUG of 2 m ensured safe mining in the No. 6 coal seam. However, a second round of filling is needed when achieving a UZHUG of 4.4 m. This study serves as a reference for safe mining above gob piles.

Numerical Analyses on Deformation and Failure Characteristics of Surrounding Rock of Gob-Side Entry Retention by Roof Cutting

Universal distinct element code (UDEC) is a simulation software based on the discrete element method, widely used in geotechnical mining. However, in the UDEC, when simulating large-scale excavation, the subsidence of the fractured zone is almost equal to the mining height, which makes the deformation value calculated in the study of gob-side entry retention too large. To solve this problem, in this paper, the double-yield constitutive model is applied to the whole caving zone to analyze the deformation and failure characteristics of surrounding rock along gob-side entry retaining by roof cutting. The results of the simulation are in good agreement with the result of drilling peeking (drilling observation by borehole televiewer) and field condition (observation and measurement in the field). Finally, by using this numerical method, the effects of roadway width, temporary support, and coal side support on the failure of the roof and the arc coal side are studied.

Statistical methods of fracture characterization using acoustic borehole televiewer log interpretation

©2017. American Geophysical Union. All Rights Reserved. Acoustic borehole televiewer (BHTV) logs provide measurements of fracture attributes (orientations, thickness, and spacing) at depth. Orientation, censoring, and truncation sampling biases similar to those described for one-dimensional outcrop scanlines, and other logging or drilling artifacts specific to BHTV logs, can affect the interpretation of fracture attributes from BHTV logs. K-means, fuzzy K-means, and agglomerative clustering methods provide transparent means of separating fracture groups on the basis of their orientation. Fracture spacing is calculated for each of these fracture sets. Maximum likelihood estimation using truncated distributions permits the fitting of several probability distributions to the fracture attribute data sets within truncation limits, which can then be extrapolated over the entire range where they naturally occur. Akaike Information Criterion (AIC) and Schwartz Bayesian Criterion (SBC) statistical information criteria rank the distributions by how well they fit the data. We demonstrate these attribute analysis methods with a data set derived from three BHTV logs acquired from the high-temperature Rotokawa geothermal field, New Zealand. Varying BHTV log quality reduces the number of input data points, but careful selection of the quality levels where fractures are deemed fully sampled increases the reliability of the analysis. Spacing data analysis comprising up to 300 data points and spanning three orders of magnitude can be approximated similarly well (similar AIC rankings) with several distributions. Several clustering configurations and probability distributions can often characterize the data at similar levels of statistical criteria. Thus, several scenarios should be considered when using BHTV log data to constrain numerical fracture models.

Statistical methods of fracture characterization using acoustic borehole televiewer log interpretation

©2017. American Geophysical Union. All Rights Reserved. Acoustic borehole televiewer (BHTV) logs provide measurements of fracture attributes (orientations, thickness, and spacing) at depth. Orientation, censoring, and truncation sampling biases similar to those described for one-dimensional outcrop scanlines, and other logging or drilling artifacts specific to BHTV logs, can affect the interpretation of fracture attributes from BHTV logs. K-means, fuzzy K-means, and agglomerative clustering methods provide transparent means of separating fracture groups on the basis of their orientation. Fracture spacing is calculated for each of these fracture sets. Maximum likelihood estimation using truncated distributions permits the fitting of several probability distributions to the fracture attribute data sets within truncation limits, which can then be extrapolated over the entire range where they naturally occur. Akaike Information Criterion (AIC) and Schwartz Bayesian Criterion (SBC) statistical information criteria rank the distributions by how well they fit the data. We demonstrate these attribute analysis methods with a data set derived from three BHTV logs acquired from the high-temperature Rotokawa geothermal field, New Zealand. Varying BHTV log quality reduces the number of input data points, but careful selection of the quality levels where fractures are deemed fully sampled increases the reliability of the analysis. Spacing data analysis comprising up to 300 data points and spanning three orders of magnitude can be approximated similarly well (similar AIC rankings) with several distributions. Several clustering configurations and probability distributions can often characterize the data at similar levels of statistical criteria. Thus, several scenarios should be considered when using BHTV log data to constrain numerical fracture models.

Az ImaGeo magszkennelés módszerei egy Mecseki fúrás nagyfelbontású értelmezésének példáján

A saját fejlesztésű ImaGeo rendszer terepen, fúrásokban és bányatérségekben is nagy felbontású, digitális földtani dokumentációt és 3D földtani-tektonikai adatfelvételt tesz lehetővé, normál és UV fény megvilágításban egyaránt. A fúrásokban kapott adatok mélyfúrás-geofizikai akusztikus (Borehole televiewer, BHTV) vagy ellenállás (pl. Formation MicroImager, FMI) adatsorok felhasználásával újra orientálhatóak és ezzel részletes földtani elemzések megvalósítása válik lehetővé a valós térben. A rendszer részei a Magszkenner, a Fotórobot és a LIPS (Lézer gerjesztésű plazma spektrométer). Jelen cikk az ImaGeo rendszer módszereinek bemutatása mellett esettanulmányt közöl az Ibafa, Ib–4 fúrás mezozoos rétegsorának magszkenneléses eredményeiről különös tekintettel a Jakabhegyi Homokkő Formáció elemzésére. A Formáció vizsgálatát az adatok 45/13°-os (dőlésirány/dőlésszög) tektonikusan kibillentett helyzetből történt visszabillentés után valósítottuk meg. A magszkennelésből származó szemcseméret, rétegvastagság, dőlésirány és dőlésszög eloszlások vizsgálata alapján a Jakabhegyi Homokkő Formáció harántolt rétegsorát 5 szakaszra lehetett bontani. A szakaszhatárok nem korrelálnak a földtani dokumentáció szakaszainak határaival. A visszabillentett dőlések DDK, D és DNy felé mutatnak, de bizonyos mélységszakaszokban a tisztán Ny-i és K-i irányok is jelentősek. Mindezeket üledékszállítási főirányként értelmezzük. A szállítási irányok a formáció egészét tekintve is széles spektrumon oszlanak el. A rétegvastagság, dőlésirány és dőlésszög adatok ciklicitás elemzését vizuálisan, mintázatok felismerése útján és geomatematikai periodicitás elemzéssel vizsgáltuk. Ezek alapján több, különböző periódushosszúságú (deciméteres, 1, 3 és 8 méteres) ciklust lehetett meghatározni. A ciklusosság megállapítható a lemezvastagságban, a dőlésszögek és a dőlésirányok eloszlásában is. A hosszabb ciklusok leginkább a dőlésirányok eloszlásában mutatkoznak. A dőlésszögekben a vizuális, mintázatokon alapuló és a geomatematikai módszer is a 0,5 m körüli ciklust mutatta ki. A geomatematikai elemzés 2 párhuzamos ciklushosszt mutatott ki a dőlésirányokban és a dőlésszögekben. Ezek 1,3 és ~4,5 m ciklushosszúságú periódusok. A vizuális elemzés feltárt egy mintegy 50 m-es ciklust is, ezt geomatematikai úton nem lehetett igazolni.

Controls on fault zone structure and brittle fracturing in the foliated hanging-wall of the Alpine Fault

The orientations and densities of fractures in the foliated hanging-wall of the Alpine Fault provide insights into the role of a mechanical anisotropy in upper crustal deformation, and the extent to which existing models of fault zone structure can be applied to active plate-boundary faults. Three datasets were used to quantify fracture damage at different distances from the Alpine Fault principal slip zones (PSZs): (1) X-ray computed tomography (CT) images of drill-core collected within 25 m of the PSZs during the first phase of the Deep Fault Drilling Project that were reoriented with respect to borehole televiewer images, (2) field measurements from creek sections at