摘要:

AbstractThe use of the gravity method to predict rock bursts in mines is based on the relationship between the development of a dilatancy process in the exploited rock mass and the time‐dependent gravity anomalies induced by this phenomenon. The differences between successive observations of anomalies and the time behaviour of their trend amplitudes as precursors of preceding changes of rock stability are interpreted. The centres of zones of induced rock density variation are determined by computing the position of singular points of the differences between anomalies.Two gravity surveys have been carried out in the Radbod coal mine (Germany). The first survey took place at the level of the Dickebank seam (depth 1030 m), the second in the Sonnenschein seam (depth 1090 m). The observations were made with Worden and LaCoste‐Romberg (D‐type) gravimeters. The differences between successive anomalies were less than 100 μGal.In the case of the Dickebank seam, the position of singular points demonstrates the effect of two approaching longwalls on a previously mined‐out seam and on the gallery in which the gravity observations were made. In the case of the Sonnenschein seam, the trend amplitudes show distinct variations in the formation of the approaching longwall below the edges of all previously mined‐out seams. In particular, the effect of a remnant pillar has caused the largest gravity gradients. This result corresponds to the existence of a zone of rock‐burst hazard known from test drilling. The computed singular points are grouped together under the remnant pillar indicating two local hazard zones.Both results, the observed development of rock instability with time and the information about the position of the disturbed rock mass relative to the mine workings, are of importance, subsurface gravity surveying can therefore be a valuable tool for predicting

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00562.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractReiter, E.C., Toksoz, M.N. and Purdy, G.M. 1992. A semblance‐guided median filter.Geophysical Prospecting41, 15–41.A slowness selective median filter based on information from a local set of traces is described and implemented. The filter is constructed in two steps, the first being an estimation of a preferred slowness and the second, the selection of a median or trimmed mean value to replace the original data point. A symmetric window of traces defining the filter aperture is selected about each trace to be filtered and the filter applied repeatedly to each time point. The preferred slowness is determined by scanning a range of linear moveouts within the user‐specified slowness passband. Semblance is computed for each trial slowness and the preferred slowness selected from the peak semblance value. Data points collected along this preferred slowness are then sorted from lowest to highest and in the case of a pure median filter, the middle point(s) selected to replace the original data point. The output of the filter is therefore quite insensitive to large amplitude noise bursts, retaining the well‐known beneficial properties of a traditional 1D median filter. Energy which is either incoherent over the filter aperture or lies outside the slowness passband, may be additionally suppressed by weighting the filter output by the measured peak semblance.This approach may be used as a velocity filter to estimate coherent signal within a specified slowness passband and reject coherent energy outside this range. For applications of this type, other velocity estimators may be used in place of our semblance measure to provide improved velocity estimation and better filter performance. The filter aperture may also be extended to provide increased velocity estimation, but will result in additional lateral smearing of signal. We show that, in addition to a velocity filter, our approach may be used to improve signal‐to‐noise ratios in noisy data. The median filter tends to suppress the amplitude of random background noise and semblance weighting may be used to reduce the amplitude of background noise further while enhancing coherent signal.We apply our method to vertical seismic profile data to separate upgoing and downgoing wavefields, and also to large‐offset ocean bottom hydrophone data to enhance weak refracted and post‐critically r

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00563.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractThe Pelton DRTMServovalve Enhancement causes the natural output of a vibrator to resemble the desired output more closely. This simplifies the control problem and reduces harmonic distortion.The traditional type of servovalve used on seismic vibrators is a flow‐control servovalve. Flow is proportional to a vibrator's baseplate velocity, with respect to its reaction mass.The new servovalve control parameter is pressure rather than flow. The differential pressure applied to a vibrator's actuator piston, multiplied by the area of the piston, equals the force applied to the vibrator's baseplate structure. This may be defined as actuator force. There is a simpler and more linear relationship between actuator force and ground force than between actuator velocity and ground force. Thus, it is better for the servovalve to control pressure into the actuator rather than flow.A flow‐control servovalve can be made to control pressure by sensing the differential pressure across a vibrator's actuator piston and applying it as a negative feedback around the servovalve main stage. This has been carried out and tested. The result is more accurate vibrator control and reduced harmonic distort

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00564.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractMT data from a detailed 84‐site grid array (28 × 12 km) in the Paraná Basin (Brazil) reveal a high degree of frequency‐independent and parallel amplitude distortion. The Palaeozoic sediments across the survey area are covered by 1.2 km of flood basalts. A deep well‐log provides some control regarding the emplacement of thin diabase sills but information regarding vertical feeder dikes is non‐existent. The degree of parallel behaviour is identified using anisotropy ratios which quantify the extent and bandwidth of the distortion characteristics.A 2D modelling study is carried out using the concept of a horizontally layered (1D) basin with superimposed, small‐scale inhomogeneities. Two likely geological distortion structures are considered. The first represents at‐ or near‐surface (0 to 100 m) inhomogeneities. The second represents thin but vertically elongate dikes. The two distortion effects are found to produce different and characteristic behaviour in the H‐polarization (TM) mode model data. The modelled E‐polarization (TE) mode data are undistorted and provide control. The model developed to represent vertical dikes gives rise to a characteristic frequency dependence which is distinct from that of the undistorted response. This model also generates subparallel distortion effects which give rise to a form of variance in the response estimates as a function of location. The behaviour occurs, importantly, in both the amplitude and phase of the distorted data. A comparison of the normalized amplitude data, observed and modelled, indicates a high degree of correspondence down to a low frequency limit (<0.1 Hz) where additional and deep contributions to the observed response become apparent. The modelling results and data characteristics indicate that the survey area is underlain by a series of closely parallel, thin intrusive d

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00565.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractElectrical measurements are an important and integrated component of geophysical investigations connected with environmental problems. As a result of an analysis of the electrical conductivity, basic experiments on sandstones at frequencies below 10 kHz show that the complex behaviour of conductivity is caused exclusively by a complex interface conductivity. Its value is determined mainly by the internal rock interface to porosity ratio, the composition of the pore fluid and connected matrix‐water interactions resulting in a specific microstructure of the interface. Therefore, it can be expected that the interface region of a soil or rock material is very sensitive to changes in composition caused by contamination. Contaminated sandstone and clay samples were investigated using a low‐frequency measurement system. The investigations are directed at the influence of different contaminants and their concentration. Results show that the complex electrical conductivity (real and imaginary parts) is influenced by properties of the pore‐filling contaminant. This influence results in a change of the level of both parts and the shape of their frequency dependence. The imaginary part in particular seems to provide important secondary information; in some cases this part alone allows a differentiation of the various contaminants. The different behaviour of various rock types shows that the effects observed are the result of interactions between pore fluid properties and the internal pore surface stru

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00566.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractA new ray‐tracing method called linear traveltime interpolation (LTI) is proposed. This method computes traveltimes and raypaths in a 2D velocity structure more rapidly and accurately than other conventional methods.The LTI method is formulated for a 2D cell model, and calculations of traveltimes and raypaths are carried out only on cell boundaries. Therefore a raypath is considered to be always straight in a cell with uniform velocity. This approach is suitable to tomography analysis.The algorithm of LTI consists of two separate steps: step 1 calculates traveltimes on all cell boundaries; step 2 traces raypaths for all pairs of receivers and the shot.A traveltime at an arbitrary point on a cell boundary is assumed to be linearly interpolated between traveltimes at the adjacent discrete points at which we calculate traveltimes. Fermat's principle is used as the criterion for choosing the correct traveltimes and raypaths from several candidates routinely.The LTI method has been compared numerically with the shooting method and the finite‐difference method (FDM) of the eikonal equation. The results show that the LTI method has great advantages of high speed and high accuracy in the calculation of both traveltimes and raypaths. The LTI method can be regarded as an advanced version of the conventional FDM of the eikonal equation because the formulae of FDM are independently derived from LTI. In the process of derivation, it is shown theoretically that LTI is more accurate than FDM. Moreover in the LTI method, we can avoid the numerical instability that occurs in Vidale's method where the velocity changes abrup

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00567.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY

摘要:

AbstractFurness, P. 1993. Gradient Array Profiles over thin resistive veins.Geophysical Prospecting41, 113–130.Gradient array geoelectric surveys are commonly used to explore for buried sheet‐like targets. It is demonstrated that under certain circumstances the responses of such features can become practically equivalent for a range of models differing considerably in resistivity and thickness. This is the dual of the familiar equivalence phenomenon associated with thin beds in geoelectric sounding operations. Hence the geoelectric response profiles over thin dipping resistive veins are conveniently studied by considering equivalent forms with vanishing thickness and infinite resistivity but with the same resistivity‐thickness product. The effects of varying the dip, depth of burial, depth extent and resistivity–thickness product of such features on the gradient array response profiles are investigated by means of a series of numerical experiments. These demonstrate several effects of significance to survey design and data interpretation.In particular it is found that while the resistivity–thickness product influences the amplitude of the response profiles it has little effect on their form. A similar behaviour is found to be exhibited by veins with extended depth extent when the dip is varied.A complete quantitative interpretation of gradient array profiles over thin dipping resistive veins is thus only possible for veins of limited depth extent. For veins with an extended depth dimension, the data is ambiguous (in a practical sense) in terms of the dip and the resistivity–thickness product. Here, the interpretation can only be expected to deduce the horizontal and vertical locations of th

ISSN:0016-8025    

DOI:10.1111/j.1365-2478.1993.tb00568.x

出版商:Blackwell Publishing Ltd    

年代:1993

数据来源: WILEY