Tamás Ormos

A new procedure for the interpretation of VES data: 1.5-D simultaneous inversion method

In the paper the principles of a new inversion method as well as the results of its application are described by using synthetic and field VES data. The basic idea of the method says that the horizontal changes in the layer thicknesses and the resistivities of the 2-D geological structure can be described by (expanding in series) functions of one variable. The coefficients of the functions are determined from the VES data by simultaneous inversion method using a least-squares technique. The local thicknesses and the resistivities of the geological structure are calculated from the coefficients from point to point along the profile. Using this method, the equivalence can considerably be reduced, and by using the functions chosen on the basis of a priori knowledge, the whole geological model can be determined by this inversion method. In the inversion the local 1-D forward modelling is used and this approximation is often applied in the VES practice. To qualify the results of the inversion, the correlation matrix for the coefficients are calculated and the error values for the local model parameters are introduced.

In-mine geoelectric investigations for detecting tectonic disturbances in coal seam structures

The methods of in-mine seam-sounding and transillumination (geoelectric tomography) for the detection of tectonic disturbances of coal seams were developed by the Department of Geophysics of the University of Miskolc in the 1970–80’s with the effective support of the former “Borsod” Coal Mines Ltd.The paper gives an overview about the theory of seam-sounding and a special geoelectric tomographic inversion, and introduces the in-mine geoelectric seam-sounding and transillumination measurement systems using vertical electrode dipoles. In the second part the paper, the results of an in-mine geoelectric measurement are presented, which was carried out in order to detect tectonic disturbances of the Miocene aged coal seams situated in Slovakia. As results of the geophysical investigation, the authors forecasted the tectonic features in the coal seam. The company confirmed the results by independent information about seam disturbances and tectonic features arising from the excavation of the investigated area.

Inversion of Refracted First Breaks using Traveltime Differences

Refraction seismic methods are widely used in near surface investigations of geological structures. In the Department of Geophysics (University of Miskolc) a series expansion based inversion method was developed for the interpretation of refraction traveltime data. The experiences that we gained using this method show that the accuracy (or the errors) of trigger time have greater importance in near surface application than in the exploration of deeper regions. In this paper an experiment is described for the development of this inversion method for avoiding the interpretation problems originated from the inaccurate source time. In the presented inversion procedure traveltime differences are used instead of the measured simple traveltime dataset. Therefore a significant improvement is detected in the results of those cases where triggering error occurred. The method is tested on synthetic and field datasets as well. Further upgrade of this theory and its implementation into the series expansion based inversion method is planned in the near future.

New rock physical model describing the pressure dependence and hysteresis of acoustic velocities

In this paper new rock physical model describing the hysteretic pressure dependence of acoustic wave velocities is presented. The model is able to consider two or more physical mechanisms responsible for the pressure dependence of the propagation characteristics (for example the closing/opening of microcracks and the change of pore volume by virtue of the change in the rock pressure). Our rock physical model was successfully used as forward modelling equations in the inversion of longitudinal velocity data measured on core samples originated from oil drilling wells and also seismic/acoustic velocity data sets published in the literature.

Application of In-mine Geoelectric Methods for Detecting Tectonic Disturbances of the Coal Seam Structure

In-mine geoelectric methods – the geoelectric seam-sounding and geoelectric transillumination – were applied for detection of fault zones in a coal mine. To determine the model parameters of the coal seam structure seam- and drift-sounding measurement were carried out along the boundary of the coal layer (roof sounding) and that of the floor (floor sounding). From the measured apparent resistivities the model parameters of the structure were determined using a joint inversion procedure (Dobroka et al., 1991).The measured data were interpreted using geoelectric tomography procedure. Good agreements were found between the location of tectonic zones predicted by the interpretation and those observed during the mining extraction.

Interpretation of IP Measurements Using an Inversion Based TAU-Transform Method

The paper presents the TAU-Transform of the Time-Domain IP curves and gives an approximate solution for TAU-transformation using inverse problem theory. A method for estimation of the type and the measure of soil contaminations will also be introduced. The determination of the contamination type (metallic or chemical) comes from the time constant spectra. The estimation of the soil contamination is based on the waited amplitude values of the time constant spectra. The application of the method is demonstrated in a contaminated industrial area in Hungary.

Laboratory P and S wave Velocity Measurements to Confirm the Developed Petrophysical Model for Acoustic Hysteresis

It is well known that acoustic wave propagation under pressure is very nonlinear and the elastic properties of rocks are hysteretic, which behavior is important for mechanical understanding of reservoirs during depletion. Pressure strongly influences the elastic parameters of rocks, thus wave velocities too. In this study longitudinal and transverse wave velocities measured in laboratory on sandstone samples under pressure are presented. The uniaxial loading of the samples was carried out by an automatic acoustic test system. Measurement data were processed by a joint inversion technique based on the developed petrophysical model which describes the relationship between acoustic P/S wave velocities and pressure for loading and unloading phases. After Birch we assume that the main factor determining the pressure dependence is the closure of pores. The advantage of the model is that it is not based on simple curve fitting, but gives physical explanation for the process with three-parameter exponential equations. The quality checked joint inversion results showed that the misfits between measured and calculated data are small, proving that the proposed petrophysical model can be applied well in practice.

New Petrophysical Model Describing the Acoustic Hysteresis

In this paper a new theory is developed which describe the pressure dependence of seismic velocity and deals with acoustic hysteresis. The model is based on the idea (accepted in the literature) that the microcracks closed during pressurization do not reopen completely during depressurization. The model was applied to acoustic P wave velocity data sets measured on core samples originated from oil-drilling wells. The model parameters of the petrophysical model were determined by inversion method. During the measurements the pulse transmission technique was used. Measurements were carried out at various incremental pressures and parameters of the model were determined by linearized inversion methods. The calculated data matched accurately with measured data proving that the new rock physical model describing acoustic hysteresis applies well in practice.