عنوان مقاله [English]
نویسندگان [English]چکیده [English]
The purpose of the geophysical studies is to obtain information about the shape, location and physical parameters of the subsurface bodies. To do this, we often need to solve the inverse problem. On the other hand, the solution to this problem is very complex because of the many solutions compatible with an acquired data set; indeed, different bodies can cause the same image on the surface.
Â Â Â The probability tomography approach allows the analysis of the experimental data without introducing any a priori information on the investigated structures. This method is able to give a geometrical representation of the buried sources of anomalies.
Â Â Â Nevertheless, the main difference between this method and the inverse problems is the absence of any response to estimate the physical parameters of the source of the anomaly. The probability tomography was originally formulated for the self-potential method, and then extended to the Electric, EM induction, Gravity and Magnetic prospecting. In Gravity formulation, it was proposed as an approach to vertically explore the subsurface in order to locate the most probable source of anomalies appearing in a field dataset collection in a given datum domain. First, the Newtonian-type integral defining the Bouguer anomaly function was solved as a sum of elementary contributions from points like mass contrast elements.
Â Â The power associated with the Bouguer effect was derived as a sum of cross correlation integrals between the Bouguer anomaly data and scanner function expressing the gravity effect from a point like mass contrast element. Using the Schwarzâs inequality property, we achieveda â-mass occurrence probability function as a suitable tool for determination of the depth to the source of a given gravity anomaly field.
Â Â Â The 3D â-mass occurrence probability function is a normalized correlation which its positive values refer to a mass excess at point q while negetive values are the results of a mass deficit at the same point with respect to the host volume.
Â Â Â The tomographic procedure consists of scanning the subsurface with the elementary source and calculating the occurrence probability function at the nodes of a regular grid. The complete set of grid values is used to highlight the zone of the highest probability of mass contrast concentrations. This range is defined as the most probable location for the source of the gravity anomaly.
Â Â Â In this study, gravity anomalies due to synthetic gravity data created for spherical models with a density contrast ofÂ 1gr/cm3, radius of 2m and at the depth of 10m were interpreted. To apply the procedure, the surface was divided into a regular grid with a sampling factor of 1m. The sphere was located at the center of the grid. The obtained results from applying tomography method on syntethic models implied that the method had a high resolution in determintion of maximum and minimum depth of subsurface anomalies. The effect of random noise was examined on the model, by 20% random noise, and showed that the effect of noise was negligible on the procedure.
Â Â Â The practical application of real data was also illustrated. The survey area was close to Abadeh, a city in Fars province, in south west of Iran. The main geological units were constructed from silt stone, conglomerate and limestone. Ore bodies of Barite were mainly out-cropped in limestone unit. The gravity station grid consisted of 200 measurement points on a grid of 5m to 10m. Applying the proposed method to real data, the horizontal and vertical extention of the anomaly were detected with satisfactory results.