Ves-Field And Processing Technology For The Case Of High Level Geological Noise

Abstract

tel: (7-095) 939-49-63 fax: (7-095) 939-43-70 E-mail (Internet): sh@geophys.geol.msu.su. Many publications in last several years were devoted to the new technology in resistivity method, called as Electrical Imaging (R.Barker) or Resistivity Tomography (H.Shima). In MSU from 1980 the similar investigation was carried out for VES technology called The total electric sounding (TES). In each separate case the TES field technology aims the investigation of a depth interval from h,,, up to h . The horizontal size of any object to be visible should be approxin?rrely equal of its depth (Fig.1). The step on the profile for detailed , investigation of the such object should be 2-5 times smaller of its size. The objects deeper h,, are not visible due to restricted penetration Ffg.1. The ba*ic model for TES depth. Other objects at the depth smaller than h,,, with proportionally small dimensions may influence and more noticeably, the smaller their depth is, because they are closer to the points of the current and potential electrode’s position. These objects (Subsurface Inhomogeneities SSI) are considered as geological noise. Distortions of VES curves, caused by SSI may be divided into two types: caused by objects near potential electrodes (P-effect) and near current electrodes (C-effect). The main features of these two effects were described in our report P129 at EAEG conference in Vienna, These effects are not clear visibly when standard VES technology is applied. The main danger of the effects is in wrong geological interpretation of VES data. The greatest influence of SSI may be in towns, in mining regions with the artificial upper layer, in the vicinity of ditches with cables, tubes and so on. After canceling of all distortions, caused by SSI, the geological effectivity of VES may be much higher. The essence of our approach is the recognition of geological noise as the constant constituent of any medium under investigation, The model of such medium is on our opinion a combination of layered medium, deep inhomogeneities and subsurface inhomogeneities SSI (Fig. 1). Two different approaches to the interpretation of profile VES data seem possible. In the first case the model includes all objects, which are noticeable in the experimental field. In the second approach considered as more realistic, the model includes only objects in the working depth interval without details about small upper objects. Then we should have some instrument to separate objects (or their influence on the field) into objects of our interest and the noise, and the last may be removed. That may be done with special field technology: l.VES locations on the profile are regular with equal distances. 2.The two pole-dipole sounding (AMN & MNB) at each location are fulfilled. 3.Step in current electrodes distance growth is constant (that is linear) and equal to the sounding step. Then places for current electrodes grounding will be the same for all VES points on the profile. The best way for that is multi-electrodic field equipment, produced by Campus, ABEM, OYO, DMT and other companies. 4.Step on the profile for the h,,,-h,, depth interval should be equal to h,,,. The field data processing to reduce SSI influence we fulfill with the IPI-2D package. Two algorithms: the principal component’s method and median polishing are applied for that (MPC and MEDIAN programs). Algorithm MEDIAN appeared in 1994 and has been changed greatly after Vienna’s conference. Its last version block-scheme is presented in Fig.2. The algorithm works in three stages. At the first stage pa field for AMN & MNB arrays is . divided into components: Horizontally-layered (HL), Rg.2. Block-scheme of data processing with Mez dian program (including MPC)