E. V. Pervago

Separation of anisotropy and inhomogeneity influence by the spectral analysis of azimuthal resistivity diagrams

The first aim of resistivitv survey is a study of any inhomogeneities. Frequently in practice the effect of anisotropy is displayed together with that of layering or inhomogeneities. It complicates data interpretation within the framework of anisotropic models, and distorts results of interpretation in the framework of layered or inhomogeneous media.

Anisotropy Determination In Heterogeneous Media By Tensor Measurements of the Electric Field

Traditional anisotropic resistivity survey requires performing a set of azimuthal measurements. In an anisotropic heterogeneous medium the possibility of an anisotropy study is restricted by the necessity to carry out the time-consuming azimuthal soundings on a dense grid of observations. On the other hand, at applying the Electrical Imaging in anisotropic media the complete neglect of azimuthal−anisotropy influence can result in noticeable errors of interpretation. The authors propose an alternative method to determine the parameters of anisotropic media based on measuring the second derivatives of the electric potential from a point current source. Such a technology (named tensor measurements) reduces a number of observation azimuths to a single one and at the same time allows obtaining all anisotropy parameters. The efficiency of different tensor arrays is illustrated on the complex 3D model of the dipping interface between an isotropic overburden and an anisotropic basement. In this case the effects produced by anisotropy, heterogeneity, and formation layering appear jointly. The vertical soundings with the tensor measurements give three apparent characteristics: mean resistivity ρam, anisotropy coefficient λa, and anisotropy strike αa. The interpretation of their cross-sections allows determining true parameters of anisotropy and structure.

Anisotropy Effects Investigations by Resistivity Method in Some Inhomogeneous Media

Resistivity method is usually applied for horizontally-layered media investigation as sounding, and for inhomogeneous media as profiling.

Distortions of VES data caused by subsurface inhomogeneities

The first (mainly used) model for VES method is based on horizontally layered structure. The second one includes different deep objects, such as faults, horsts, grabens, domes, etc. We introduce the third model, which includes horizontal layers, deep objects, and many subsurface objects of different resistivities, referred to as subsurface inhomogeneities (SSIs).

Arrays for Tensor Measurements of the Electric Field

The study of resistivity anisotropy in heterogeneous media is a complex problem that needs developing the special field technology and interpretation technique. Traditional anisotropic resistivity survey requires performing a set of azimuthal measurements. In the case of the anisotropy estimation in heterogeneous media the application of such a technology is essentially restricted by the field-operating complexity for profiling and sounding. On the other hand, at applying the Electrical Resistivity Tomography (ERT) in anisotropic media the neglect of azimuthal−anisotropy influence can result in noticeable errors of interpretation. The alternative method of anisotropy determination is based on measuring the second derivatives of the electric potential from a point current source. This technology consists in the tensor measurements of the electric field using groups of specially distributed transmitting and receiving electrodes (tensor arrays). The tensor measurements allow obtaining the orientation and value of the resistivity anisotropy from the observations realized for a single orientation of the array without its rotation. In this report we analyzed the feasibility of different modifications of the tensor array for the determination of anisotropy parameters and their field applicability for profiling and sounding. The sensitivity and stability of the anisotropy estimation with the tensor arrays were shown for a horizontally layered anisotropic medium and vertical anisotropic layer in an isotropic half-space.

A New Approach to Resistivity Anisotropy Measurements

Summary This work introduces a new approach for the estimation of resistivity anisotropy parameters from measurements of the second electrical potentials derivatives. This approach allows to estimate anisotropy parameters by measuring on a single arbitrary azimuth instead of traditional azimuthal resistivity survey. Stability of anisotropy parameters’ estimation from the second potentials derivatives and opportunities of new technology application for resistivity profiling and sounding are shown at several models of inhomogeneous anisotropic media.

Interpretation of Resistivity Sounding Data - Distorted by Geological Noise

The traditional resistivity sounding method deals with isolated sounding sites located casually on profile or area and executed with logarithmic step in distante growth. This technology has very restricted possibilities.

Electromagnetic Technology for Determining Technical Conditions of Oil and Gas Pipelines

E005 Electromagnetic Technology for Determining Technical Conditions of Oil and Gas Pipelines A. Mousatov* (Mexican Petroleum Institute) E. Nakamura (Mexican Petroleum Institute) V. Shevnin (Mexican Petroleum Institute) O. Delgado (Mexican Petroleum Institute) & E. Pervago (Mexican Petroleum Institute) SUMMARY In this paper we present an electromagnetic technology for non destructive surface inspection of metallic pipelines that includes the determination of pipeline position and depth assessment of technical condition of an insulation coating and estimation of operating state of a cathodic protection system. This technology is based on the approximation of a metallic pipeline by a heterogeneous transmission line. In the

Electric Field Modeling In Arbitrary Anisotropic Layered Media Using the Set of Fast Hankel Transformations of Integer Orders

The technique for computation of the electric potential in an arbitrary anisotropic multilayered medium produced by point source is proposed. The solution is presented as the set of Hankel Transformations of integer orders and based on the analytical recurrent equations obtained for the potential spectrum. For the potential conversion in the space domain, we applied the algorithm of the Fast Hankel Transform in logarithmically spaced points that provided high accuracy and velocity in the modeling.

Problems in interpretation of resistivity sounding data, distorted by geological noise

The traditional resistivity sounding method deals with isolated sounding sites located casually on profile or area of investigation and executed with logarithmic step in distance growth. This technology has very restricted possibilities. Therefore it has begin to exchange practically simultaneously in different countries into new sounding technology, called multielectrodic sounding, or resistivity tomography, electrical imaging, etc. Its main advantage - more dense survey along profile - is clear visible, while the other - overlapping system of measurements is not so evident. Our group in MSU began to apply multi-electrodic resistivity sounding to overcome distorting influence of geological noise. From the end of the 80-th we have been working in areas with high resistivity contrast along both vertical, and horizontal direction. Such sounding data have different features of distortions or differences from response for horizontally layered models. At that period we developed theory of distortions of DC electric field by deep and near-surface inhomogeneities (Electrical..., 1994) and software to establish VES data distortion with the help of various indicators, noticeable on VES curves. We have found, that more than 70 % of all VES curves are distorted. That means, that the distortions are practically constant features of resistivity sounding. Carrying out the large volumes of VES field data and their interpretation (up to 1000 VES sites a year) we analyzed misfit errors of VES curves interpretation. The average value of misfit error (average RMS or D , where D is dispersion) consists 8-12%, that seems rather high value and needs to be explained.