Darko Vasić

Pulsed eddy current nondestructive testing of ferromagnetic tubes

In sinusoidal eddy current nondestructive testing (NDT) of thick ferromagnetic tubes, wall thickness is measured with the coils displayed for 2-3 tube diameters (remote field eddy current technique). The tube inner diameter is measured at higher frequency with another pair of coils displaced for around 1 tube diameter (electrical caliper). In this paper, we give a thorough analysis of the excitation frequency and the distance between the coils for measurement of the tube inner diameter and wall thickness as a background for the application of pulsed eddy current (PEC). We propose application of one pair of coils displaced for 1-2 tube diameters for measurement of those tube parameters employing the features of response to the pulsed excitation. Results of our experimental work confirm that PEC technique provides a significant improvement of present eddy current systems for NDT of the ferromagnetic tubes.

Validation of a coil impedance model for simultaneous measurement of electromagnetic properties and inner diameter of a conductive tube

In eddy-current nondestructive testing of conductive tubes such as oil-well casings, the measured tube wall thickness and inner diameter must be compensated for variations of electromagnetic properties of a tube material -magnetic permeability and electrical conductivity. Commercial systems for eddy-current nondestructive testing of oil-well casings incorporate multiple coils and multifrequency excitation for measurement of the inner diameter and the electromagnetic properties. This paper investigates the feasibility of the electromagnetic properties and inner diameter measurement with only one coil excited at one frequency. The authors have derived a coil impedance model and developed an inversion procedure for the determination of tube properties. Measurements have been performed on several tubes made of different materials. In order to validate the model and the optimization algorithm, measurement results were compared with the predictions of the model and finite-element analysis. Applicability of the method has been confirmed for the investigated frequency range (1-50 kHz).

Application of Stochastic Inversion to Casing Effect Correction in Through Casing Induction Logging

Electromagnetic induction logging is routinely used for the measurement of the electrical conductivity of rock formations from within an uncased borehole in order to determine producibility of hydrocarbons. Because of its usefulness in the open-hole environment, it was proposed to use this method inside steel-cased wells. The casings are magnetic and for five or six orders of magnitude more conductive than a typical rock formation. Thus, the measurements in through casing induction logging must be corrected for the variations in casing properties. It was suggested to employ existing casing inspection tools for this purpose. In this paper, we develop a stochastic approach to the inverse problem of determination of the casing properties using these tools. We numerically analyze the application of the inversion procedure to the model of a typical casing inspection measurement and investigate if the information obtained from the modeled data is applicable to the correction of the rock conductivity measurement for the casing effect. We conclude that the measurement uncertainty of the tool cannot exceed 1000 ppm if it is to be used for the casing effect correction.

Noise Analysis of the Pulse-Width Modulator for Quasi-Digital Sensors

Majority of available integrated and smart sensors are based on quasi-digitalization concept. A pulse modulation of the analog sensor output signal can be accomplished by using a fairly simple relaxation oscillator circuit. Pulse-width modulation (PWM) is commonly used in such applications, due to its ratiometric output that greatly reduces the error caused by the changes in the time constant of the oscillator. However, most of the literature that deal with the pulse-width modulators do not include the analysis of noise effects, which is important factor in optimizing the readout of PWM signal. In this article, we analyze the effects of noise in the simple dual-slope pulse-width modulator and provide an accurate noise model that can easily be adopted to any other type of pulse-width modulators. The model can be used to predict the jitter of the edges of output PWM signal and to analyze the effects of that jitter on the uncertainty of the measured duty-cycle for various PWM readout techniques.

On Feasibility of Inductive Conductivity Measurement of Formation Surrounding a Metal Casing

Bypassing hydrocarbon-layers when casing the borehole can result in recovering of only small part of estimated original oil in place. The inductive measurement of subsurface rocks (formation) conductivity through casing is required in order to successfully reevaluate existing wells and detect bypassed oil. The main problem in the cased- hole inductive measurement is strong signal attenuation and high variations in the properties of the steel casing. In this study, we investigate the feasibility of the inductive measurement in the presence of steel casings with realistic electromagnetic properties. We discuss the method for cancellation of the casing effect and inadequacy of the casing factor for the same purpose. We analyze effects of the excitation frequency and mutual separation of a transmitter-receiver coil pair on the expected measurement sensitivity.

Robust Estimation of Metal Target Shape Using Time-Domain Electromagnetic Induction Data

Discriminating metal parts of buried hazardous targets from ordinary metallic clutter is a very difficult and time-consuming task. For that purpose, electromagnetic induction (EMI) sensors composed of multiple transmitter/receiver coils in multiaxis arrangements are commonly used. In this way, data of high fidelity and spatial diversity are obtained so that the target can be characterized in terms of its geometric and electromagnetic properties. However, this often increases sensor size and reduces its portability, which is a problem for humanitarian demining applications, where compact, robust, and lightweight sensors are needed. If such sensors are to be used for target characterization, robust estimation algorithms are required, capable of coping with limited spatial information content and uncertainties related to sensor positioning and its coil geometry model. In this paper, we present a robust concept for estimating the general shape of magnetic metal targets using time-domain EMI sensors with single-axis coil geometries. We introduce the signature matrix, a parameter derived from time-dependent eigenvalues of the targets magnetic polarizability tensor, and use it for shape estimation. The proposed method was evaluated both through simulations and experiments, using two different sensor platforms (laboratory-based experimental sensor platform and a commercial metal detector mounted on a mobile robot). The obtained results clearly indicate that the target shape can be estimated from sensor data of limited spatial diversity and under the uncertainties of sensor positioning and coil geometry.

Active induction balance method for metal detector sensing head utilizing transmitter-bucking and dual current source

A central problem in a design of frequency domain electromagnetic induction sensors used in landmine detection is an effective suppression of a direct inductive coupling between the transmitter and the receiver coil (induction balance, IB). In sensing heads based on the transmitter-bucking configuration, IB is achieved by using two concentric transmitter coils with opposing exciter fields in order to create a central magnetic cavity for the receiver coil. This design has numerous advantages over other IB methods in terms of detection sensitivity, spatial resolution, sensor dimensions and suitability for model-based measurements. However, very careful design and precise sensing head geometry are required if a single excitation source is used for driving both transmitter coils. In this paper we analyze the IB sensitivity to small perturbations of geometrical properties of coils. We propose a sensor design with dual current source and active induction balance scheme which overcomes the limitations of geometry-based balancing and potentially provides more efficient compensation of soil effects.

Scaled Experimental Verification of Single-Well Induction Conductivity Measurement Through Nonmagnetic Casing

The electromagnetic induction measurement of dimensions, integrity, or material properties of conductive objects is especially challenging in the case when the electrical conductivity of these objects spans five to seven orders of magnitude. Such an application is the conductivity measurement of the surrounding rocks from within a metal casing of an oil well. The rock conductivity measurement is required in order to determine and revaluate hydrocarbon-bearing layers. We present the first experimental verification of the single-well through-casing induction measurement on a scaled laboratory model of a borehole lined with a stainless steel or copper casing surrounded with a low-conductive medium. The measurements are in the frequency range of 2–16 kHz (10–80 Hz in case of the actual borehole). We explain the theoretical background of the method, describe the scaled model and experimental procedure, and discuss the experimental results. The measurement results are in agreement with the theoretical predictions (relative error less than 15%), and the equivalence of the results for the steel and copper casings (average relative discrepancy less than 10%) corroborates that the rock conductivity measurement can be corrected for variations in the casing properties (dimensions, electrical conductivity, and magnetic permeability).

A digital tachometer for high-temperature telemetry utilizing thermally uprated commercial electronic components

The high-temperature environment of an oil well requires reliable downhole instrumentation, typically based on simple and robust circuits, low power consumption, and smart measurement algorithms. In this paper, we present an electronic design and measurement methodology for a simple microcontroller-based digital tachometer, optimized for downhole spinner-flowmeter applications and multichannel telemetry. We have developed and experimentally evaluated an effective algorithm for direction-sensitive rotational speed measurement, which combines quadrature pulse decoding with the method of dependent count. The algorithm employs a temperature compensation technique, making it suitable for the high-temperature application of commercial-grade electronic parts. Our results confirm the functionality and reliable operation of the tachometer, targeted at high-temperature short-term oil-well logging applications.

A Framework for Low Data Rate, Highly Distributed Measurement Systems

Specific types of distributed measurement and virtual instrumentation systems periodically acquire data from a large number of geographically distributed field terminal devices. Such systems are used in environmental monitoring or in monitoring of various commonalities such as electrical power distribution, gas or water networks. In this article we present a framework for low data rate, highly distributed measurement systems based on the publicly available GSM and GPRS services. By using the framework, it is possible to build a system which is capable to perform local measurements on a large number of geographically distributed measurement points, to collect measured data by using the mobile network, to store them on the server computer and to publish gathered data by providing XML web services interface. The framework is robust, scalable and suited for long-term autonomous operation. The tests showed that the framework is stable and operates properly.