Subsurface Resistivity Measurements Using Square Waveforms - Instrumentation and Measurement Technology Conference, 1997. IMTC/97. Proceedings. 'Sensing, Proce

This work analizes the effect of inductive and capacitive coupling from the injecting circuit to the detecting circuit in resistive field surveys. Experimental results demonstrate that if a square waveform is injected into the soil and synchronous sampling is used to sample the flat zone of the detected voltage, then the interference is greatly reduced. Furthermore, square waveforms are easier to generate than sinusoidal waveforms, so that they offer a new approach to subsurface resistivity measurements. between the injecting and detecting circuits in ac measurements. In all electrical surveys, electromagnetic induction between current and voltage cables must be avoided. This is easily achieved by using dipole-dipole arrays [2], but it is interesting to devise a method able to reduce this interference independently of the electrode configuration. II. PROBLEM STATEMENT I . INTRODUCTION. The detection of buried structures from the surface without drilling the soil is of interest in archaeology and in other situations such as detection of water and contaminants from leaking underground pipes. There are several techniques that can be applied in these cases: Ground Penetrating Radar (GPR), Time Domain Reflectometry (TDR), etc. Our work is centered in geoelectrical prospecting methods, which consist on injecting current to the soil with a pair of electrodes and detecting the drop in voltage with another pair of electrodes. The parameter that provides the information of the buried structure (anomaly) is the apparent resistivity p, which is given by p, = kAV / I [l], where AV is the detected potential, I is the injected current and k is a geometric factor, which depends on the configuration of the electrode array. It is important to recognize that any possible error in the measured voltage will affect p, and can hinder the detection of the anomaly. Some error sources are the position of the electrodes, telluric noise (which has its main influence in dc measurements) and the electromagnetic coupling In electrical impedance measurement we inject a current (frequency fs) and detect a drop in voltage whose amplitude is modulated by the impedance sensed. Modulation produces an upward translation of the message spectrum. Demodulation, therefore, implies a downward frequency translation in order to recover the message from the modulated wave. A common demodulation technique providing a good signal to noise ratio is homodyne detection. This method can be applied to impedance measurement as shown in Fig. 1 .