Channel Estimation in Through-The-Earth Communications with Electrodes

One of the main points in the design of a communication system is to know the channel behaviour for a given frequency band in terms of amplitude and phase. Free space com- munications channels have been characterized thoroughly for a large range of frequencies. But in the fleld of Through-The-Earth (TTE) communications, the work about channel characterization found in the literature is very scant and focused in magnetic propagation (induction loops). This paper presents a method of channel characterization for TTE communications with electrodes, and the tests and results done in real fleld measurements. TTE communication systems have been developed for establishing communication in conflned areas as tunnel, mines or caves. Due to rock conductivity, electromagnetic waves sufier from skin efiect attenuation. Because of that, TTE communication works in VLF-LF range. There are two possible medium access solutions: current injection by means of ground electrodes (1) and magnetic induction by inductive loops. In current injection a pair of electrodes is located underground and other pair in surface. In TTE communication not only it is necessary to study the electrical noise present. A chan- nel characterization is of paramount importance at time to model and simulate a communication system. A channel sounding allows obtaining the parameters of the channel impulse response ex- pression h(t), considering in this case a time invariant channel. The channel transfer function states that for a given input x(t), the output y(t) is the convolution of x(t) and the impulse response h(t). Knowing h(t), it would be possible to predict the distortion that the receiver will sufier and to design equalizers or matched fllters to compensate it. On the other hand, if h(t) is not known, a large number of parameters (channel attenuation, SNR, phase distortion) critical in system design are left to trial and error, which efiectively multiplies time needed to design such systems and most important, designed systems are usually far from optimum. The characterization of TTE communication channel with inductive loop has been studied theoretical and experimentally (1,2). In the electrodes case, several studies in the literature ofier theoretical formulae that model the communication channel with electrodes. However, they are valid for big electrodes, with emitter and receiver in surface and with a large separation between them. This situation is not suitable for TTE communication system where the electrodes separation is relatively small, also its size. Moreover, the electromagnetic waves travel though the rock in this case. For the channel object of study, electrodes efiect is also considered, since it is not possible to isolate its impact in the measurements. Furthermore, in a real TTE communication application the electrodes contribution in the channel will be present. The electrode impedance generally presents a very high value that limits the current injected and so after the electric fleld generated (3). In this paper a basic channel characterization method developed for TTE communication applications with electrodes is presented. This method has been applied in several locations, presenting here a couple of measurements done in surface. 2. CHANNEL SOUNDING METHODS Many methods for communication channel estimation have been developed. They are based on setting x(t) = -(t) (Dirac delta function). The output y(t) is directly the impulse response h(t). In practice, it is not possible to obtain a true delta (inflnite amplitude and inflnitesimal duration). Therefore, the measurement methods try to approximate an impulse function. They are grouped