Stationary and portable magnetohydrodynamic (MHD) generators are used in the Soviet Union for deep crustal electromagnetic soundings to depths of tens of kilometers. MHD sources produce tens of megawatts of power and transmit tens of thousands of amperes, but can only be fired at infrequent intervals. An alternative method of attaining a high signal-to-noise ratio is to transmit multiple low-power pulses and stack the repeated signals for some period of time. Energy may be also be stored during the transmitter off time and released in a narrow pulse, a technology known as energy storage or pulse-compression. Varying the pulse width has two opposing effects on the signal-to-noise ratio (SNR). As the transmitter pulse width is increased the received signal increases, but the number of transients that can be stacked in a given period of time decreases. The optimum ratio of transmitter on-time to off-time is 1 for near-zone soundings (source-receiver separation less than the depth of investigation), increasing somewhat for large source-receiver separations. Analysis of signal-to-noise ratio (SNR) demonstrates that there are disadvantages in employing high-power sources such as MHD generators which can not deliver repetitive pulses. Low-power conventional transmitter technology, with signal stacking and adaptive noise cancellation at the receiver, can result in superior results at a much lower cost. In addition, energy storage or pulse-compression technology appears to offer relatively little advantage, and only a moderate gain in SNR is achieved.
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