An amplitude-distance curve has been computed for short period P waves in the range A=30"-102" (and h=O) using the amplitudes of explosion signals only. Effects of source asymmetry can thus be discounted and the problem of deciding what amplitude to measure is reduced because the maximum amplitude of explosion signals always occurs in the first few cycles. To increase the consistency of the measurements all the amplitudes have been measured by the same person.Assuming the effect of distance is constant over a series of discrete intervals of distance, the amplitude-distance curve with its confidence limits has been estimated by least squares using data from 65 stations which recorded some or all of the explosions fired at six sites. The most striking features of the curve are: (1) a sharp maximum at about A = 35", (2) a minimum followed by a sharp increase at A = 75", and (3) a sharp minimum at A =93". Over the remainder of its length the curve is similar to the Gutenberg and Richter curve. Station magnitude corrections have also been estimated.

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This paper describes a method whereby three resistances are measured for a four electrode configuration, and relations between these three resistances and their corresponding apparent resistivities are derived. The practical application of the resistance relation as a means of detecting observational and instrumental errors is indicated. The concept of apparent resistivity is examined and by means of examples it is shown that the apparent resistivity can take negative values. Finally, the possibility of using the triple resistivity method as a means of distinguishing between the effects of lateral and vertical resistivity changes on depth probes is discussed.

This paper deals with the four electrode method of electrical resistivity prospecting using direct current. It is shown that three resistances can be measured for any four electrode configuration, and a simple relation between them is derived. The particular case of the Wenner electrode configuration is discussed and a further relation derived between the three apparent resistivities corresponding to the three resistances. The practical value of the (resistance relation as a means of avoiding experimental errors is indicated.In general the three apparent resistivities are different in magnitude although there are certain particular cases where they are equal to one another. Their actual values are determined for a number of simple problems and the possibility of having a negative apparent resistivity is illustrated.Finally, examples of field tests are given in which the three resistances were measured, and it is concluded that the method can often provide a simple way of distinguishing between the effects of lateral and vertical resistivity variations.