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沉积物(岩)的粒度呈正态分布,在对数概率图上表现为几个直线段。在描述各组分直线段时,都是用其倾角或斜率来说明分选性的。但是不同的对数概率纸上,斜率与分选性的关系是不同的。标准偏差值是斜率的函数:σ=α/tgθ,其中,α为概率尺度上15.9%至50%间的长度与φ尺度上单位长应的比值。可见,每个线段的标准偏差值可通过测量其倾角来求得,从而使不同概率图上的粒度曲线的分选性可以相互比较。
The grain size of sediments (rocks) is normally distributed and appears as several straight line segments on a logarithmic probability plot. In describing the straight line segments of each component, the inclination or slope is used to illustrate the sortability. But on different logarithmic probability papers, the relationship between slope and sortability is different. The standard deviation value is a function of the slope: σ=α/tgθ, where α is the ratio of the length between 15.9% to 50% on the probability scale and the unit length of the φ scale. It can be seen that the standard deviation of each line segment can be obtained by measuring its inclination, so that the sortability of the particle size curves on different probability plots can be compared with each other.