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TABLE 3-2
Comparison of data of Magara (1968, p. 2474, table 11) with estimated quantities (-)from
eqs. 3.5a and 3.8 using the value of b determined from the sonic data
Depth At At Porosity
(m) Wft) (Clslft) f f

1029 145 134 0.390 0.36
1610 127 121 0.332 0.30
1809 130 117 0.266 0.28
2151 109 111 0.246 0.25
2296 114 108 0.243 0.24
2444 110 106 0.231 0.23
2607 102 103 0.218 0.22
3063 99 97 0.196 0.19
3205 95 95 0.188 0.18
3505 104 91 0.159 0.16
3701 s92 89 0.146 0.15

The values of the scale length b determined from the sonic log in Tertiary
mudstones seem to lie commonly between 2500 and 4000 m (8000 and
13,000 ft). The scale length of Hedberg’s (1936) data - 1550 m or 5000 ft -
is unusually short. Older sequences tend to have shorter scale lengths. Athy’s
Palaeozoic mudstones have a scale length of about 700 m (2300 ft). Creta-
ceous mudstones of western Canada have one of about 520 m (1700 ft)
(Magara, 1973, p. 11, fig. 6). But, as can be seen, the value of the scale length
is not solely determined by age.

Fig. 3-8. Family of transit-time - depth curves for various values of the scale length.