Method and apparatus for determining permeable zones in subsurface earth formations

DETAILED DESCRIPTION I claim: 1.
Apparatus for investigating subsurface earth formations traversed by a borehole, comprising: a.
means for supporting a plurality of electrodes adapted for movement in a borehole, the plurality of electrodes including at least one current emitting electrode adapted for emitting current into a borehole and the formation surrounding a borehole; b.
means for emitting a main and an auxiliary current from the at least one current emitting electrode, the main current flowing primarily in the formation surrounding a borehole and the auxiliary current being confined primarily to a borehole and the formation immediately surrounding a borehole; and c.
means for detecting said main and auxiliary currents to produce first and second resistivity parameters, the first resistivity parameter being related to the auxiliary current and the second resistivity parameter being related to the main current and the first resistivity parameter.
2.
The apparatus of claim 1, wherein the first resistivity parameter, R.
sub.
1 *, is determined according to: R.
sub.
1 * = K.
sub.
1 R.
sub.
s /R.
sub.
m = K.
sub.
2 wherein: R.
sub.
s is representative of the shallow resistivity of the formation; R.
sub.
m is representative of the resistivity of the mud cake; and K.
sub.
1 and K.
sub.
2 are constants.
3.
The apparatus of claim 1, wherein said second resistivity parameter, R.
sub.
2 *, is determined according to: ##EQU2## wherein: R.
sub.
s is representative of the shallow resistivity of the formation; R.
sub.
d is representative of the deep resistivity of the formation; R.
sub.
m is representative of the resistivity of the mud cake; and K.
sub.
1, K.
sub.
2 and K.
sub.
3 are constants.
##EQU3## 4.
The apparatus of claim 1 further including means for producing a third resistivity parameter related to the first and second resistivity parameters, the third resistivity parameter providing an indication of permeable and impermeable zones in the subsurface earth formation.
5.
Apparatus for investigating subsurface earth formations traversed by a borehole to determine the presence of permeable zones in the formation comprising: a