Common variables
Rw Resistivity of formation water
Rt True formation resistivity
Sw Saturation woth water
Rm  Mud resistivity
Rmf Mud filtrate resistivity from fluid & mud that filters into the formation invaded zone
dh borehole diameter
Rmc mud cake resistivity from the side of the borehole
Rxo Flushed zone resisitivity, a portion of the invaded zone
Ri Annulus zone resistivity, the outer portion or transition part of the invaded zone
di diameter of flushed zone portion of invaded zone
dj diameter of complete invaded zone, includes di
Sxo water saturation of flushed zone
hmc thickness of mud cake
Drj radius of outer boundary of invaded zone
a tortuosity factor in Archie Equation
m cementation exponent in Archie Equation
n saturation exponent in Archie Equation
Ri invaded zone resistivity
Rmf >> R resistivity relation for freshwater muds
Rmf ~ R resistivity relation for saltwater muds
Rt true formation resistivity derived from deep resistivity log
Tms mean surface temperature
Tf formation temperature
gG geothermal gradient; slope of Tf/Depth
D depth
SSP static spontaneous potential, correction from raw SP
Rwe equivalent formation water resistivity
Vshale volume of shale
SPshale SP value of shale generally set to zero
PSP psuedostatic spontaneous potential; maximum SP of a shaly formation
fs sonic derived porosity
Dtma interval transit time in the matrix
Dtlog interval transit time in the formation
Dtfl interval transit time in the fluid
Cp compaction factor for unconsolidated sands in Wyllie formula for sonic porosity
Dt interval transit time
f=fsx.09 Hilchie (1978) formula for correction of hydrocarbon effect on sonic porosity log for where oil is present
f=fsx0.7 Hilchie (1978) formula for correction of hydrocarbon effect on sonic porosity log for where gas is present
rb Rho b, bulk density
rma Matrix density
rfl density of fluid (salt=1.1 or fresh=1.0 g/cm3; gas=0.7)
fD density derived porosity
fN neutron porosity
fNDgas porosity of the gas bearing formation
fw water-filled porosity
fND  neutron density crossplot derived porosity
e dielectric constant from the log
ew dielectric constant of water
ema dielectric constant of rock matrix
p  constant that varies with water salinity; value of 1.2 can be used, or can be calculated from nearby H2O zone by substitution of phiND for phiw