DETERMINATION OF SUBSURFACE POROSITY

The disclosure is directed to an apparatus and method for determining the water-filled porosity of formations surrounding a borehole. Means are provided for deriving a first quantity which is a measure of the travel time of microwave electromagnetic energy propagating over a predetermined distance in the formations, this first quantity being designated as tp1. Means are also provided for deriving a second quantity which is a measure of the attenuation of the microwave electromagnetic energy propagating over the predetermined distance in the formations, the second quantity being the attenuation constant, alpha , in the preferred embodiment. Further means, responsive to the second quantity, are provided for generating a third quantity, designated tpwl, which is representative of the travel time of microwave electromagnetic energy propagating over the predetermined distance in water having a lossiness determined from the second quantity. Finally, means responsive to the first and third quantities are provided for generating a porosity-indicative quantity, designated phi w, as a function of the first and third quantities. In the preferred embodiment of the invention, means are provided for deriving a fourth quantity, designated tpm, representative of the travel time of microwave electromagnetic energy propagating over the predetermined distance in the formation matrix. In this embodiment, the means for generating a porosity-indicative quantity is also responsive to the fourth quantity for generating the porosity-indicative quantity as a function of the first, third, and fourth quantities.