Determining anisotropic resistivity

A system to determine the vertical resistivity of a subsurface formation, comprising:

• a downhole logging tool having a plurality of spaced antennas, at least one of which is a transverse antenna, at least two of which are tilted antennas, and at least two of which are axial antennas;
• and
• a processor capable of:
• forming voltage ratios using measurements involving the transverse and/or the tilted antennas of the downhole logging tool;
• forming conditioning factors by raising the voltage ratios not involving the transverse antenna to some power, wherein the sum of the exponents of the conditioning factors equals one;
• computing a ratio using a voltage ratio involving the transverse antenna and the conditioning factors;
• and
• determining the vertical resistivity of the subsurface formation using the computed ratio.

A system having a non-transitory computer-readable medium having a set of computer-readable instructions encoded thereon that, when executed, perform acts comprising:

• obtaining measurements involving the transverse and/or the tilted antennas of a downhole logging tool;
• forming voltage ratios using the measurements;
• forming conditioning factors by raising the voltage ratios not involving the transverse antenna to some power, wherein the sum of the exponents of the conditioning factors equals one;
• computing a ratio using a voltage ratio involving a transverse antenna and the conditioning factors;
• and
• determining the vertical resistivity of the subsurface formation using the computed ratio.