Deep Induction-log readings are often influenced by drilling fluids. If invasion by mud filtrate is deep and R_t/R_(xo) is small, the Induction log gives values that are higher than the true resistivity. Some water-bearing intervals that are severely invaded could confuse the log analyst by appearing as oil-bearing during interpretation for water saturation. This paper describes a method that allows the analyst to compute invasion diameter (d_i) and true formation resistivity (R_t) from the measurements made with the Induction-Spherically Focused (ISF)~4 -R_(xo) combination tool. The ISF log has proven to be a most useful logging tool under conditions of R_(xo) > Rt_i, R_(mf) > R_w, and shallow invasion. The method covers mainly the conditions when R_(xo) is greater than R_t, When R_(xo) is less than R_t the Induction log becomes less accurate for R_t evaluation. An accurate invasion correction chart has been constructed to determine values of d_i, R_(xo)/R_t, and R_t. Without this correction a serious error could result in the calculation of water saturation (S_w), especially in a deeply invaded formation and with R_(xo) R_t.
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