Effects Of Skin-Effect Correction And Three-Point Deconvolution On Induction Logs

Skin-effect correction and three-point deconvolution are commonly applied to the induction sonde signals. The original objectives are to make the apparent resistivity reading closer to the true resistivity of the formation and to improve the vertical resolution of the induction sonde. However, it is shown in this paper that these objectives are not always met. Skin-effect correction results in true formation resistivities only in thick beds. When the resistivity contrast between the shoulder bed and the resistive bed is small (e.g., 10 to 5 ohm-m), the three-point deconvolution process has no significant effect on the resistivity log. For moderate resistivity contrast (e.g., 10 to 1 ohm-m), the three-point deconvolution process has a desirable effect of improving the resolution of the bed boundary. Quantitatively speaking, however, reading of the resistivity in a thin bed still needs correction even when the data are processed by the three-point deconvolution algorithm. When the resistivity contrast is high (e.g., 100 to 1 ohm-m), the three-point deconvolution process causes high-resistivity streaks to appear in the log. For very high resistivity contrast (e.g., 100 to 0.5 ohm-m), infinite resistivity streaks appear in the log, and their appearance is entirely due to the three-point deconvolution. An algorithm is developed whereby the original raw data may be retrieved from the deconvolved data.