Estimating depth of investigation in dc resistivity and IP surveys

In this paper, the term "depth of investigation" refers generically to the depth below which surface data are insensitive to the value of the physical property of the earth. Estimates of this depth for de resistivity and indu ced polarization (IP) surveys are essential when interpreting models obtain ed from any inversion because structure beneath that depth should not be in terpreted geologically. We advocate carrying out a limited exploration of m odel space to generate a few models that have minimum structure and that di ffer substantially from the final model used for interpretation, Visual ass essment of these models often provides answers about existence of deeper st ructures. Differences between the models can be quantified into a depth of investigation (DOI) index that can be displayed with the model used for int erpretation. An explicit algorithm for evaluating the DOI is presented. The DOI curves are somewhat dependent upon the parameters used to generate the different models, but the results are robust enough to provide the user wi th a first-order estimate of a depth region below which the earth structure is no longer constrained by the data. This prevents overinterpretation of the inversion results. The DOI analysis reaffirms the generally accepted co nclusions that different electrode array geometries have different depths o f penetration. However, the differences between the inverted models for dif ferent electrode arrays are far less than differences in the pseudosection images. Field data from the Century deposit are inverted and presented with their DOI index.