From the Hydrogeological Conceptual Model to the Apparent Model with Electrical Resistivity Measurements: Case Studies in the Choushuichi and Ilan Fan-delta in Taiwan

Abstract:

In the study we used the resistivity measurements across two fan-delta systems, the
Choushuichi and Ilan fan-deltas, to establish three-dimensional (3D) hydrogeological
apparent models. The resistivity measurements includes the half-Schlumberger
surveys conducted during the year of 1990-2000 across the entire areas, and the
two-dimensional resistivity data collected recently for the purpose of characterizing
the recharge zone boundaries between the upper-fan gravels and the lower-fan clayey
sediments. Core records from the monitoring wells in the area were used for the
training data to help determining the resistivity ranges of the gavel, sand, and muddy
sediments in the fan-delta. The resistivity measurements were inverted and converted
into 1-D data form and interpolated for rendering a three dimensional resistivity
volume that represents the general resistivity distribution in the fan-delta systems.
Water resistiviy data from the observation wells were used for calculating the
formation factors (FI) within the models. We compared the FIs with indexed core
records near some of the resistivity surveys sites and concluded the range of the FIs
for different materials in a statistical sense. Lastly we transfer the resistivity models
into the gravel-sand-clay geological models with the computed formation factors.
Since the spatial distribution of non-invasive resistivity measurements is more dense
than the locations of geological cores. These “apparent” models are better to represent
the detailed sedimentary structures than the traditional over-simplified conceptual
models built from only the correlations of core records. Hence we are able to quickly
construct the 3D discretized apparent models with three geologic materials for further
modeling purposes.