Application of a streamer resistivity survey in a shallow brackish-water reservoir

Abstract

To delineate the resistivity structure of sub-bottom sediments in a shallow brackish-water reservoir in the western coastal area of Korea, we carried out a streamer resistivity survey using a dipole–dipole array. First, through numerical testing, we confirmed that the resistivity method with a dipole–dipole array could be applied in a shallow marine environment, when the resistivity contrast between water and the underlying sediments ranges from a factor of 3 to 5. Also, inversion with a water layer explicitly included is more effective than the conventional inversion method in resolving power, which we confirmed by observing that the inversion results for synthetic datasets matched better when a water layer was included in the inversion procedure.

After constructing a data acquisition system composed of a resistivity meter, GPS, and echo sounder, and developing data processing software, we conducted a streamer resistivity survey and inverted the data obtained to identify the hydrogeological sequences and sediment characteristics at the bottom of the shallow brackish-water reservoir. Drill logs identified three sediment layers, including silty sand, fine sand, and mixed sand. The resistivity distributions from inversion matched the resistivity ranges measured on materials obtained by sampling near the drilling points. We constructed a contour map of the top of the mixed-sand layer, using semivariogram analysis. Comparing these results with the drilling results, the depth to each layer, and the measured and estimated resistivity range of the materials, also corresponded to resistivity profile. From this study, we are assured that the streamer resistivity method would be a useful tool for surveying shallow brackish-water reservoirs.