The purpose of this study is to estimate groundwater aquifer formation-strength parameters including shear modulus, bulk modulus, Poisson's ratio, and Young's modulus by using geophysical well logs. A new dispersed-shale index equation was developed by using the natural gamma-ray log and the compensated formation density log to solve a confusing problem of the compaction factor setting in the calculation of sonic porosity for an unconsolidated groundwater aquifer. A useful Poisson's ratio estimation method was employed to estimate groundwater aquifer formation-strength parameters when shear-wave transit time data is lacking in groundwater wells. Hydrogeologic parameters are characterized in estimating formation-strength parameters. Five wells in the southwestern coastal area of Yun-Lin, Taiwan, were logged, and four shallow aquifers were identified from log-derived hydrogeologic characteristics less than 200 m in depth. The formation-strength parameters for aquifers between 310 and 500 m in depth were calculated in two wells because complete formation density and compressional-wave transit time data were available. The results of the aquifer's formation-strength parameters demonstrate that both shear modulus, ranging from 0.15 to 0.42 * 10(6) psi, and Young's modulus, ranging from 0.40 to 1.07 * 10(6) psi, increase with depth, whereas bulk compressibility, ranging from 1.2 to 2.6 * 10(-6) psi(-1), decreases with increasing depth.