Abstract: The Wenner four-electrode method, employing independently loaded electrodes, effectively avoids polarization and contact resistance and is widely used for geoelectrical testing. However, probe insertion disturbs the specimen structure, and varying penetration depths cause differences in current field attenuation, limiting its reliability and application in hard geomaterials. To address this, an indoor four-electrode penetration testing system is developed and experiments on sandy and clayey soils are conducted to examine resistivity variation with penetration depth. Results show that the resistivity growth rate decreases with increasing penetration depth and tends to zero, strongly influenced by water content and dry density. A threshold tangent factor (ψ) and minimum penetration rate threshold (V) are introduced to establish soil-specific threshold models, with their governing mechanisms analyzed and a nonlinear regression model constructed. Based on resistivity attenuation and threshold models, a nondestructive evaluation model (S-I model) is proposed and validated.