Undrained shear strength is a very critical property to evaluate resistance of ground for geotechnical engineering.

Undrained shear strength can be inferred by void ratios. 

Field void ratios with depth are analogically observed from field shear wave velocities with relationship between void ratios and shear wave velocities. 

In similar way, field void ratios can be predicted by electrical resistivities. 

Finally, undrained shear strength-tomography can be developed by this technique.