What does a negative Poisson's ratio of soil and rock mean?

[Yusei]

Hi
We conducted downhole seismic tests and we're able to get p- and s-wave velocities. When we calculated the Poisson's ratio at each depth, there were negative values at various depths. Most materials at these depths are soil and there are some depths where the material is rock. What could could these results mean? What does a negative Poisson's ratio of soil and rock mean?
Thank you.

 

[geotechguy1]

Is there any reason to suspect you might have say, repeated layers of buried material that might have a negative Poisson's ratio? Wood maybe? That's about the only thing I could think of.

Did you muck up the calculation and end up with a negative instead of a positive? Share a bit of the data and how you did the calc to derive Poisson's ratio.

 

[Yusei]

I calculated Poisson's ratio using the formula:

Here are my calculations in one of the boreholes:

 

[EireChch]

something is wrong with you data, acquisition system, geophones, or calcs. Soil and rock can have negative v.

 

[geotechguy1]

The Vp to Vs ratio suggests the top 10m is extremely dry interestingly, wonder if it's an unsaturated soil mechanics thing? Vp in soil is very dependent on the degree of saturation / water content right? Wonder if its an unsaturated soil mechanics thing. Although I'm not sure you expect a dry soil to expand in tension? Although I guess we say it dilates / expands in shear..although that should only be near a shear surface.

 

[geotechguy1]

https://www.researchgate.net/figure...ree-of-saturation-and-b-matric_fig4_344811412

In this research they test a silt at various water contents and poissons ratio is always positive. So that suggests that idea is out. A negative ratio doesn't really make sense. I wonder if that formulae is strictly applicable to soils? Or if the assumptions inherent in deriving it aren't applicable?

I'm not sure I'd buy the data being bunk... although you could check that. Do you know from other measurements where ground water is and what the degree of saturation is?