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I have run a 60 m refraction seismic line on a mountain summit. The environment was noisy (wind would be the principal agent) and quite difficult to pick first arrivals. It resulted in 800 m/s.

A borehole has shown the profile stands the same after 0.5 m depth of biologic weathering with no fault nor joints to 10 m. Sample taken only at 1 m shows UCS 5 MPa, 10% water absorption which leads to 0.16 porosity, 17.5 kN/m$^3$ bulk density. My geologist partner has identified it as rhyolite or andesite, and attributes the low density to weathering action that can extend to several meters.

A backhoe couldn't excavate more than 1.0 m. So my concern of rippability expected based on Seismic velocity Vp.

Can an igneous rock have a Vp of less than 1000 m/s on this situation and need blasting?

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That seems way too low, even for crustal rocks. Recall: Vp = sqrt[(K + 4G/3)/𝜌]

If you have a rough idea of the mineralogy of the rock (you do if you know that it's andesite or rhyolite), you can estimate the bulk modulus (K) and shear modulus (G) with a Voigt-Reuss-Hill average and use a reasonable density value (maybe 2500 kg/m^3) to get a reasonable number for Vp.

This wikipedia page has a table of velocities for common rocks that might be helpful.

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  • $\begingroup$ Bulk density has been measured (17.5 kN/m3). Rock type has been estimated visually and according to a general geologic map (I'm sure it is an igneus rock. It could be also ignimbrite) . All tests results has been shown above. ¿Would you mention some bibliography reference (a book) for K and G parameters for rocks? $\endgroup$ – Carlos Javier Aug 12 at 15:29
  • $\begingroup$ I'll look around for some references on those elasticity values and report back. Are you confident in the density value you have for the rock? 17.5 kN/m^3 is 1.78 g/cm^3 which is very low, unless it is VERY heavily weathered, like a saprolite. $\endgroup$ – g.z. Aug 12 at 19:28
  • $\begingroup$ This link may be helpful. I had to use my university vpn to access it. If you can't access it let me know, and maybe I can email you the paper? web.b.ebscohost.com/ehost/pdfviewer/… $\endgroup$ – g.z. Aug 12 at 19:38
  • $\begingroup$ By the way, after reading the paper in the link, it looks like I was wrong about velocities. For relatively shallow depths (<45 m), it looks like s-wave velocities can vary between .075 to .425 km/s. So your p-wave velocity might actually be okay based on how weathered the rock is. $\endgroup$ – g.z. Aug 12 at 19:45
  • $\begingroup$ 1) Yes. I am confident about density estimation based on four (4) dried rock cores -one of them diameter 74.6 mm, height 144.8 mm, and mass 1135 g-. 2) No, I could not access the document linked. 3) What about rippability considerations, would you have any document at hand to share? $\endgroup$ – Carlos Javier Aug 13 at 21:36

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