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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.

Rock type has been estimated visually and according to a general geologic map (I'm sure it is an igneous rock. It could be also ignimbrite) . All tests results has been shown above, and I'm confident about them.

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/m3) 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, 2019 at 15:29
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    $\begingroup$ 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, 2019 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, 2019 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, 2019 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, 2019 at 21:36
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Andesite can present very large vugs, unconnected massive pores, then it may explain a mass density < 2 kg/m$^3$. Since the pores may not be connected, you may have porosity of 16%, while the pore space in reality is much bigger, up to 75%.

If it is basaltic andesite it may be very hard, with the the added Si content, if it just andesite it is relatively hard to drill.

If drilling in such environment, the proper diamond drill bit is a must. I am not aware if you can have backhoe with diamond teeth.

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