<p>That seems way too low, even for crustal rocks. Recall: V<sub>p</sub> = <span class="math-container">$\sqrt{[(K + 4G/3)/]}$</span></p>

<p>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 <a href="https://pangea.stanford.edu/courses/gp262/Notes/9.EffectiveMediumTheories.pdf" rel="nofollow noreferrer">Voigt-Reuss-Hill</a> average and use a reasonable density value (maybe 2500 kg/m<sup>3</sup>) to get a reasonable number for V<sub>p</sub>.</p>

<p><a href="https://en.wikipedia.org/wiki/P-wave" rel="nofollow noreferrer">This Wikipedia page</a> has a table of velocities for common rocks that might be helpful.</p>

CC BY-SA 4.0
2020-05-06T11:46:41.933Z
@CarlosJavier you should edit those details in to your original question.
Spencer
CC BY-SA 4.0
2019-08-12T15:29:44.627Z
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?
Carlos Javier
CC BY-SA 4.0
2019-08-12T19:28:03.030Z
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.
g.z.
CC BY-SA 4.0
2019-08-12T19:38:59.817Z
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?

https://web.b.ebscohost.com/ehost/pdfviewer/pdfviewer?vid=1&sid=336739d9-26f9-47fc-a4a6-4f12b819ed40%40sessionmgr103
g.z.
CC BY-SA 4.0
2019-08-12T19:45:25.967Z
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.
g.z.
CC BY-SA 4.0
2019-08-13T21:36:08.663Z
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?
Carlos Javier
CC BY-SA 4.0
2019-08-09T22:54:04.487Z
2020-05-06T13:31:39.513Z
2020-05-06T13:31:39.513Z
g.z.
<p>Andesite can present very large vugs, unconnected massive pores, then it may explain a mass density &lt; 2 kg/m<span class="math-container">$^3$</span>.
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%.  </p>

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

<p>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.</p>

CC BY-SA 4.0
2020-05-08T06:47:29.107Z
2020-05-08T06:47:29.107Z
EarlGrey
<p>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.</p>

<p>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<span class="math-container">$^3$</span> 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.</p>

<p>A backhoe couldn't excavate more than 1.0 m. So my concern of rippability expected based on Seismic velocity V<sub>p</sub>.</p>

<p>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.</p>

<p>Can an igneous rock have a V<sub>p</sub> of less than 1000 m/s on this situation and need blasting?</p>

<geology><geophysics><seismology>
Can an igneous rock have a Vp of less than 1000 m/s and need blasting to excavate?
Carlos Javier
# Can an igneous rock have a Vp of less than 1000 m/s and need blasting to excavate?

(Asked by: username_0 on 2019-08-09)

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?

**Tags:** geology, geophysics, seismology


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## 2 answers

### Answer by username_1 on 2019-08-09. Score: 1

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

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](https://pangea.stanford.edu/courses/gp262/Notes/9.EffectiveMediumTheories.pdf) average and use a reasonable density value (maybe 2500 kg/m3) to get a reasonable number for Vp.

[This Wikipedia page](https://en.wikipedia.org/wiki/P-wave) has a table of velocities for common rocks that might be helpful.

> **Comments:**
> **username_0**: 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?  
> **username_1**: 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.  
> **username_1**: 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?  https://web.b.ebscohost.com/ehost/pdfviewer/pdfviewer?vid=1&sid=336739d9-26f9-47fc-a4a6-4f12b819ed40%40sessionmgr103  
> **username_1**: 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.  
> **username_0**: 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?  
> **username_2**: @username_0 you should edit those details in to your original question.  

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### Answer by username_3 on 2020-05-08. Score: 1

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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