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