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I've heard long ago that the rock material deeply below surface are checked by a device that acts like radar - it sends radiowaves into the ground, and geologists find out from reflections that what type of rock can be found there.

What's the name of this device or method, and how is it possible to differentiate rocks from radiowave reflection?

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  • $\begingroup$ This question seems rather vague. I think a little basic research would have paid off, and enabled you to ask a more penetrating question. $\endgroup$ – kwinkunks Apr 18 '14 at 23:15
  • $\begingroup$ @kwinkunks I got the answer, winwaed was right. $\endgroup$ – Zoltán Schmidt Apr 18 '14 at 23:29
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Do you mean Ground Penetrating Radar? This is typically limited to very shallow depths though - eg. archaeological investigations, police forensics (finding graves), and civil engineering site investigations. The latter can go to tens of meters and would be used for planning foundations, excavations, etc (I know of an example where it was used for site characterization for a planned rowing lake). The depth limit is typically due to attenuation in brackish groundwaters. Ice applications can typically go further due to the lack of conducting liquid water.

Or are you thinking of Reflection ('Active') Seismic Surveys, which work on a similar principle but using sound waves. Sound waves can travel to the far side of the Earth (cf. large earthquake 'P' waves), but reflection surveys are typically looking at the upper crust (few km) and rarely go beyond the base of the crust (aka the 'Moho') which is a strong seismic reflector. Reflection seismic surveys are widely used in the oil business.

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  • $\begingroup$ I guess it's GPR. I remember that it was used in an episode of Mythbusters. $\endgroup$ – Zoltán Schmidt Apr 18 '14 at 22:24
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    $\begingroup$ Although correct about GPR being for relative shallow depths, you seem to forget studies of sediments on the order of 10s of metres and ice (order km). The examples you mention would typically deal with 200 MHz and higher whereas impulse radars with antennas down to 5 MHz can see much deeper as mentioned. $\endgroup$ – Peter Jansson Apr 18 '14 at 22:32
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    $\begingroup$ A noteworthy limitation of GPR is strong attenuation by conductive groundwater, making it hard to use in brackish conditions. $\endgroup$ – kwinkunks Apr 18 '14 at 23:13
  • $\begingroup$ @Peter I was thinking of 10s of meters with the reference to civil engineering. I wasn't are of ice applications. I'll amend the answer and also mention the brackish limitation. $\endgroup$ – winwaed Apr 18 '14 at 23:29
  • $\begingroup$ Another related method to mention is shallow seismic refraction (SSR), which is a subset of active seismic. SSR is often higher-frequency and can be used to differentiate surficial and subseafloor sedimentary material from densely-packed till and bedrock, and also to find utilities and infrastructure. $\endgroup$ – Ian May 7 '15 at 23:56

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