Perhaps you should try some spelling variations:
"ground penetration rate" » "grout penetration rate", "cloud penetration rate", "rod penetration rate", "ground penetration a", "ground penetration chapter", "ground penetration tester"
"ground penetrating radars" » "ground penetrating radar", "ground penetrating bars", "ground penetrating beams"

Try widening your search to All Fields.

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• If you are trying to use Boolean operators, they must be ALL CAPS: (("ground penetration rate") OR ("ground penetrating radars")).
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