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

Try widening your search to All Fields.

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (((("ground penetration rate") OR ("grout penetration rate"))) OR ("ground penetrating radars")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"ground penetration rate\") OR (\"grout penetration rate\"))) or (\"ground penetrating radars\"))".
• Removing quotes may allow a broader search: (((( ground penetration rate ) OR ( grout penetration rate ))) or ( ground penetrating radars )).
• Adding a wildcard symbol may retrieve word variants: (((("ground penetration rate") OR ("grout penetration rate"))) or ("ground penetrating radars"))*.