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

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 radar") OR ("ground penetration rate"))) OR ("ground penetrating radars"))) OR ("ground penetration raman")).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((((\"ground penetration radar\") OR (\"ground penetration rate\"))) or (\"ground penetrating radars\"))) or (\"ground penetration raman\"))".
• Removing quotes may allow a broader search: (((((( ground penetration radar ) OR ( ground penetration rate ))) or ( ground penetrating radars ))) or ( ground penetration raman )).
• Adding a wildcard symbol may retrieve word variants: (((((("ground penetration radar") OR ("ground penetration rate"))) or ("ground penetrating radars"))) or ("ground penetration raman"))*.