Perhaps you should try some spelling variations:
"ground penetration radar" » "grout penetration radar", "cloud penetration radar", "rod penetration radar", "ground penetrating radar", "ground penetration rate", "ground penetration der"
"ground penetration raman" » "grout penetration raman", "cloud penetration raman", "rod penetration raman", "ground penetration human", "ground penetration rate", "ground penetration a"
"ground penetration bmn" » "grout penetration bmn", "cloud penetration bmn", "rod penetration bmn", "ground penetration human", "ground penetration body", "ground penetration gene"
"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 penetrating radars"))) OR ((("ground penetration raman") OR ("ground penetration bmn")))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "((((\"ground penetration radar\") or (\"ground penetrating radars\"))) or (((\"ground penetration raman\") OR (\"ground penetration bmn\"))))".
• Removing quotes may allow a broader search: (((( ground penetration radar ) or ( ground penetrating radars ))) or ((( ground penetration raman ) OR ( ground penetration bmn )))).
• Adding a wildcard symbol may retrieve word variants: (((("ground penetration radar") or ("ground penetrating radars"))) or ((("ground penetration raman") OR ("ground penetration bmn"))))*.