Perhaps you should try some spelling variations:
"ground penetration raman" » "grout penetration raman", "cloud penetration raman", "rod penetration raman", "ground penetration human", "ground penetration rate", "ground penetration bmn"
"ground penetrating radar" » "ground generating radar"

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 penetrating radar") OR ("ground penetration raman")).
• Removing quotes may allow a broader search: (( ground penetrating radar ) or ( ground penetration raman )).
• Adding a wildcard symbol may retrieve word variants: (("ground penetrating radar") or ("ground penetration raman"))*.