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

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