In a move to strengthen its position in digital media and mobile advertising, WPP announced on Monday that it has acquired semantic advertising solutions group Crystal Semantics.The purchase was made by the media group's marketing technology arm, 24/7 Media.London-based Crystal Semantics uses technology to match advertising to the meaning of a page of web content in an attempt to reduce the risk of inappropriate advertising placement."With the acquisition, the combined companies will provide distinct advantages to both advertiser and publisher clients through improved advertising relevance and enhanced brand protection," WPP said.Crystal Semantics will add to WPP's digital revenues - totalling well over $5bn in 2012 - which account for around a third of group sales.The stock was 0.76% higher at 1,324p by 09:06 on Monday.BC