* Order book $623 mln at end Aug from $689 mln end June
* Says $240 mln of orders "about to be signed"
* 95 pct of 3D capacity already booked for Q4 at Q3 prices (Adds detail)
OSLO, Sept 13 (Reuters) - Seismic surveyor PGS, which scans the ocean floor for hydrocarbon deposits, suffered a dip in orders during the summer but expects a surge in demand as oil firms rush to book available capacity, it said on Thursday.
"Oil companies are proactive in securing capacity and the market momentum going into 2013 looks good," the firm said in a presentation to investors.
PGS, a key beneficiary of the recent oil exploration boom, said its order book had dipped to $623 million by the end of August from $689 million at the end of June but deals worth $240 million were "about to be signed".
It added that 95 percent of its 3D capacity, its flagship service, was already booked for the fourth quarter at prices on on a par with the third quarter and its booking level was already at around 70 percent for the first quarter of next year.
PGS shares have risen 37 percent in the past three months and 56 percent in the past year as oil companies rush to speed up offshore exploration and production to meet increasing global demand and take advantage of oil prices holding above $115 per barrel.
The boom has created a shortage for everything from rigs and seismic surveys to people, pushing prices higher and providing lucrative benefits to oil services firms.
In July, PGS raised its 2012 earnings forecast for the third time in three months.
Discussing the broader industry, PGS predicted rising demand for seismic surveys as key regions from the Gulf of Mexico to the North Sea, Australia and Brazil push ahead with exploration.
"From 2006 to end 2011, demand for seismic has grown by more than 70 percent measured in square kilometres (and) growth in 2012 is expected to be in excess of 10 percent," it said.
However, global seismic capacity is only rising by 6 percent in both 2012 and 2013, pushing capacity utilization higher, it added. (Reporting by Balazs Koranyi; Editing by Mike Nesbit)