(Adds details on other dams still at risk, weather)
By Lisandra Paraguassu and Reese Ewing
BRASILIA, Nov 17 (Reuters) - Brazilian miner Samarco said on Tuesday it is conducting real-time monitoring and emergency repairs on two of its tailings dams that suffered damage in the wake of the Nov. 5 collapse of another dam that killed 11 and left another 12 missing.
Officials of Samarco, an iron ore venture owned by miners Vale and BHP Billiton, said at a news conference on Tuesday that the company's Santarem dam was most at risk of collapse and that its teams were racing to transport 500,000 cubic meters (653,975 cubic yards) of stone to shore it up.
Samarco Operations Director Kleber Terra said he expected repairs to Santarem as well as at the Germano dam to take 45 to 90 days and that the rock was coming from Samarco's own mine.
But local weather forecaster Somar expects heavy rains to move into the region on Tuesday.
Jose Vasconcelos, civil engineer at the company said Santarem had been overrun by roughly 40 million cubic meters of mud and water released after the Nov. 5 collapse of the Fundao dam above it.
He estimated 20 million cubic meters of mud and mine debris settled in the valley below, but about 5.5 million cubic meters of that material remains contained by the Santarem dam.
Soon after the collapse of Fundao, the company had reported that Santarem had also given way, but upon later inspection officials discovered that Santarem's dike was mostly intact, but its dike had suffered considerable damage.
The company is trying to conduct repairs as quickly as possible before the region enters the rainy season, which could complicate repair efforts and further erode the damaged dike.
Heavy rains are moving across Brazil's southeast including the Rio Doce River Valley in Minas Gerais, where the disaster is unfolding, and down river in Espirito Santo state, where the mud is just now arriving 12 days after the dam burst.
The company's Germano dam, which is the biggest and oldest of Samarco's tailings dams, also suffered damage when the Fundao dam gave way.
But the Germano has been deactivated for years and engineers say it is almost completely dry and much more stable than Santarem is or Fundao was.
Samarco officials said they were continually monitoring the dams with radar, lasers and drones.
"There has been no movement detected in respect to the dikes," Terra added. (Reporting Lisandra Paraguassu; Writing by Reese Ewing)