i'm using (ArcGIS 10.1) and i want to remove a negative values from DTM in order to keep just a ground topographic surface.
I would point out that, if your terrain dips below sea-level, negative values do represent real elevation values. You did not state if you wanted to set these values to a uniform value (eg., 0) or to NoData. Removing data is different than recoding it.
This can easily be accomplished in the raster calculator using a Con or SetNull.
To set negative values to a uniform value,
Con("elev" < 0, 0, "elev")
Or, to set negative values to NoData
SetNull("elev" < 0, "elev")