This disclosure describes a method of assessing fractures in an underground formation using temperature profile measured by a DTS. The fractures can be identified by a temperature profile established by the feedback signals obtained from the DTS, but the DTS measurements are made at least 36 hours after a stage is fractured and during shut-in, thus allow crossflow and other noise sources to abate. Further, the size of the temperature differential is indicative of fracture spacing or density, not volume of fluid flow as thought in the prior art. Thus, these methods can be used to optimize the next stage or next well in a fracturing program.