injected flow in that case corrections of the pressure drops due to viscous flow along the well have to be performed the interpretation of well pressure with respect to hydraulic fracture growth can be rendered difficult due to damage around the well bore disturbing the fluid pressure gradients typical analysis of pressure decline shut in and fall off data g plot etc can help estimating a number of characteristics of the fracture treatment efficiency closure stress etc economides and nolte 2000 5 a number of simplifying assumptions are usually made in order to analyse pressure records acoustic emission monitoring also sometimes call passive seismic consists of recording using precise accelerometers the seismic signal produced by the growth of the fracture an array of geophones is used and analysis of the signal enables to locate the seismic source the velocity of seismic waves in the rock mass must be known usually operators calibrate a velocity model by recording the signal generated by a known source typically during the perforation of the casing this method can provide good results in homogeneous rock mass if the array of sensor covers well the fracture area in the case where natural joints are present in the vicinity of the hydraulic fractures multiple shear events along the natural joints can occur this can u2018 blur u2019 the overall picture given by the analysis of the recorded signals a tiltmeter which is tightly installed in the rock measures at its location changes of inclination in two orthogonal directions with a precision down to nanoradians the tilts recorded are a direct measure of the small strain rotation at the.