Review of Methods of Measuring Stress and its Variations

Methods of measuring stress and its variations are briefly reviewed with particular interest in precise measurements at depth. Stress-relief methods are widely used techniques in the engineering field. Wireless strain-cells indicate the possibility of stress-relief methods for deep wells. However, elastic coupling of [rock mortar strain-meter] must be precisely estimated including thermal stress e#ects, before application to depth. The hydraulic fracturing method has been widely used in the geophysical field. However, serious suspicions about the interpretation of reopening pressure have also been raised in the past ,* years. As possible answers to these suspicions have been proposed recently, it is necessary to check past results again and cross-check the results of di#erent methods. Non-hydrofracture methods are free from problems associated with the permeation of pressure fluid into artificial fracture and borehole wall. Furthermore, some non-hydrofracture methods do not need the assumption of principal stresses having a constant direction, which can be an advantage over hydrofracture methods for long-term observations of stress variations. Corebased and borehole-wall-fracture-based methods, unfortunately, cannot now be considered precise measurement methods. However, these methods have the potential to estimate stress, particularly at great depths.