A New Acoustic Energy-Based Method to Estimate Pre-Loads on Cored Rocks

A New Acoustic Energy-Based Method to Estimate Pre-Loads on Cored Rocks

Murat Karakus (University of Adelaide, Australia), Ashton Ingerson (University of Adelaide, Australia), William Thurlow (University of Adelaide, Australia), Michael Genockey (University of Adelaide, Australia) and Jesse Jones (University of Adelaide, Australia)
DOI: 10.4018/978-1-5225-2709-1.ch008
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Abstract

The Acoustic Emission (AE) due to the sudden release of energy from the micro-fracturing within the rock under loading has been used to estimate pre-load. Once the pre-load is exceeded an irreversible damage occurs at which AE signals significantly increase. This phenomenon known as Kaiser Effect (KE) can be recognised as an inflexion point in the cumulative AE hits versus stress curve. In order to determine the value of pre-load (sm) accurately, the curve may be approximated by two straight lines. The intersection point projected onto the stress axis indicates the pre-load. However, in some cases locating the point of inflexion is not easy. To overcome this problem we have developed a new method, The University of Adelaide Method (UoA), which use cumulative acoustic energy. Unlike existing methods, the UoA method emphasises the energy of each AE, the square term of the amplitude of each AE. As the axial pre-load is exceeded, the micro cracks become larger than the existing fractures and therefore energy released with new and larger cracks retain higher acoustic energy.
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Background

Acoustic emissions are bursts of high frequency elastic waves emitted by the localised failure of a material when it is subjected to a loading (Villaescusa et al., 2002; Karakus, 2014; Karakus et al., 2015, 2015; Yuan and Li, 2008). The generation of AE within rock materials occurs when they are subjected to a stress of sufficient magnitude to induce pore compression and micro cracking (Seto et al., 1997; Yamamoto, 2009). Therefore, as a rock material is subjected to a stress that exceeds the maximum previously applied stress, irreversible damage occurs within the rock and the rock forms a new ‘stress memory’ (Lavrov, 2002; Kurita and Fujii, 1979; Karakus, 2014; Karakus et al., 2015; Yamshchikov et al., 1994).

Key Terms in this Chapter

Felicity Ratio (FR): The ratio of the estimated stress to the pre-loaded stress.

The University of Adelaide (UoA) Method: The method that uses acoustic emission energy to locate point of inflexion by the authors of the chapter in 2014.

Master Cores: 63.5 mm core samples extracted from main borehole.

Acoustic Emission (AE): Bursts of high frequency elastic waves emitted by the localised failure of a material when it is subjected to a loading. The generation of AE within rock materials occurs when they are subjected to a stress of sufficient magnitude to induce pore compression and micro cracking. The phenomenon behind the use of the AE technique is that a lack of micro cracking, and accompanying AE activity, occurs when a rock material is loaded at levels below its previous stress state.

Kaiser Effect (KE): The phenomenon behind the use of the AE technique is that a lack of micro cracking, and accompanying AE activity, occurs when a rock material is loaded at levels below its previous stress state (s m ). At the previously ‘memorised’ maximum stress (s m ), there is an abrupt increase in the level of micro cracking and collapsing of pores within the material. This closure and propagation of fractures is associated with a significant increase in AE activity is known as Kaiser Effect, first discovered by Joseph Kaiser in the mid-20 th Century.

Threshold Voltage: A voltage level used to eliminate outside noises during loading of rock samples.

The Inflexion Point: The inflexion point marks the point of the KE with the corresponding stress representing the previously applied stress (See Figure 1 ).

Cumulative AE Hits: Total number of acoustic emission signals over threshold voltage level during loading of rock samples.

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