## 1 Introduction  
Spherical storage tanks are used in many industries, including the petrochemical, chemical, water management, and aerospace industries. They can store substances such as gasoline, oxygen, waste water, and liquefied petroleum gas (LPG) [1; 2; 3]. Along with the usefulness of storage tanks in industrial processes comes great responsibility. Installation of storage tanks in a facility can put environmental and public safety at risk when these tanks contain hazardous substances. Leaks through the cracks in storage tanks containing hazardous substances can pollute the air, land, and water [4]. This contamination of the environment can be fatal. Factors that can lead to leakage include cracks, corrosion, pipe failure, and poor maintenance. To avoid health risks, tank conditions must be monitored. Among the many ways to inspect the condition of a storage tank, the most popular and efficient are nondestructive testing methods, which have the primary advantages of not taking components out of operation during the integrity test, and not cutting them apart or altering them during the test [5]. The typical nondestructive testing methods include ultrasonic, radiographic, eddy current, electrical resistance measurement and acoustic emissions testing [6; 7; 8; 9; 10; 11]. Among these methods, acoustic emission and electrical resistance measurements can detect early stage as well as already existing damages in an object. In the case of the electrical resistance method, when the failure stress is approached, the opening of the cracks cause variation of the electrical resistance. This variation indicates the initiation or presence of a crack and void in the structure. Similarly, when damage takes place in an object, it releases elastic waves which can be recorded as acoustic emission signals. These elastic waves (acoustic emission) also provide valuable information about the crack and plastic deformation in the object. Therefore, these two techniques can be used separately or in combination to address the issue of crack and leak detection in composites. In combination, acoustic emission testing can be used as primary means to detect cracks in an object, and then electrical resistance measurement of the object can be used to verify the results of acoustic emission testing. In this work, as the objective is to classify cracks of spherical tanks, acoustic emissions (AE) are just used as a means of nondestructive testing to record data regarding integrity testing of the spherical tank. AE testing has two main advantages: (1) No external energy is supplied in the AE recording; instead, the sound generated within the object is recorded. (2) AE testing can record dynamic changes in material, giving it the critical ability to discern growing and stagnant defects [5; 12]. These advantages under real-time conditions make acoustic emission testing a favorable choice to acquire data regarding tank structural integrity. Moreover, AE testing provides wide-band signal recording, which makes the spectral analysis of the signals much easier. In this way, plastic deformation and microscopic leaks of a material can be investigated in real time. Few quantitative approaches are described in the literature regarding the detection of spherical tank leakage as a classification problem. A study of leakage detection in a tank storing water for firefighting is presented in [13]. The authors examined AE waves acquired from a tank that had small holes at its bottom due to corrosion. They analyzed the fundamental characteristics of AE waves to identify the waves associated with the holes. A similar study [14] used wavelet neural networks to identify AE signals from different types of tanks. After employing a wavelet denoising algorithm, the signals were decomposed to a certain level, and the node energy distributions and extracted feature vectors from the corrosion signals were provided to the neural network to carry out the experiment. In the current study, we considered the case of a spherical tank installed in chemical industry.