## 1 Introduction  
A coal burst, also called a rockburst, is a typical dynamic disaster during coal mining, with the distinctive features of sudden occurrence accompanied by strong vibration and destruction, which has become the most serious challenge of safety in deep mining [1]. Most mining countries, including China, Poland, the USA, Australia, and South Africa, are suffering from coal burst disasters [2; 3; 4]. According to statistics, there are approximately 138 burst-prone coal mines in China as of 2021, and their number keeps increasing every year [5]. Great achievements have been made on the mechanism, prediction, and control of this phenomenon, and dozens of theories have been proposed from different perspectives over the past decades [6; 7; 8; 9; 10]. A coal burst is a dynamic ejection of coal and/or surrounding rocks into the excavation. Sometimes it is accompanied by floor heating or roof caving. Due to the cause of the occurrence, coal bursts are divided into a stress coal burst, stroke (roof) coal burst, and mixed stress-stroke coal burst. Some authors also distinguish a fault rock burst. [11; 12; 13; 14]. Research shows that the majority of coal burst disasters are controlled and induced by the adjacent strong massive roof strata in China as the longwall mining method is used extensively [15; 16; 17]. Thus, the presence of strong massive roof strata close to the coal seam is taken as an indicator of coal burst hazard in practice and then, the analysis of the roof break law and monitoring of the stress changes during roof movement are of great significance for coal burst risk evaluation and prevention [18; 19].  
The monitoring of the precursor information before a coal burst is always a key issue and research hotspot since the first occurrence, and different kinds of devices and technologies have been developed and applied, of which micro-seismic monitoring, coal seam-stress monitoring, drilling bits, acoustic emission, electromagnetic emission are most popular methods used in China coal mines [20; 21]. Due to the complexity of coal burst, each technology has obvious pros and cons and can hardly describe the stress distribution in a wide area. Based on microseismic technology, seismic wave velocity computed tomography (CT) has been introduced and successfully used in coal mines and proved to be a powerful tool for the evaluation of dynamic hazards during underground mining [22; 23; 24; 25; 26]. Both \"passive\" and \"active\" velocity tomography have numerous reported studies and overcame lots of technical difficulties. Passive CT uses mining-induced tremors as the seismic sources and, therefore, can continuously and timeously carry out research with little limits but low resolution [27], while active CT is relatively time-consuming because of preparing artificial seismic sources, in most cases blasting but with high accuracy, and more importantly, active CT could be implemented before the longwall face excavation [28]. At present, for a specific longwall face, the passive and active CT were used independently, and few took both methods, that led us only to have partial features of stress concentration. In this paper, we combined the active and passive CT into one monitoring system to obtain stress information and assess the coal burst risk comprehensively in kilometer longwall face with a hard-thick roof so as to provide a basis for the design of prevention parameters and ensure the safety during the fracture and movement of the hard-thick roof.