# Feasibility of Identifying Shale Sweet Spots by Downhole Microseismic Imaging
## 1 Introduction  
The sweet spot of geological formations is the concurrence of several favorable geologic contents such as fracturing, thermal history, gas content, reservoir thickness, or matrix rock properties, and, economically, areas that are most likely to be the developed parts of continuous accumulation [1]. [PERSON] and [PERSON] [2] defined that a shale sweet spot is a region likely to yield high gas production, associated with a low Poisson's ratio. With an increase in light hydrocarbon saturation, the compressional wave velocity decreases and shear wave velocity increases, thereby resulting in a low velocity ratio (Vp/Vs ratio) or Poisson's ratio. By analyzing Poisson's ratio distributions derived from conventional seismic processing of surface seismic data, they identified potential high-yield areas in shale formations. The Poisson's ratio, or the Vp/Vs ratio, serves as a critical indicator of reservoir lithology and fluid properties [3, 4]. Widely utilized in active seismic exploration, the Vp/Vs ratio facilitates hydrocarbon identification and stratigraphic interpretations from reflection seismic data [5, 6]. [PERSON] and [PERSON] [7] reviewed three methods for estimating the Vp/Vs ratio to elucidate rock properties from seismic data: AVO analysis coupled with the post-stack inversion of AVO attributes [8], the prestack inversion of P-wave gathers directly to impedances [9], and the joint prestack inversion of P-wave and converted wave data to impedances [9]. Despite the effectiveness of these techniques in identifying sweet spots within reservoirs, their high cost and lengthy processing times restrict routine use [10, 11]. In passive seismology, 3D passive seismic traveltime tomography is recognized as a crucial technique for understanding seismotectonics and seismogenic processes across extensive regions [12; 13; 14]. The methodology has been comprehensively reviewed by experts such as [PERSON] [15], [PERSON] [16], and [PERSON] and [PERSON] [17]. Additionally, passive or microseismic traveltime tomography has proven effective in hydrocarbon and geothermal exploration, enhancing reservoir monitoring [18; 19] and delineating reservoirs at a lower cost compared to traditional 3D seismic surveys [20]. [PERSON] et al. [21] utilized 4D microseismic tomography to analyze space-time changes in response to fluid pressure variations. [PERSON] et al. [22] demonstrated the potential of microseismic traveltime tomography for reservoir imaging and property estimation in an oil field using induced seismicity. [PERSON] et al. [23] conducted a case study that inferred high-resolution 3D Poisson's ratio variations through passive seismic tomography by estimating P- and S-wave velocities. [PERSON] et al. [24] applied traveltime tomography to microseismic data from a mining operation to achieve a detailed 3D velocity structure. Numerous studies [25; 26; 27; 28; 29; 30; 31] have also highlighted the promise of the tomographic measurement of Poisson's ratio or Vp/Vs ratio anomalies from earthquakes or induced seismicity for geothermal resource identification. Microseismic activity induced by hydraulic fracturing is typically monitored using geophone arrays positioned in nearby wells. To infer shale sweet spots characterized by a low Poisson's ratio, the effort involves the determination of microseismic event locations and 3D velocity models or a Poisson's ratio model with microseismic arrival time picks. It is well known that the locations of microseismic events are coupled with velocity models [32]. To address the coupling problem, [PERSON] [33] introduced joint inversion and alternating inversion with parameter separation; both methods can produce accurate models. An alternating strategy that has been commonly utilized in studies [15; 17; 22; 23; 34; 35; 36; 37] is a scheme of estimating a velocity model by fixing event parameters and then updating the velocity model based on the event location parameters to solve the location-velocity coupling problem.