## 1 Introduction  
Fractures are found throughout crustal rocks, and fracture detection is of critical importance for hydrocarbon exploration, water resource management, CO\({}_{2}\) capture and storage, and geo-thermal exploitation. Aligned fractures can cause seismic anisotropy, making it possible to obtain fracture information from seismic data. Fracture-induced anisotropy is an integrated result of various fracture parameters (fracture density, scale, aspect ratio, etc. ), fluid properties (fluid modulus and viscosities), and background properties (modulus, porosity, and permeability). Background matrix porosity has been thought to greatly influence P-wave velocity anisotropy [1, 2]. Because fractures are present in low, mid, and high porosity hydrocarbon reservoirs, understanding the effect of background porosity on seismic anisotropy could further improve seismic fracture characterization in reservoirs of different porosities. To date, theoretical and experimental studies have been conducted to relate the physical properties of rock to seismic velocity and anisotropy, and various equivalent medium theories have been proposed [1; 3; 4; 5; 6]. These theories were based on different assumptions and were derived using different methods, resulting in different predictions. Thus, there is a desire to verify and calibrate these theoretical models through laboratory experiments. Laboratory experiments typically use synthetic samples with controlled and known rock physical properties and fracture parameters. A number of experimental studies have been conducted to observe elastic wave velocity and anisotropy in fractured rocks [7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22]. These studies have investigated how parameters, such as fracture density, scale, and aspect ratio, can influence elastic wave velocity and anisotropy, providing significant insights into the physical mechanisms of elastic wave propagation in fractured rocks. However, natural rocks have a complex depositional and diagenetic/tectonic history, and fracture parameters (fracture geometry and density) cannot be quantitatively controlled, making them too variable for experimental investigations concerning the effect of a single variable on seismic wave propagation. Thus, synthetic samples must be used in laboratory experiments to observe seismic wave propagation in cracked rocks. Despite variations in the background porosity of fractured reservoirs, few experimental studies, both in laboratory and field observations, have focused on the effect of background porosity on seismic anisotropy in these reservoirs. [PERSON] (2003) discussed P-wave anisotropy variation with background porosity in water-saturated rocks with aligned fractures in a low-frequency range using numerical simulations [1]. When there is no background porosity, P-wave anisotropy (\(\varepsilon\)) is very small. In the absence of background porosity, fluid is 'isolated' in fractures. The 'isolated' fluid acts to'stiffen' the fractures and reduce fracture compliance. This results in a relatively low P-wave anisotropy for the overall fractured rock. As the characteristic porosity increases, there is a sharp increase in P-wave anisotropy. When surrounded by pores, the fluid in these fractures can escape during compression; therefore, the fracture is more compliant than it would be in 'isolated' conditions. As porosity continues to increase, P-wave anisotropy flattens and decreases gradually. Other theoretical studies have also discussed the influence of porosity on anisotropic parameters [2; 23]. However, no experimental studies have been conducted to test and verify this, owing to the difficulties faced in manufacturing artificial fractured rocks with low porosity. This study experimentally examined the influence of background porosity on P-wave anisotropy in fractured rocks under water saturation conditions. A set of artificial fractured sandstone samples with the same fracture density (0.06), but different porosities (ranging from 15.3% to 30.8%), were built.