## 1 Introduction  
The physical and structural deformation of stone architectural heritage is generally caused by natural weathering and short-term damage. Fortunately, the rocks are considerably durable because they have been able to endure several million years of geological activity; therefore, long-term weathering does not considerably impact the stability of a stone architectural heritage. However, structural defects, environmental changes, and intentional damage are important factors that cause the short-term damage of a stone architectural heritage. Therefore, evaluating and monitoring the durability of constituent rocks against weathering and damage are essential to maintain the stability of a stone architectural heritage that is exposed outdoors, thereby preserving its original form. The most extensively known methods for evaluating the weathering of a stone architectural heritage include visual inspection and non-destructive testing. Although visual inspection based on a preliminary survey is very useful for precisely categorizing and mapping the defects, the scope of visual inspection is limited because it depends on the visual resolution and because it only detects the surface [1, 2]. Further, non-destructivetesting is advantageous to quantitatively determine the physical and structural defects that are observed from the surface to the inside of the structure. Some common testing methods for stone architectural heritages include ultrasonic measurement [3; 4; 5], infrared thermographic analysis [4; 5; 6; 7; 8; 9], ground-penetrating radar [10; 11], rebound hardness measurement [12; 13], and drilling resistivity testing [14; 15] among various other methods. One of the most extensively used testing methods for detecting defects and for evaluating the weathering of a stone architectural heritage is ultrasonic measurement using the P-wave velocity, which is highly sensitive to cracks, schisty, mineral composition, water content, and erosion [16; 17]. However, ultrasonic measurements are influenced by various factors such as the petrological characteristics and shape of the stone heritage, the external environment, the measurement conditions, and the level of experience of the user. Thus, researchers have attempted to improve the reliability of these measurements by conducting experiments related to the moisture content [18; 19; 20; 21] and anisotropy [22; 23; 24] of the rocks, the surface roughness [25], the direct and indirect correlation of ultrasonic waves [26; 27; 28; 29], and the effects of the probe [30], couplant [25], and measuring operator [31]. Such studies have made a significant contribution to set the conditions for performing ultrasonic measurements. However, there are still several issues related to analysis and evaluation of weathering considering the material characteristics and the conservation conditions of a stone architectural heritage. This is because even a stone architectural heritage can include different types of stones; further, if a stone architectural heritage shows poor conservation condition, new stones may be used to replace the weathered stones during restoration. Furthermore, different types of rock exhibit different physical properties; even identical types of rock may exhibit different absolute strengths depending on its origin. Therefore, the weathering grade standards that consider the type of rock and the conservation condition of a target heritage should be established to evaluate the weathering of a stone architectural heritage based on ultrasonic measurements [32; 33; 34; 35; 36; 37]. In this study, the conservation condition of the surface of the stone block foundation of the Sungryemun Gate, one of the most representative stone buildings in the Republic of Korea, was primarily examined. The weathering of each block was quantitatively evaluated by performing ultrasonic measurements. First, a detailed visual inspection, a simple testing, and an analysis of contaminants were conducted to diagnose the conservation condition.