# A New Look at the Stress State Across the Bohai Strait, China
## 3 Materials and Methods
### HF Measurements in Xiangli and Changdao  
To obtain stress data near the key structural region of the strait and construct a cross-strait stress transect, we conducted HF tests in two boreholes (i.e., Changdao and Xiangli) drilled recently. The Xiangli (XL) and Changdao (CD) boreholes were drilled in the southern area of the Bohai Strait (Figure 1). The XL borehole, located in Xiangli Village, Penglai City, Shandong Province, is approximately 10,000 m south of the Bohai Sea. It is located on the east side of the Shandong segment of the TLFZ and the south side of the ZBFZ. The topography around the XL borehole is a low-relief hilly terrain with elevation variations of less than 20 m. The depth of this borehole is 500.10 m, with a diameter of 76 mm. The strata of the XL borehole are primarily composed of plagioclase genesis, granitic genesis, and granite. The CD borehole is located in a small basin on South Changdao Island, Changdao County, Shandong Province, with minor topographic variation of less than 50 m. It is located on the eastern side of the TLFZ but near the southern margin of the ZBFZ. The depth of the CD borehole is 100 m, with a diameter of 95 mm. Drill cores and loggings reveal that no faults were exposed. The strata are primarily composed of amphibole-plagioclase gneiss and relatively intact core samples. The strata in these two boreholes (also the PD and GZ boreholes) are granitic rocks. According to rock mechanics experiments on the rocks, the rocks in the XL and CD boreholes exhibit slight anisotropy; therefore, the rocks can be considered nearly elastic, homogeneous, and isotropic, largely aligning with the assumptions of the HF method. In addition, the two boreholes are subvertical, with dips less than \(5^{\circ}\), meaning that the vertical stress can be considered to be parallel to the axis of the borehole. These conditions initially ensure the reliability of HF testing in these boreholes. HF field tests in this study were conducted by using a single-loop HF system [56], which consists of an automated control pump (35 MPa max. pressure; 15 L/min max. flow rate), a downhole shut-in valve, a 1.0 m test interval, pressure transducers, and a flowmeter. HF tests were immediately conducted after drilling was completed. Before the HF test, target intervals were preselected based on integrated analysis of optical televiewer logs and core samples, with exclusion of zones including natural fractures or faults. In detail, the test intervals were selected free of fractures, microcracks, and veins, and are far away from fractured zones over 2 m, referring to previous practices [56]. The testing procedures in these two boreholes followed the ISRM recommendations, through which non-conforming hydraulic fracturing curves and unclear impression images were excluded, resulting in ISRM-compliant datasets. During the HF test, pressure/flow curves are displayed in real-time on the control system and digitally recorded at 20 Hz. After the fracturing test, an impression packer with an automatic orientation device is run down to the same depth (within the same fracturing test interval). Under pressure, the rubber wall of the packer is forcibly pressed into the fracture, thus recording a fracture imprint on the packer surface to determine the \(S_{\text{H}}\) direction. Finally, 15 valid and 4 valid HF tests were conducted in the XL and CD boreholes, respectively. Representative pressure-time curves obtained from the XL borehole are presented in Figure 2. It can be found that the HF curves obtained conform to ISRM specifications for HF test protocols [50]. Using the impression test method, five and two traces of the induced fractures were obtained from the XL and CD boreholes, respectively (Figure 3). To determine the magnitudes of \(S_{\text{H}}\) and \(S_{\text{h}}\) using Equations (2) and (3), \(P_{\text{s}}\) and \(P_{\text{r}}\) should be calculated reliably and accurately.