## 4 Methodology  
### Gravity Field Survey and Bouguer Gravity Anomaly Data  
Through east-to-west-oriented active fault segments in the northern and southern parts of the Eskisehir, Graben played an important role in the formation of the depression area [4,11,14]. Therefore, discussing the fault system controlling this structure with edge detection using microgravity data makes a significant contribution to the geological setting of the region. For this purpose, five lines approximately 4 km apart were chosen along a 30 km long region. The lines are in a north-south direction and perpendicular to the fault systems, and the distance between the gravity measurement points was designed to be 200 m following the Nyquist rule (Figure 1a). The Nyquist rule states that the shortest wavelength that can be measured is twice the distance between the measuring points, which is 0.4 km in our study [42]. Gravity measurements were made at an average of 100 points along each line. Three measurements of sixty seconds were taken at each measurement point, and the measurement with the least noise and standard deviation was used for each point. A CG-5 automatic gravimeter, used to collect gravity data, calculated standard deviation and error values automatically for every sixty seconds long measurement. The reliability and accuracy of each measurement were analyzed by base measurements. If there was a large difference between the two base measurements, it was accepted that there was a problem with the device, and the measurements between the two base measurements were repeated after the device was checked. To correct the drift error, twelve base points were established along the lines within the measurement area (Figure 1a). The first base point was chosen twenty points north of the beginning of the line 1. In addition, high-precision location coordinates were obtained with an RTK (Real Time Kinematics) GPS device. The measurement location code, measurement time, weather conditions, and other environmental conditions were entered into a gravity measurement form at each measurement point. Each working day began with a measurement at the base point, and ended with a base measurement. To minimize the drift error in the measurements, the measurements were taken by returning to the base point every two hours. Each station's corrected Bouguer anomaly value was obtained by applying corrections for elevation (free air and Bouguer plate corrections), drift, latitude, tide, and terrain. A Bouguer anomaly map of the study area was obtained by gridding single station data using the Kriging method (Figure 3) (Data S1 [43]). In order to apply Bouguer plate correction and terrain correction, the average density of the study area was chosen to be 2.67 gr/cm\({}^{3}\) considering major surface geological units, which is also the average density of the upper crust. The physical and mathematical foundations of the gravity corrections have been discussed extensively by numerous authors, so just brief information is given about the corrections [44; 45]. An RTK GPS device connected to an active CORS (Continuously Operating Reference Stations) network was used at each measurement point in order to make the most accurate elevation corrections. The sensitivity of the device is about ten centimeters horizontally and about thirty centimeters vertically. The precise elevation reading of each measuring point is used for elevation corrections. After each measurement, the height of the device was also measured by a laser meter. Therefore, the exact height of the sensor inside the device was calculated. By the elevation data, the correction values were applied in the most accurate way according to the conditions at hand.