## 7 Radial Boreholes and Newtonian Noise Subtraction Sensors  
Effective Newtonian noise subtraction techniques require numerous high-sensitivity sensors to instrument the rock mass surrounding the test masses. These sensors must be distributed in a pattern with a size comparable with the wavelength of the seismic waves of interest. At 2 Hz, with a seismic speed of 5 km per second, the characteristic dimension is of the order of a kilometer. This is a large and expensive endeavor involving tens of kilometers of boreholes that may be justified only when the detector has reached sufficient sensitivity at low frequency. Experience shows that it takes years to build a gravitational wave detector and even more to reach the stage when Newtonian noise may become a limitation. It is thus necessary that the Newtonian noise mitigation can be staged and/or implemented at a later time, when better understanding of the requirements is gained. The required Newtonian noise cancellation system can be achieved with maximized effectiveness, minimized borehole length, and minimized cost by directional drilling of boreholes radiating from tunnels located near the three corners. Directional boring techniques developed for oil extraction and ore exploration [54] allow an optimal and easily expandable sensor geometry. Boreholes radiating from near the corner stations are sketched (in a plane) in Figure 7. This geometry minimizes the drilling length and allows centralized signal collection. Implementation will require custom triaxial inertial sensors with remote anchoring/release and alignment mechanisms. Boreholes hundreds of meters in length can be drilled from one or two dedicated tunnels to instrument the volume surrounding the corner stations. Each borehole will host many sensors spread out at suitable distances to satisfy the optimal sensor distribution requirements. The mono-axial sensing heads inside modern low-frequency seismometers are small and require only small changes to fit within standard diameter boreholes, i.e., 130-170 mm but precision self-alignment capability will be necessary. The installation must be performed remotely by crawlers capable of installing and aligning or retrieving the sensors. An installation procedure might be as follows. A mono-axial sensor is brought in place by a cable-controlled crawler. The sensor's outer housing is equipped with spring-loaded expansion brakes for rigid connection to the borehole walls while a spherical bushing allows smooth rotation of the sensor head. Both the brakes and the bushing are of the normally locked type. They are released as needed with compressed air provided by the crawler. After positioning, the crawler locks the brakes to the walls, orients the sensor to level and align it, then locks the rotation. Finally, the crawler returns to fetch the next sensor that is connected to the previous one before starting its alignment procedure. The sensors are daisy chained along the borehole and installed in groups of three to get three-axial sensing while the spacing between the triplets is determined by the sensing requirements. Boring produces vibrations. The tunnels hosting the drilling equipment must be at a distance, to be determined experimentally, from the detectors that the seismic attenuation chain can neutralize them. Figure 7: Schematic view (not to scale) of radial boreholes and inertial sensor positioning around the corner station for optimal Newtonian noise subtraction. A tunnel in the front and one on the back of the corner station are hypothesized here. The angular density per steradian, the length of the boreholes and the longitudinal positioning of the sensors are chosen to optimize the sensor density for Newtonian noise subtraction while minimizing the borehole length. A crawler positions, levels and aligns the sensors at the desired distance along the borehole (see text for details).