Misalignment angle correction of borehole seismic sensors: The case study of the Collalto seismic Network

One of the most critical problems affecting seismological data acquisition is related to possible misorientation of three-component seismic sensors. This generally happens when their orientation cannot be measured directly, as in the case of sensors deployed in boreholes. We describe here the sensor reorientation procedure of the Collalto Seismic Network, a microseismic monitoring network located in northeastern Italy that consists of 10 broadband three-component stations deployed in boreholes. We apply a procedure based on the misfit minimization of a complex trace recorded by a given station with respect to a reference station for which orientation is known. The main advantage of this methodology is that the reorientation of seismic sensors can be viewed as a linear inverse problem in the complex domain, which ensures that the preferred solution corresponds to the global minimum of a misfit function. Furthermore, it is also possible to simultaneously use more than one seismic event to better constrain the solution of the inverse problem. In this article, we further compare the orientation results obtained for a seismometer-seismometer sensor pair with those obtained using an accelerometer-seismometer sensor pair. We finally show the reorientation results for all the stations of the network, obtained using eight teleseismic events that occurred between January 2012 and May 2014.