## 2 Experimental Investigations and Data Acquisition
### Operational Monitoring on a Hybrid Tower In Situ  
The operational measurements were carried out on an operating wind turbine, with a measuring system permanently installed in the plant. In collaboration with the associated partner (_MAX BOGL_), a wind turbine with a conventional foundation near _Neumark in der Oberpfalz_ (_Karholz_) was selected for this purpose, which had already been analyzed in other research projects (e.g., _\"MISTRALWIND\"_, _\"IM Wind\"_[10]). This turbine was monitored for almost one year, from 25 February 2021 to 22 February 2022.  
The foundation of this tower provides a spacious tensioning chamber in which the master unit of the measurement system was set up and installed. Free empty ducts between the ground level and the tensioning chamber were used for the tendon routing so that, for example, a power cable could be routed down into the tensioning chamber and measurement cables and antennas could be routed upwards.  
The areas around the anchorages of the tendons on the foundation and transition piece were selected as sensor locations. On the one hand, the anchorages were identified as potential locations for frequent damage. On the other hand, it is possible to monitor several tendons with a single sensor only at these locations. This opportunity can be exploited to reduce the total number of sensors within a generalized monitoring concept.  
A total of eight acoustic emission sensors were installed on the tower structure. Figure 4a,b show the relevant sensor locations. In the tensioning chamber, four sensors were installed at two measuring locations between the anchorages of the lower tendon ends. These sensors were positioned at the quarters of the tower's diameter and centrally between two anchorages. Two _VS30_ sensors with a preamplification of 0 dB and 34 dB were used at each measuring location [35]. These sensors are specifically sensitive in the range from 25 to 80 kHz. This pair of amplifications was chosen because the background noise level could not be determined in advance, and the actual magnitude of the background was unclear.  
The measured data were recorded locally and stored onsite. The data were recorded as continuous time series with a sampling rate of 100 kHz without additional filtering. As immediate data analysis was not required, remote data transmission was unnecessary, and local storage on hard drives was chosen instead. During the measuring period, the hard disks had to be exchanged several times in the course of maintenance work due to the large amounts of data generated. It was not always possible to schedule the replacement without delay when the hard drives were full, which resulted in data gaps lasting several weeks in some cases. A total data volume of 29 terabytes was recorded over the entire measurement period.  
The environmental conditions and operating parameters of the wind turbine (including temperature, wind direction, wind speed, rotor speed, blade pitch angle, and nacelle direction) were recorded by a _SCADA_ system by default (_Supervisory Control and Data Acquisition_). This system enables the wind turbine operator to control and monitor its performance remotely. These data were provided at a sampling rate of 15 min.