## 3 Long-Term Durability of Sensor Coupling and Mounting and of the Measurement Equipment
### Long-Term Sensor Coupling
#### 3.1.3 Examples of Long-Term Coupling for AE Monitoring  
Infrastructure, in particular bridges or bridge components such as bridge decks, cables or connections, is one example where AE monitoring has been investigated with respect to long-term performance. However, the effective monitoring periods in most applications presented by, e.g., [87; 88; 89; 90], were on the order of hours or days and not months or years. A few publications present long-term AE measurements and their analysis. The first is from a 10-year rock monitoring operation in the Gran Sasso mountain in Italy [91] with the aim to observe whether AE activity also exists in non-volcanic areas with seismic activity. Analysis showed that, for an AE sensor (type Bruel and Kjaer model 8312) mounted on the end of stainless-steel rod fixed by cement in a rock-drill hole, about 90% of all signals recorded (after an 82 dB amplifier) in the frequency range between 100 kHz and 1 MHz were noise from electromagnetic sources. A second sensor, with the metal case coupled electrically but not acoustically to the first, provided a simple approach for noise discrimination. The second sensor only recorded the electromagnetically induced signal, and the other recorded all signals. Unfortunately, the paper does not discuss the long-term performance of the AE sensors, of its coupling and of the measurement setup. A second example is a four-month monitoring operation of a bridge across the Yellow river in China based on modal analysis from accelerometer data [92]. There were fifteen accelerometers installed on each of the two separate bridge lanes. Again, no information on long-term accelerometer and data recording system performance or problems encountered in the course of the test was provided. The authors, however, note the following (cite): _\"The influence of temperature, moisture, wind, traffic load, etc., need to be simulated in the finite element model and verified in the measured data\"_. A third example is a two-year AE monitoring operation of two parallel steel-arch highway bridges [93]. An AE monitoring system (type Sensor Highway IITM System) with sixteen channels and AE sensors (type R15I-LP-AST) was installed on each of the two bridge arches. Sensor positions were set as far apart as possible, and their couplings were tested by lead pencil breaks (Hsu-Nielsen sources). Sensors mounted ten feet apart proved sufficient for monitoring the relevant parts, while still providing source location. Remote AE system operation was via a 3G modem. The AE equipment was powered by a solar panel (520 W), with four 110 Ah batteries. Due to problems believed to be caused with respect to the solar panel from ice, snow, de-icing salts and other road debris, as well as vandalism, installing an alternative power source is recommended in the conclusions of the report. For data transmission, the authors explicitly stated (cite) the following: _\"A land-based internet connection is recommended to insure that the monitoring system is always accessible for a remote login and/or data upload. Wireless connection are less reliable that land-based internet connections and often suffer communication interruptions. Furthermore, wireless modem antennas are susceptible to factors such as vandalism, damage during bridge maintenance and equipment failure\"_. Two sets of data were collected under service conditions: the first from November 2012 to October 2013 (from one bridge only), and the second from November 2013 to October 2014 (from both bridges). Separate fracture test specimens acoustically coupled to the bridge structure provided a data base to discriminate AE noise signals from fracturesoccurring in the bridge. This discrimination required a combination of several criteria. For analysis, analyzing bridge AE limited amounts of data for easy of handling is recommended. Therefore, the continuous data were broken down into one day segments for detecting fracture events.