A seismic receiver serves as an indicator of oscillations of the environment that surrounds it. The seismic receiver is a vibration-sensitive element with an amplifier typically encased into the watertight housing.
The oscillations of seismic receiver body are transferred to the vibration-sensitive element. Therefore, the seismic receiver should be installed at the measuring location, so that it can transfer the oscillations of environment/structure to the sensor housing. There are different techniques to harmonize the impedances of two environments, when acoustic oscillations or seismic waves cross the interface.
The seismic receivers are installed on:
- steel structures – for diagnostics and monitoring of metal structures and bridges
- concrete structures – for measuring the vibrations of dams, tall structures, etc.
- metal and non-metal pipelines – for analyzing and monitoring the condition of pipeline transport
- ground – for measuring the ground oscillations caused by seismic and man-made impact.
In each case, the sensor installation has specifics.
It is least difficult to install a seismic receiver onto metal or concrete surface. In this scenario, the density of the body material of the sensor and the object hosting it is commensurable, as well as the acoustic speed within them – the wave does not reflect from interface at comparatively high oscillation frequencies.
The installation of sensors in soils ranging from water-bearing to dry sandy requires a matching link. In this case, the oscillations are transferred from the environment to the matching link, and from there to the sensor housing. Without the matching link, the enviroment will oscillate, and the sensor will “stand still”.
Installation of Seismic Receivers on the Ground
The seismic receiver installation on the ground using the matching link – the added mass – has simple rules: the lower the soil density, the greater must be the area of added oscillating mass, and dimensions of the intermediate link must be much less that the wavelengths of measured oscillations.
Recommendations for Installing the Seismic Receivers on Metal Trusses
The seismic receiver is directly secured on the controlled structure with screws and nuts. The signal cable must be securely (to exclude vibration) attached to the structure along the entire length without slack and tension. To exclude climate impacts on the seismic receiver body, it shall be isolated from environmental exposure (temperature fluctuations, wind, humidity), e.g. by using light external casing filled with heat insulation material.
Bear in mind that each frequency has its own nodes and antinodes of oscillations. Therefore, to analyze the behavior of the whole structure during the tests, the measurements must be carried out at several points at once.
Recommendations for Installing the Seismic Receivers on Concrete
For diagnosing oscillations of concrete structures, the analysis shall be performed at lower frequencies. The attachment methods are the same as for installation on metal, given that the screws can be tested. Alternatively, the seismic receivers can be glued with further heat- and damp-proofing with bitumen.
For monitoring seismic and man-made impact on tall buildings, one needs to know the dominating direction of external effects:
- acoustic emission waves from planes
- seismic waves from the railway transport
- the lowest and highest rigidity direction
To learn that, it is necessary to perform monitoring in three directions at different heights. For measurements along three axes simultaneously, it is convenient to use three-component sensors, such as BC 1313.
Recommendations for Installing the Seismic Receivers on Pipes
For monitoring the integrity of pipelines under seismic impacts, the sensors are installed directly onto the pipe using clamps or epoxy compounds with isolation and sealing. ZETCORR mobile leak detectors utilize BC 121 sensors, which have rectangular magnets on the base for fast and secure attachment to the pipe.
Recommendations for Installing the Seismic Receivers on Ground
When installing the sensors on the ground for monitoring seismic condition of the area, the impedances of two environments must be matched. This involves mounting of seismic receiver onto the slab with further installation on the ground (see figure above). Also, the seismic receivers can be installed on the ground using legs (pins) screwed to the sensor base (sensor flange has mounting holes).
Recommendations for Installing the Seismic Receivers in Soil
When installing the seismic receivers in soil, a hole is dug, the sensor is earthed, and then the soil is compacted. Installation depth – until firm soil or permafrost.
The groundwater does not affect the seismic receivers if they are watertight. When installing in soil, it is critical to place the sensor somewhere without water current.