Transfer standards
If the seismic sensor cannot be returned to laboratory for calibration, on-site calibration requires a device capable of producing reference measurements to be compared with those provided by the seismic sensor.
Two concepts are therefore conceivable. On the one hand, there is the vibrating table-type device simulating ground displacements in horizontal and vertical directions, on the same principle as equipment found in metrology laboratories. Such field systems have been developed and occasionally used [Lennartz Electronic GmBH, Rapberry Shake Co.]. Displacements are measured by comparator-type instruments integrated into the device, using a metrological configuration that takes into account the positions of the table and the seismic sensor to be calibrated. Implementing such a vibration table requires the sensor to be temporarily removed from service, and installed on the table in the appropriate orientation for calibration. The main constraints of these vibratory devices are their size, their mass, their fragility (risk of damage during transport) in terms of the mechanical elements that materialize the mobile translation links ensuring the quality of guidance (straightness, pitch, roll and yaw), the need to manage the metrology of the integrated measuring instrument, and especially the handling of the sensor that can cause disturbances or damages. If the sensor has a thermal insulation, then it must be removed and replaced after calibration.
The other concept is based on the use of a sensor as a transfer standard with or without an excitation source, co-located with the sensor to be calibrated. Unlike the first concept, where the sensor's excitation signal comes directly from the vibrating table's motions, here the signal is of natural seismic or external mechanical origin (using e.g. a vibration generator), propagating in the ground. The advantages of this method are, its low cost (depending on whether or not an external generator is used), a relatively small footprint compared to the size of the sensor (particularly suited to small spaces), the absence of any auxiliary equipment other than the standard in the calibration zone, and the absence of handling of the sensor to be calibrated while being protected with a thermal insulation. One important benefit here is that the sensor to be calibrated neither needs to be moved nor modified and can stay in undisturbed measurement operations during the calibration.
As part of the InfraAUV project, an investigation of different sensors for selection as transfer standard was performed, and the following specifications aim to identify the requirements for a suitable transfer standard.
<Feature | Description |
Dimension and weight | Weight: the sensor should not be to heavy (equal to or less than the masse of a seismometer) to facilitate handling and installation. Dimension depends mainly on the space available around the seismometer to be calibrated. Either the space is large enough and therefore the transfer standard can be easily be co-located (side by side), or if it isn’t, and a smaller transfer standard sensor will have to be used. Beyond the size of the sensor, the feasibility of co-locating sensors depends on the available floor space guaranteeing the same propagation quality.
Weight and dimension are also an issue considering the calibration capabilities in the metrological laboratories. Shaker tables are limited regarding the load capacity and the table dimensions. |
Transportation | The transfer standard should be used on site several times a year. It must be transported either by car or by plane, without risk of damage. The sensor should be robust with a specific packaging. Mechanical or electrical locking of the sensitive parts are a feature that helps to limit risks. |
Handling | Easy to handle (lifting handle included) for easier transportation and installation. |
Bandwidth (Fmin, Fmax in Hz) at – 3dB | Similar bandwidth as the sensor to be calibrated. |
Resonance frequency (Hz) | None below k.Fmax (k>2) |
Voltage sensitivity (V/(m/s), V/(m/s^2)) | ≥ sensitivity of the sensor to be calibrated |
Transverse or Cross axis sensitivity (%) | As low as possible. Estimated by the manufacturer ideally. |
Linearity/distortion (%) |
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Axis configurations | Several configurations are possible. They should be considered in relation to the type of sensor to be calibrated, the space available, and the cost. §- Single sensor with three orthogonal axes xyz §- Single sensor with reconstructed XYZ (native uvw) §- Single sensor with single axis (horizontal & vertical) |
Control of the measurement axis alignment | Should have some easily identifiable scribe marks, accessories to identify the orientation of the horizontal axis in order to minimize the effect of the misalignment between the two sensors. |
Control of the tilt | Sensors must have at least a level indicator to minimize as much as possible tilting effects |
Noise level ( dB (m/s)/√Hz) | If we generate a vibration signal, then we can expect that the signal to noise ratio will be high and then the noise level of the standard is not critical. If we use only the ambient signal, noise level will be critical. It should be at least equivalent to that of the sensor to be calibrated. |
Levelling | Must be equipped with a mean of detecting and controlling horizontal adjustment. This mean must offer good visibility during on-site implementation. |
Fidelity | Depends on the expected uncertainty |
Operating environment | The same requirements as the seismometer to be calibrated. |
Temperature storage | - 40°C to 70°C |
Temperature operating | - 20°C to 50 °C |
Thermal sensitivity | Calibration (in laboratory and on site) will be carried out once the thermal equilibrium has been reached. Thermal sensitivity is a factor that can be critical in case of local thermal fluctuation (comparing the calibration time at low frequency), and if on site temperature differs significantly from that in the laboratory during primary calibration. Coefficien t for thermal sensitivity should be supplied by the manufacturer. |
Humidity sensitivity | As low as possible. Estimated by the manufacturer ideally. |
Barometric pressure sensitivity | As low as possible. Estimated by the manufacturer ideally. |
Magnetic field sensitivity | As low as possible. Estimated by the manufacturer ideally. |
Power supply effects | As low as possible. Estimated by the manufacturer ideally. |
Supply voltage | Will have its own power supply or be powered with the local energy (between 9 VDC up to 36 VDC) |
Electrical protection | Over-current (fuse available) |
Warm-up time (s) | Up to one day |
intro sentence and reference to report
Streckeisen STS2.5
Geotech GS13
Nanometrics Trillium 360 GSN
References
Lennartz electronic GmBH: the CT-EW1 Calibration Table for broadband and short period seismometers. CT-EW1.idcs (shopify.com)
Multi-sensor calibration table. Rasberry Shake company. Multi-Sensor Calibration Table - Earthquake and Infrasound Sensors (raspberryshake.org)