| Home | Highlights | Intro | Objective | System | More |

| General Issues And Design Goals | Basic System Components |

Basic System Components > Front-end unit

Seismic Sensor

General aspects

As explained elsewhere, the acceleration sensor can be used for different objectives. Regarding the placement of the sensor, specific rules dependent on the parameter to be measured need to be observed. For geophysical observations, the sensor unit is normally attached to a rigid structure close to the basement of a building. For earthquake engineering purposes, the sensor unit may be fixed directly to a specific segment of the civil structure under observation.

Often, the sensors must be placed in positions close to the ground, positions not favorable to radio propagation. Thus, there is good reason for the sensor and the master unit (with its integrated antenna) to be designed as physically separate units.

The RED System employs modern semiconductor sensor technology. Sensors of this type are adequate candidates for the design of really compact and robust strong motion instruments.

One sensor technology in particular stands out for its remarkable cost-efficiency. Readers interested in details regarding this specific type of surface micromachined sensor technology may visit the following website: www.analog.com/publications/whitepapers/products/seismart.html

Another manufacturer has developed an integrated acceleration sensor based on a different micromachined technology. With this alternative approach, a unique signal-to-noise ratio can be achieved--more than ten times better than the low cost product mentioned above. This type of sensor allows digitizing with much higher resolution. However, there is no such thing as a free lunch: the device with the much lower noise figure is available at a much higher price.

Integrated sensors of this sort have been on the market for several years and have proven their reliability and stability. A detailed discussion about the pros and cons of modern silicon sensors, especially in comparison with more classical strong motion instrumentation, is given elsewhere on our website: sensor

Choice of sensor elements

The RED System’s user can choose between two different sensor elements belonging to different categories of precision:

• Class S (standard) sensors for a course classification of really strong and potentially dangerous events, and
• Class H (high resolution) sensors, capable of evaluating faint seismic events beyond human perception

Two different types of silicon acceleration sensors

Table 1: Classification of the RED System´s sensor elements

Notes:
A) referring to a restricted signal bandwidth of 0.5 to 10 Hz
B) defined as clipping level vs. intrinsic noise (peak-to-peak)

The RED System´s seismic sensor unit contains three separate channels, i.e. one sensor element assigned for each of the three coordinates X, Y, and Z. The whole sensor is integrated into a compact, weatherproof aluminum box with the dimensions 80 x 75 x 55 mm (3.4 x 3.0 x 2.3 inches). This compact unit can be fixed in different locations depending on the user’s objectives.

Photo with sensor box

Each of the three class B sensor elements is directly connected to a DC-coupled preamplifier. By employing ultra-low-noise semiconductors, an amplifier noise figure in the order of 1 dB has been achieved in the relevant frequency domain. This is pretty close to the theoretical limits, which means that an optimal SNR has been achieved with the high-quality class B sensor element.

Another remarkable feature of the sensor/preamplifier combination is its extremely low power consumption: less than 4 mW per axis.