Introduction to earthquake sensors

Selecting the right earthquake sensor for an application is not necessarily straight forward. There are several types of sensors on the market, and all come in different versions and price ranges, with integrated recorders and without integrated recorders. This guide focuses on the different types of sensors, and their primary applications.

Sensors and Recorders

First, let’s take a quick look at the complete chain of instrumentation. An earthquake sensor in itself is simply a device which measures vibration and converts it to voltage or current. In order to interpret the output of the sensor it is necessary to connect the sensor to a seismic recorder which can capture the output of the sensor and convert it back to something meaningful, like acceleration or velocity, and store it digitally.

Earthquake sensors can either be integrated directly with a seismic recorder in a single device, like is the case for Seisodin Tilia instruments, or they can be isolated devices which must be connected to a stand-alone seismic recorder through a cable.

Types of earthquake sensors

Earthquake sensors are typically separated in 3 categories; accelerometers, seismometers and geophones. Accelerometers measure acceleration in units of g or cm/s^2, while geophones and seismometers measure velocity in units of cm/s.

Seismometers are extremely sensitive at low frequencies, some extending as far down as 0.002Hz, but are typically limited in upper frequency and have relatively low clip levels (maximum velocity which can be measured). That makes them suitable for measuring distant or weak earthquakes. The low clip level is an issue if the earthquake is nearby the sensor, as the seismometer may clip and not record the entire earthquake. This is where the accelerometer becomes relevant; with it’s much higher clip level it is able to record the full amplitude of nearby earthquakes. Often a seismic station will have both seismometers and accelerometers installed. Modern Force-Balance Accelerometers have very wide bandwidth and high dynamic range, making them quite exceptional sensors even as stand-alone stations. They are often installed in Strong Motion Networks – regional networks of strong motion accelerographs typically deployed in areas with frequent, high-amplitude earthquakes. Sensors based on Geophones typically have higher clip levels than seismometers, but their low-frequency typically ends at 1Hz. While that makes these sensors less relevant for seismic monitoring, their much lower cost make them attractive for very dense monitoring networks.

	Accelerometer	Geophone	Seismometer
Measures	Acceleration	Velocity	Velocity
Unit	g or cm/s^2	cm/s	cm/s
Clip Level	High	Mid-Low	Low
Low Frequency Response	Great	Poor	Excellent
High Frequency Response	Excellent	Excellent	Poor
Example of sensor	Tilia T130	Tilia V210	Tilia S310

Types of accelerometer products

For seismic monitoring applications and structural health monitoring, generally two types of accelerometers are available; Force-Balance accelerometers and MEMS accelerometers. The key difference between the two types lies in the performance and the price. The Force-Balance Accelerometer (FBA) has brilliant performance in dynamic range, noise floor and stability, while the MEMS sensor is more cost-effective, but has lower specifications. While a good force-balance accelerometer can measure accelerations as small as tens of nano-g, a good MEMS accelerometer can measure acceleration in micro-g.

	Force-Balance (FBA)	MEMS
Seisodin Sensor	Tilia T130	Tilia T100
Dynamic Range @ 1Hz	155dB	105dB
Dynamic range @ 0-50Hz	130dB	91dB
Bandwidth	0-200Hz	0-250Hz
Full-scale	±4g (~±3920cm/s^2)	±4g or ±2g (~±3920cm/s^2 or ±1960cm/s^2)
Cost (relative)	$$	$

Types of geophone products

The Geophone is a proven sensor type which has been around for decades. It is a simple construction, which has a resonant frequency in the lower end of the passband, defining the lowest possible frequency to be measured. In seismic monitoring applications this frequency is typically 4.5Hz or 10Hz, though other values exist. Above that resonant frequency the sensors response to velocity is flat, making it suitable for measuring higher frequencies. Some products, like the Tilia V210, achieves lower frequency response by implementing electronic circuits which can extend the frequency response of the sensor. In the case of the Tilia V210, the response is extended down to 1Hz where as the Tilia V245 use un-extended geophones, and hence have a cut-off frequency around 4.5Hz.

	4.5Hz Geophone	Range-extended Geophone
Seisodin Sensor	Tilia V245	Tilia V210
Dynamic Range @ 1Hz	130dB	130dB
Bandwidth	4.5-315Hz	1-315Hz
Full-scale	±25cm/s, ±10cm/s, ±1cm/s	±25cm/s, ±10cm/s, ±1cm/s
Cost (relative)	$	$$

As is evident from the table above, the difference between a sensor based on a standard 4.5Hz Geophone and one applying an electronic range extension to the same 4.5Hz geophone is only in the bandwidth – what range of frequencies can be measured. While the difference may seem small, it does make a great difference in seismology where lower frequencies are of great interest, as well as in vibration monitoring where the bandwidth is specified by local and international standards.

Types of seismometers

	Short Period Seismometer	Long Period Seismometer	Broadband Seismometer
Seisodin Sensor	Tilia S310	–	–
Dynamic Range (typical)	130dB	>150dB	>150dB
Bandwidth (typical)	0.1 – 50Hz	0.01 – 50Hz	0.002 – 50Hz
Full-scale (typical)	1cm/s	1cm/s	1cm/s
Cost (relative)	$	$$	$$$

Applications

	Accelerometer FBA	Accelerometer MEMS	Geophone 1Hz	Geophone 4.5Hz	Seismometer Short Period	Seismometer Broadband
SENSOR	T130	T100	V210	V245	S310	–
Strong Motion Networks	✅
High Density Local/Regional Network	✅		✅		✅
Regional and Global Networks	✅					✅
Earthquake Monitoring of buildings	✅	✅
Dynamic Structural Health Monitoring	✅
Simplified Structural Health Monitoring		✅
General Structural Health Monitoring		✅
On-site Earthquake Alarm	✅	✅	✅
Vibration Monitoring			✅	✅
Earthquake Shutdown System	✅	✅	✅		✅
P-wave Detection	✅	✅			✅	✅