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Low-noise and Low-power Solutions for Seismic Wave Detection Applications

Seismic wave refers not only to the movement caused by earthquake, but also to the disturbance caused by any force exerted on the ground, even such a small force as a person walking on the ground. Seismic wave detection application is a typical industrial precision data acquisition system, and it needs to take into account both low power consumption and ultra-low noise.
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Low-noise and Low-power Solutions for Seismic Wave Detection Applications

LHA9954 low for seismic wave detection applications Noise, Low Power Solutions

1. Introduction
Seismic wave refers not only to the movement caused by earthquake, but also to the disturbance caused by any force exerted on the ground, even such a small force as a person walking on the ground. Seismic wave detection application is a typical industrial precision data acquisition system, and it needs to take into account both low power consumption and ultra-low noise.
A large amount of information can be extracted from seismic wave data. By deploying a large number of node devices and interconnecting them, the large amount of data extracted from each node device has a wide range of applications after scientific analysis or modeling, such as geophysical research, energy exploration, construction or Health monitoring of engineering structures and other fields. Here are some common applications:
1.1. Earthquake early warning system
Earthquakes are events triggered by the movement and collision of tectonic plates. The energy from the collision travels around the Earth's inner surface in the form of seismic waves. These waves have multiple directions and are divided into body waves and surface waves.
There are two types of body waves: longitudinal waves (P waves) and transverse waves (S waves). P waves travel in the direction of propagation as a series of compressional and sparse waves. Due to the nature of their propagation, P waves diverge spherically. Although its wave energy attenuation is the greatest of all wave types, it has the fastest speed, between 5 km/s and 8 km/s. The fast energy decay also makes it the least destructive class of waves. P waves can propagate not only through surfaces, but also through water or fluids. The S wave, also known as a shear wave, arrives right after the P wave. It travels along the Earth's surface at about 60% to 70% of the speed of P waves. Such waves travel perpendicular to the direction of propagation and to the Earth's surface. S waves have less energy attenuation and are more destructive than P waves. P waves and S waves are collectively called body waves.
Surface waves are 10% slower than body waves, but are the most destructive. It's worth noting that the velocity of a seismic wave has a lot to do with the type of soil it travels through. Surface waves consist of Rayleigh waves and Love waves. A Rayleigh wave is a surface wave that propagates near the Earth's surface as a ripple, causing a prograde (in the direction of propagation) or retrograde (opposite the direction of propagation) rotation. Due to the nature of its motion, it is also known as ground roll. Love waves travel in a direction normal to the direction of propagation, but parallel to the Earth's surface.
S-waves and surface waves are the more destructive seismic waves, but they travel more slowly than the least destructive P-waves. Utilizing this feature can realize an earthquake early warning system that detects early signs of earthquakes. This way, all types of systems have a short time to respond, preventing earthquakes from causing major damage. Residential and commercial buildings will be able to shut down electrical systems and natural gas lines in the moments before severe ground shaking occurs. Using a network of seismic sensors deployed at multiple locations around the protected area will help increase the allowable reaction time. In addition, false alarms from non-seismic sources are minimized.
1.2. Remote Seismic Network
Volcanology and seismology studies deploy seismic sensors in harsh (and sometimes dangerous) terrain. Monitoring internal processes within a volcano requires earthquake motion monitoring at multiple points. After certain phases of volcanic activity, these locations can become dangerous and make it impossible for seismic sensors to retrieve them. Low-cost, low-power seismic sensors will reduce research costs while maintaining a long lifetime. Another similar situation is the signature of plate motion, which also requires the deployment of large numbers of seismic sensors along fault lines.
1.3. Structural Health Monitoring
Structural health monitoring is applied to engineering disaster prevention and building safety. For example, changes in engineering geological environment and stress conditions of supporting structures may cause different degrees of damage or rupture inside, or even induce catastrophe. Use microseismic monitoring technology to monitor some major tunnel projects and bridges during their use in real time, to grasp the micro-crack precursors and damage levels in the structure, and take timely countermeasures to ensure operational safety during use. In the event of damage, a widely distributed network of seismic sensors can locate areas of structural damage, reducing the risk and cost of visual inspections.
1.4. Energy exploration
Seismic exploration is a common method for geophysical research and energy exploration. Its principle is to explore the underground geological conditions by using the propagation rules of seismic waves excited by artificial methods in strata with different elasticity. By recording and analyzing data characteristics of node equipment, the depth and shape of these interfaces can be determined more accurately, and the lithology of the formation can be judged. It is the main geophysical method for exploring oil and gas structures and even directly looking for oil.
At the just-concluded two sessions in early 2022, Premier Li Keqiang delivered a government work report, raising "energy security" to a strategic level as important as "food security". Continuous high-quality exploration of new oil and gas resources is the key to ensuring China's energy security and realizing modernization.


2. Program introduction
2.1. Sensor
Commonly used sensors in the industry are geophones. It is an Earthquake Velocity Sensor which is lightweight, durable and does not require any power source to work. A modern geophone has a magnet attached to its housing and surrounded by a coil. The coil is suspended by springs and can move on magnets. The velocity of this motion relative to the magnet induces an output voltage signal. Geophones are implanted in the ground along the array, and measure the time it takes for seismic waves to reflect back from a discontinuity, such as a slice, as shown in Figure 1.

Figure 1. Seismic source and receiver array


2.2. Data collection
To accurately measure and analyze the weak seismic wave signals reflected from the deep underground, the requirements for the entire data acquisition system are extremely demanding. The minimum output signal amplitude of the detector is only tens of μV, so it is required that the total root mean square noise of the signal chain should be less than 1.0 μV rms. In the case of external excitation, the signal amplitude can reach tens of mV, or even 1~2V, so the dynamic range of the signal chain is required to reach 140dB. In addition, due to the rich frequency of seismic wave signals, the limited flat low-pass bandwidth of the signal chain is required to be in the range of 300 Hz to 400 Hz. The small, to achieve ultra-low distortion, so THD needs to be better than -120dB. Because the seismic monitoring instruments are buried on the ground and powered by batteries and need to work for a long time, the power consumption should be controlled within 20mW.

2.3. Signal chain scheme
Legendsemi's latest 32-bit ultra-high resolution ADC LHA9954 provides a perfect solution to the challenges faced by the seismic wave detection signal chain. This product has excellent performance:
● Integrated Programmable Gain Amplifier: 1, 2, 4, 8, 16, 32, 64
● Integrated wideband digital filter: Sinc + Finite Impulse Response (FIR) + Infinite Impulse Response (IIR) (optional)
● Ultra-low noise: 135.4dB SNR (250 SPS, PGA = 1)
● Ultra-low total harmonic distortion (THD): –120dB
● Very low power consumption: 12mW (high resolution mode), 7mW (low power mode)
● Very high common mode rejection ratio (CMRR): 127dB
● Two-channel multiplexer: second channel for self-test


3. Test results and analysis
3.1. Typical circuit

A typical detector front-end application circuit based on LHA9954 is shown in Figure 2, where LHA9954 works under a bipolar ± 2.5V analog power supply, and it can also work under a unipolar 5V analog power supply.

Figure 2. Typical circuit of detector signal chain


The detector input signal is composed of a differential mode filter (components C4 and R1 to R4) and a common mode filter (components C2, C3 and R1, R2) filtering. Differential-mode filters remove high-frequency differential-mode components from the input signal, and common-mode filters remove common-mode high-frequency components from both input leads. Resistors R5 and R6 bias the signal input to the mid-supply point (ground). For single-supply operation, set the bias to a low-impedance mid-supply point (AVDD/2=2.5V).
Optional diode clamps protect the LHA9954 inputs from high-level voltage transients and overloads. The diodes provide additional protection if possible high-level input transients and surges exceed the ratings of the ADC's internal ESD diodes.
The LHR3050 5V reference provides the reference for the ADC. An optional filter network (R7 and C5) reduces in-band reference noise for improved dynamic performance. However, the RC filter network increases the filter settling time (from seconds to minutes), depending on the dielectric absorption characteristics of capacitor C5. Capacitor C7 is mandatory to provide high-frequency bypassing of the reference input; place C7 as close as possible to the LHA9954 pin. Resistor R7 (1kΩ) will cause a 1% system gain error. Multiple ADCs can share a reference, but if shared, separate reference filters are used for each ADC.

3.2. Evaluation board scheme
The main performance of the signal chain scheme in this paper is tested using the evaluation board shown in Figure 3.

Figure 3. Geophone Signal Chain Evaluation Board


ADC configuration and data display and acquisition are realized by supporting host computer software, as shown in Figure 4.

Figure 4. Host computer data collection

 

3.3. Result analysis
1) Noise performance

Table 1. High resolution mode SNR (dB) and input referred noise (µVRMS) 

Data Rate (SPS)

PGA (signal-to-noise ratio, decibel) (1)

  PGA (input referred noise, µV RMS)

1

2

4

8

16

32

64

1

2

4

8

16

32

64

250

135.3

134.8

134.4

132.1

128.2

123

117.6

0.606

0.322

0.169

0.11

0.086

0.078

0.073

500

132.2

131.9

131.2

129

125.1

120

114.6

0.868

0.452

0.243

0.156

0.123

0.11

0.103

1000

129.1

128.9

128.3

126

122.1

117.1

111.6

1.243

0.637

0.338

0.221

0.173

0.154

0.146

2000

126

125.7

125.2

123

119.1

114.1

108.5

1.769

0.915

0.486

0.314

0.245

0.217

0.209

4000

122.5

122.2

121.7

119.7

116

111

105.3

2.64

1.373

0.724

0.458

0.353

0.311

0.299

 

Table 2. Low Power Mode SNR (dB) and input referred noise (µVRMS) 

Data Rate (SPS)

PGA (signal-to-noise ratio, decibel) (1)

  PGA (input referred noise, µV RMS)

1

2

4

8

16

32

64

1

2

4

8

16

32

64

250

128.8

128.5

128.5

127.3

125.2

120.9

115.5

1.435

0.732

0.365

0.203

0.13

0.101

0.091

500

125.5

125.4

125.3

124.3

122

117.8

112.5

2.042

1.024

0.526

0.291

0.185

0.142

0.129

1000

122.5

122.4

122.3

121.1

118.8

114.8

109.6

2.949

1.494

0.759

0.414

0.263

0.205

0.187

2000

119.2

119.1

118.9

117.9

115.6

111.7

106.4

4.603

2.312

1.175

0.635

0.394

0.3

0.269

4000

115.5

115.3

115.2

114.3

112

108

102.9

8.381

4.2

2.115

1.115

0.661

0.475

0.409

 

2) Total Harmonic Distortion

PGA = 1, Vin = 2.36V@31.25Hz Sine Wave, Temperature = 25℃, First 5 Harmonics


PGA = 2, Vin = 1.18V@31.25Hz Sine Wave, Temperature = 25℃, First 5 Harmonics

PGA = 4, Vin = 590mV@31.25Hz Sine Wave, Temperature = 25℃, First 5 Harmonics
Note: The low frequency 1/f noise comes from the signal source

3) Power Consumption
HR mode power consumption comparison

Power

LHA9954

An international model

G=1

12mW

18mW

G=16

12.8mW

22mW

 

LP mode power consumption comparison

Power

LHA9954

An international model

G=1

7.2mW

12mW

G=16

7.6mW

14mW

 

Comparison between Standby and Power Down modes

LHA9954

Standby

Power Down

IAVDD

0.35uA

0.35uA

IDVDD

26uA

0.3uA

An international model

IAVDD

1uA

1uA

IDVDD

25uA

1uA

 

4. Summary
The 32-bit high-precision ADC LHA9954 used in this scheme is mainly aimed at precision measuring instruments such as energy exploration, earthquake monitoring, and automated test equipment. The product has 100% independent intellectual property rights, and its core indicators have reached the international advanced level. At the same time, it has ultra-low power consumption. It provides a perfect solution to the challenges faced by my country's energy exploration and breaks the decades-long exclusivity of foreign chip manufacturers. Monopoly, escorting China's energy security.