Measuring Seismic waves with ADALM1000 and a Geophone

Commercial systems used for seismic exploration usually require expensive analog instrumentation. In this Blog entry, a low cost vertical geophone is connected directly to the analog input channel of an ADALM1000 data acquisition system. So what's a geophone? Geophones are commonly made using a spring mounted magnetic mass moving up and down in a coil of wire to generate a voltage. The response of the coil is proportional to the velocity of the ground movement.

Normally the low level geophone signal first goes through a high gain instrumentation amplifier suitable for small amplitude signals like seismic vibrations, and then a low pass filter stage. Finally, the amplified and filtered signal is digitized by a low resolution ADC.

I recently acquired a used/refurbished vertical geophone online. The cost was less than half the cost of a new one. I wanted to see if it would work at all connected directly to the analog input without any filtering or amplification, figure1. One terminal of the transducer is connected to the +2.5 V mid-range voltage output to center the signal in the range of the analog input. The other terminal is connected to the channel A input in Hi-Z mode. No other passive or active components are used. I don't have the datasheet for my geophone (seems to be marked SM24DQ) but similar ones I've seen online have a sensitivity specification of 28.8 Volt/meter/sec (~0.73 V/in/s). The maximum output voltage seems to be about 3 Vp-p if I shake it up and down.

Figure 1, Geophone connected directly to ADALM1000 channel A

With the very high dynamic range of the 16 bit ADC front-end of the M1k, the amplification stage can be eliminated. The 100 KSPS sampling rate along with digital filtering in the ALICE StripChart software fills in for the analog filtering stage. The software gain is set to 200 which would be approximately the same as using an analog gain of 200 ahead of an 8 bit ADC. The stripchart software performs an average over all the samples taken at 100KSPS between the sample points used for the display. For example for a 50 Sps rate 2000 16 bit ADC samples are averaged to get one sample point. This in effect filters out signals at 50 Hz and above.

I live in a seismically very quiet part of the world so there is not much in the way of visible geologic activity to show. In the stripchart screen shot in figure 2 I'm tapping very gently on the desk top to show the geophone response. You would see a very similar size response when walking around or jumping up and down on the concrete floor. You can see the baseline noise level between taps on the screen. With the software gain set to 200, each vertical grid box on the chart is about 3 mV as seen at the geophone terminals. The horizontal scale is 1 Sec per grid.

Figure 2, Geophone response to lightly tapping on desk-top

So it would seem that the raw M1k analog input, with the help of a lot of oversampling and digital filtering can be used directly with a geophone for seismic exploration.

By the way, the ALICE StripChart tool is getting a few new features added to make it a better interface for displaying the geophone signals. There is now a stripchart_init.ini file that can be used to configure the interface on start-up. In the file you can now set the vertical and horizontal size (in pixels) of the display grid, the colors of the background canvas, grid lines and waveform traces. You can also set the channel gain and offset.

As always I welcome comments and suggestions from the user community out there.

Doug