Seismic noise levels analysis is useful to characterize network performance; to detect problems with seismic stations and to characterize the frequency dependent noise levels due to background site conditions.
To answer completely to your question more detail on the measurement are necessary, and first of all the real observed differences in terms of relative ratios. In my opinion variations of few percentages are acceptable for “field measurements in exact site”. Anyway, I send you a general response to give response to yours question.
In physical sense, the seismic measurement is the result of a chain of many transfer functions (TF). Namely, using their frequency (f) domain representations:
source S(f), regional propagation model M(f), local site propagation model L(f) and apparatus A(f). A(f) can be further splitted in the sensor TF, I(f) and data-logger TF, DL(f). so that our measurement in frequency domain is the product of them (or convolution in time domain).
When we perform a seismic measurement we must take into account the contribution of each component (TFs).
Now we consider the measurement with “two seismic sensors, of different brand but same bandwidth, in the exact same location”. In this case we can assume that S(f) (the transfer function of a set of sources, (which are homogeneously distributed around our measurement point) and M(f) are the same. In other words, the input at the measurement site can be assumed the same for the two sensors. If we install the sensor at “exact same location” (==really a small distance between them) with the same terrain-coupling procedure, we can assume that L(f) is again the same for the two sensor. Then the measurement should depend on only the apparatus transfer function A(f)=I(f)*DL(f).
If the cited conditions are satisfied, the answer to your question could be find in the responses of sensor and/or data logger. In particular, sensor response I(f) mainly depends on: the type of sensor (velocimeter, accelerometer) and its constructive modality (passive or active: generally, force balanced type). DL(f) is mainly depends on the Analogic/Digital conversion system, which involves the frequency response of: cable and connectors, the embedded electronic of amplifiers of data-logger system, the sampling rate and decimation procedure.
Assuming that the sensors are of the same type and have the same constructive modality we should invoke the effect of data-logger response. Assuming the same sampling rate of A/D converter and decimation procedure (you should verify this condition, also due to different producers of stations) the main effect should be produced by the data-logger (embedded) electronic that, generally, it manifests as the presence of continuous component in recording (also variable in time). This effect is relatively relevant in presence of low noise site recording.
If the data-logger and cable are the same we must invoke the effect of sensor response. You must verify if the manufacturer responses in your bandwidth are the same. Finally, an operational consideration: using sensors with different proper period it is necessary to wait to reach the optimal recording condition. Also in this case spurious response, like variable continuous component and mainly in low frequency, could be observed. It is important, to perform high quality seismic noise measurements, to use a very long recording windows (i.e. almost 1-2 days).
It is important to verify whether the two devices have the same sensitivity,
Roberto De Franco has explained a good part of the question, I will add a supplement to clear more precisely the question because I am founder of analog De-correlation technology that you can see one two of it's application at this researchgate.
In general, what you measure is the correlation or cross multiplication of all the individual frequencies that you perceive.
First - earthquake signal is an spectrum or a bandwidth composed of multi-frequencies plus the environment noise or background noise, and all the individual frequencies of earthquake noise and background-noise are cross multiplied on each other because of the nonlinear characteristics of each sensor, that will provide an specific measurement result for each sensor.
Here we see that our measurement is altered by a different sensor, however the right earthquake signal is hidden or highly altered by dynamic noise or nonlinear noise.
Now, to be able to distinguish our target (earthquake noise) from other noises and sources, we should DE-correlate or DE-multiply or DE-modulate the measured signal.
That's why we find the importance of the DE-correlation technology on earthquake observation and measurements.
Second - by De-correlating the measured earthquake signals, we will see the right frequency or the main frequency of the earthquake, the right or the main frequency of earthquake is the resonant frequency of the earthquake cavity,
third, - with multi measurements distanced on time we will see when the earthquake will happen with a great precision.
We will see exactly the location of the earthquake and the exact level of earthquake.
De-correlating earthquake signals will guide us to abort it successfully before happening.
To be able to perform all these measurements and prevention, the collaboration of several research and observation centers will accelerate the regional and worldwide mapping with exact details of when, where, and what level the earthquakes will happen.
For our side, we are able to provide the De-correlators for the measurements.
Without separating and distinguishing the main signal of the earthquake from it's sidelobes and other noises, the mapping and measurements loose their precisions and meanings.
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