## 4 Discussions  
Though the mechanical parameters of the expansive grout have an influence on the characteristic parameters of gauge combinations, the relative difference between the maximum and minimum in the range of mechanical parameters discussed in this paper is still small, according to Tables 2-4 and Tables 6-8. In addressing whether borehole strainmeters can be used for quantitatively measuring seismic strain waves independently from the mechanical parameters of expansive grout, the influence of different modulus on bandwidth is discussed here.  
As shown in Figure 10, the 1% bandwidths of \(FV_{2}\), \(FS\), \(FV_{4}\) and \(TV\) increase with the increase of \(\
u_{2}\). It can be seen in Table 9 that no matter how the mechanical parameters change in the range discussed in this paper, even if the surrounding rocks are saturated corresponding to higher Poisson ratio, the 1%  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & & & \(E_{2}\) = 30 GPa, \(\
u_{2}\) = 0.25, \(\rho_{2}\) = 2.4 g/cm\({}^{3}\) & & \\  & **\(\theta_{1}\)(\({}^{\circ}\))** & **Zero** & **Peak Value** & **\(f\) for Max** & **abs(max)** \\  & & **Frequency Gain** & **(\(\
u_{2}\)-) \(\
u_{2}\) (\(\
u_{2}\)+)** & **(\(\
u_{2}\)-) \(\
u_{2}\) (\(\
u_{2}\)+)** & **\(\
u_{2}\)-(\(\
u_{2}\)+)** \\ \hline \multirow{8}{*}{_FS_} & 0 & (0) 0 (0) & (0) 0 (0) & (0) 0 (0) & 0 (0) \\  & 15 & (2.96) 2.90 (2.82) & (3.25) 3.20 (3.16) & (3624) 3718 (3938) & 0.192 (0.185) \\  & 30 & (5.12) 5.02 (4.88) & (5.62) 5.55 (5.48) & (3624) 3718 (3937) & 0.332 (0.321) \\  & 45 & (5.92) 5.80 (5.63) & (6.49) 6.40 (6.32) & (3624) 3718 (3937) & 0.384 (0.370) \\  & 60 & (5.12) 5.02 (4.88) & (5.62) 5.55 (5.48) & (3624) 3718 (3937) & 0.332 (0.321) \\  & 75 & (2.96) 2.90 (2.82) & (3.25) 3.20 (3.16) & (3623) 3718 (3937) & 0.192 (0.185) \\  & 90 & (0) 0 (0) & (0) 0 (0) & (0) 0 (0) & 0 (0) \\ \hline \multirow{8}{*}{_TS_} & 0 & (5.12) 5.02 (4.88) & (5.66) 5.58 (5.51) & (3707) 3801 (4020) & 0.332 (0.321) \\  & 15 & (4.44) 4.35 (4.22) & (4.89) 4.82 (4.76) & (3671) 3766 (3890) & 0.288 (0.278) \\  & 30 & (2.56) 2.51 (2.44) & (2.80) 2.76 (2.72) & (3542) 3635 (3855) & 0.166 (0.160) \\  & 45 & (0) 0 (0) & (0) 0 (0) & (0) 0 (0) & 0 (0) \\  & 60 & (2.56) 2.51 (2.44) & (2.83) 2.79 (2.76) & (3707) 3801 (4020) & 0.166 (0.160) \\  & 75 & (4.44) 4.35 (4.22) & (4.85) 4.79 (4.72) & (3576) 3670 (3890) & 0.288 (0.278) \\  & 90 & (5.12) 5.02 (4.88) & (5.59) 5.51 (5.44) & (3541) 3635 (3855) & 0.332 (0.321) \\ \hline \multicolumn{8}{l}{Note: (\(\
u_{2}\)-) \(\
u_{2}\) (\(\
u_{2}\)+) refers to (0.15) 0.25 (0.35), respectively. Abs (max) is the absolute maximum of the difference between (\(\
u_{2}\)-) and \(\
u_{2}\), and between \(\
u_{2}\) and (\(\
u_{2}\)+), respectively.} \\ \end{tabular}
\end{table}
Table 7: Influence of the [PERSON]’s ratio of the expansive grout on theoretical amplitude spectrum of gauge combinations \(FS\) and \(TS\) for the incident S wave.  
\begin{table}
\begin{tabular}{c c c c c c} \hline  & & & \(E_{2}\) = 30 GPa, \(\
u_{2}\) = 0.25, \(\rho_{2}\) = 2.4 g/cm\({}^{3}\) & & \\  & **\(\theta_{1}\)(\({}^{\circ}\))** & **Zero** & **Peak Value** & **\(f\) for Max** & **abs(max)** \\  & & **Frequency Gain** & **(\(\
u_{2}\)-) \(\
u_{2}\) (\(\
u_{2}\)+)** & **(\(\
u_{2}\)-) \(\
u_{2}\) (\(\
u_{2}\)+)** & **\(\
u_{2}\)-(\(\
u_{2}\)+)** \\ \hline \multirow{8}{*}{_FS_} & 0 & (0) 0 (0) & (0) 0 (0) & (0) 0 (0) & 0 (0) \\  & 15 & (2.90) 2.90 (2.90) & (3.16) 3.20 (3.25) & (3526) 3718 (3906) & 0.075 (0.077) \\  & 30 & (5.02) 5.02 (5.02) & (5.47) 5.55 (5.63) & (3526) 3718 (3906) & 0.130 (0.134) \\  & 45 & (5.80) 5.80 (5.80) &bandwidth of all gauge combinations for borehole strainmeters is still beyond 200 Hz. This means that it is possible to measure high-frequency microseismic strain waves quantitatively.