## 2 Methods
### Dataset  
A geological core drilling test bench based on a dual-motor drive is used to collect the main parameters during the drilling process [13], as shown in Figure 3. These parameters include the WOB, TOR, Q, RPM, and ROP. The experiment uses cubic concrete blocks measuring 150 mm \(\times\) 150 mm \(\times\) 150 mm, granite, and limestone to simulate the strata. Each different concrete block is used as a unit, with an 80/20 split for the training set and testing set. As shown in Figure 4, the concrete blocks are made from the same batch in order to reduce errors in the experiment. The sensor parameters are listed in Table 1, the sample parameters are listed in Table 2, and the properties of the geologic core drilling diamond-impregnated drill bits are shown in Table 3.  
Figure 3: Geological core drilling test rig based on a dual-motor drive.  
Figure 2: Research flow chart of ROP prediction based on the BP neural network.  
In order to investigate the relationship between drilling parameters such as drilling pressure, torque, slewing speed, mechanical drilling speed, pump volume, and depth of advance, the experimental rig adopts a constant-speed drilling model. The mechanical drilling speed is controlled at 1.39 m/h, 2.78 m/h and 4.17 m/h respectively. The rotary speed is 600 rpm, the pump volume is 600 L/min, and the drilling footage is 300 mm. This paper collected a total of 13,887 datasets through the geological core drilling test bench. Some of the data are shown in Table 4. Drilling data at the different ROPs are shown in Figure 5.  
From Figure 5, the trends of the TOR and WOB in the constant-speed drilling mode are similar, and the torque and drilling pressure slowly increase and eventually stabilize with the increase in feed. However, due to the different RPMs, the matching TOR and WOB drilling parameters are different for the same experimental samples drilled at the  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Range** & **Precision** \\ \hline RPM (control) & 0\(\sim\)1500 rpm & \(\pm\)0.05 rpm \\ ROP (control) & 0\(\sim\)4.26 m/min & \(\pm\)0.71 mm/min \\ Q (control) & 0\(\sim\)50 L/min & \(\pm\)0.5 L/min \\ WOB (measurement) & 0\(\sim\)15 kN & \(\pm\)75 N \\ TOR (measurement) & \(\sim\)50–50 N-m & \(\pm\)0.05 N-m \\ Flow pressure (measurement) & 0\(\sim\)4 MPa & \(\pm\)0.002 MPa \\ \hline \hline \end{tabular}
\end{table}
Table 1: Sensor parameters.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Sample** & **Water:Line:Sand** & \
\begin{tabular}{c} **Uniaxial Compressive** \\ **Strength (MPa)** \\ \end{tabular} &
\begin{tabular}{c} **Drillability** \\ **(m/h)** \\ \end{tabular} \\ \hline Concrete blocks 1 & 1:2:2 & 30 & 3 \\ Concrete blocks 2 & 2:5:7 & 60 & 4 \\ Limestone & - & 80 & 6 \\ Granite & - & 180 & 9 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Sample parameters.  
Figure 4: Experiment sample.  
same drilling depth. At an ROP of 1.39 m/h and an RPM of 600 rpm, the TOR tends to stabilize at 600 N, and the WOB tends to stabilize at 2000 N-m. At an ROP of 2.78 m/h and an RPM of 600 rpm, the TOR tends to stabilize at 1000 N, and the WOB tends to stabilize at 3000 N-m. At an ROP of 4.17 m/h and an RPM of 600 rpm, the TOR tends to stabilize at 1500 N, and the WOB tends to stabilize at 4000 N-m. Therefore, it can be seen that by exploring the drilling parameters matching the lithology of each experimental stratum, we can find the complex relationships among them. Through the drilling parameters, it is possible to predict the characteristics of the stratum lithology in geological core drilling as well as to predict the real-time mechanical drilling speeds, thus achieving the purpose of optimizing the drilling speeds.