**7. MIMO-GPR "Yakumo"**

We developed a MIMO GPR system "Yakumo" shown in **Figure 8**, for scanning a large area [13–15]. Yakumo was developed for surveying 1–2 m in depth, which is relatively deep compared to the similar multi-static GPR systems. Yakumo is a SF-CW radar that uses 50 MHz–1.5 GHz, which is a relatively low frequency compared to MIMO-GPR for pavement inspection. Since this device operates in a wide frequency bandwidth, it can select optimal frequency.

**Figure 8.** *MIMO GPR system "Yakumo".*

#### **Figure 9.**

*The antenna arrangement of the MIMO GPR system "Yakumo".*


#### **Table 2.**

*The technical specifications of the MIMO GPR.*

**Figure 9** shows the antenna arrangement of this system, which is equipped with eight transmitting and receiving antennas, and **Table 2** shows the technical specifications. Antenna feeding point separation in the same row is 240 mm but a minimum of 120 mm in the transverse direction between transmit and receive antenna by the staggered position.

Yakumo is a multi-static radar, but by measuring the radio waves transmitted from one transmitting antenna with all receiving antennas, it is possible to acquire complete three-dimensional (3D) subsurface information by looking at the target from different angles. This leads to advanced 3D imaging.

### **8. Measurement example**

An example of C-scan imaging by Yakumo is shown in **Figure 10**, which was acquired in a rice paddy field in winter time [14]. The radar was scanned in the horizontal direction of the figure, and six images of 2 m swarth width are superimposed vertically. The two white lines that can be seen in the C-scan image are agricultural drainage. Due to the high accuracy of the position control, the water pipes for drainage are correctly visualized in a straight line.

### **9. CMP measurement**

Common midpoint (CMP) technique is used for estimating vertical profile of the velocity of electromagnetic wave in subsurface geological layers. In order to acquire

**Figure 10.** *C-scan imaging by Yakumo. The two white lines are agricultural drainage.*

**Figure 11.** *Combinations of the transmitting and receiving antennas to acquire CMP data sets.*

the CMP data by using a conventional GPR system, we move the transmit and receive antenna simultaneously to the opposite direction so that the reflection from the CMP point stays at one position. We fit theoretical arrival time of the reflected wave from the target at the midpoint position and estimate the velocity and the depth of the reflecting layer simultaneously by the use of a velocity spectrum. MIMO GPR can achieve CMP measurement by selecting a combination of antennas so that the center of the array is the midpoint (midpoint) of the transmitting and receiving antennas, as shown in **Figure 11**. CMP measurement can be performed without moving antennas by MIMO GPR [15, 16].

We show an example of simultaneous CMP and profile measurements performed by Yakumo near Sendai Airport, which was damaged by the tsunami of the Great East Japan Earthquake in 2011. This site was a rice paddy field, but the tsunami invaded, and then, the surface soil was releveled. **Figure 12** is the CMP data, and **Figure 13** is the velocity spectrum obtained by the CMP analysis. Spectral peaks are seen at four different depths, detecting four stratified geological boundaries. **Figure 14** shows a continuous display of the velocity obtained by the CMP analysis along the survey line. Under the assumption that homogeneous soil moisture is almost uniform, the distribution of geological boundaries can be detected. These are considered to contain information from geological deposited by the Great East Japan Earthquake in 2011 to past tsunami deposits from more than 1000 years ago.

**Figure 12.** *CMP profile measured by Yakumo near Sendai Airport.*

**Figure 13.** *The velocity spectrum of the CMP profile in Figure 12a.*

**Figure 14.** *Continuous display of the velocity obtained by the CMP analysis along the survey line.*
