Satellite data shed light on massive sinkhole in Tokyo

Country/area: Japan

Organisation: Nikkei Inc.

Organisation size: Big

Publication date: 17 Dec 2020

Credit: Nobutaka Hiramoto, Jun Yamazaki, Konori Fujita, Takeshi Yamada, Hiroshi Sagimori, Kaoru Yamada, Chihiro Uchiyama, Kenichiro Kamata, Hiroshi Kunou, Shohei Yasuda, Masayuki Shimizu, Gaku Ito

Project description:

Satelite data shed light on deep underground

A section of road collapsed suddenly in a quiet residential area of Tokyo. By analyzing satellite Interferometric SAR data, Nikkei found that the ground level around the sinkhole warped soon after deep underground construction work took place. The cave-in happened following these movements in the surface.

In Japan, companies can develop the “deep underground” space without the consent of landowners above. This is the first time that detailed data have indicated that deep underground construction may affect residents’ daily lives. The day after Nikkei’s report, the highway operator acknowledged impact of the construction.

Impact reached:

Impact on Japan’s infrastructure

Residents protested and demanded explanations from the highway operator. The Outer Ring Road that related to the incident is a highway designed to cut traffic congestion in the metropolitan area, but construction is halted because of the accident. No one knows when the construction will be completed. The maglev bullet train project, which will connect Tokyo and Osaka, has been forced to review its safety measures because it too plans to use deep underground space.

Blind spots in the law revealed

On December 21, the Minister of Land, Infrastructure, Transport and Tourism said, “It is regrettable that the shield tunnel construction has caused damage to residents.” In Japan, in 2001 it became possible to develop “deep underground,” which is deeper than 40 meters, without the consent of the landowners. The goal was to effectively use underground space. However, the impact on the earth’s surface became apparent for the first time in 20 years, forcing Japanese society to rethink the safety of this measure.

Compensation may grow

After the news broke, the highway operator acknowledged the impact of the accident, apologized, and announced a policy of compensation for residents. Before construction it had said “the ground surface will not be affected.” The company, which did not have Interferometric SAR data, announced that it had confirmed a pinpoint subsidence using a ground survey after Nikkei’s report.

Nikkei’s analysis confirmed a wider range of depressions existed in the area, ranging from directly above the tunnel site to the other side of a nearby river. The highway operator had not been able to find the ground swellings that Nikkei showed. Members of the panel of experts investigating the accident have called for new investigations using Interferometric SAR, which may grow the compensation beyond the highway operator’s assumptions. 

Techniques/technologies used:

Half a year with Interferometric SAR

Nikkei analyzed Interferometric SAR data of an area around the collapse that measured 530 meters east-west, and 870 meters north-south. Changes were analyzed and visualized in millimeters every 11 days starting from April 8, when tunnel construction had not yet begun underground.

Some of the Interferometric SARs that are used for commercial purposes can only measure fluctuations of several centimeters or more. In this analysis, Nikkei worked with TRE Altamira of Italy to employ a technology that can measure the ground plane in millimeters by accurately using the position and orientation of satellites.

Visualization on a map

Using deck.gl (a library that visualizes geographic data created by Uber), we created an analysis tool that displays changes on a map. At first, all the data was put on the map, but after consulting with the engineers and the reporters, Nikkei eliminated erroneous data. Nikkei indicated depressions of 1 cm or more with red dots, and uplifts of 1 cm or more with blue dots.   

Expressed by animation

By showing an animated time-series of the changes, it became clear that the ground began sinking soon after the machine passed deep underground. Prior to that there were almost no changes for half a year. The condition of depressions and uplifts is also represented on the chart. It is clear that the maximum subsidence depth exceeds 3 cm. A 24-second video showing the overall impact of the construction and what happened in chronological order helped readers to understand what the events. Based on the analysis, Nikkei spent a month verifying the accuracy of the data. Visualizations and videos were completed in almost one day. 

What was the hardest part of this project?

We opened a new page for investigative journalism by publishing a more thorough report than that of the highway operator and the government in a shorter amount of time. Using millimeter-scale fluctuation data instead of satellite images to uncover facts is a world-leading example of satellite journalism.

The hardest aspect of this project was the verifying the accuracy of the data.

Show data to the parties

We showed data to multiple government officials and members of the expert committee examining the incident to confirm whether there were any discrepancies with the highway operator’s investigation results. The various parties endorsed Nikkei’s results, saying, “This Nikkei investigation cannot be denied. You can present it with confidence.” Nikkei also received an inquiry as to whether it could sell the data.

Speaking with 100 households

We visited about 100 households and received detailed accounts from half. Nikkei noted these accounts and inspected cracks in buildings and surrounding structures. Residents felt vibrations and heard noises at the same time when the data showed depressions and uplifts occurring. In addition, Nikkei confirmed that manholes had risen up, utility poles were tilted, and walls were cracked in areas where the data showed the greatest depressions.

Survey with experts

Nikkei also showed the data to experts in geology and tunnel construction who have no direct interest in the accident investigation. They confirmed the possibility that the analysis results were plausible considering the surface conditions in the area. Furthermore, Nikkei went to the area with one of the experts and confirmed that the data and the actual ground surface fluctuations were linked.

What can others learn from this project?

This project demonstrates the possibilities for satellite data usage in journalism and the importance of teamwork.

Usefulness of alternative data

The use of satellites in journalism has focused on images, but we have demonstrated that millimeter-scale data from Interferometric SARs can also be a powerful tool. Interferometric SARs can visualize changes that the human eye overlooks and help find the truth behind them. It is a new step in reporting using alternative data.

Interferometric SAR is used, for example, to measure the ground subsidence due to excessive oil excavation at oil drilling sites. The technology of capturing changes using fixed point observation fits well with journalism, which seeks the truth in a timely manner regardless of the weather.

Because it is satellite data, measuring equipment does need to be installed on the ground, and fluctuations can be verified retroactively even in dangerous places that people cannot enter . Alternative data extends the horizon of data journalism.

Gift of teamwork

Fortunately for our team, Nikkei’s R&D department had an established hotline with an Italian company that had satellite data for that area. In addition, the R&D department and reporters have been collaborating on a daily basis to grow Nikkei’s innovative initiatives. Through these collaborations, the data to be analyzed was accessible the day after the sinkhole occurred.

In addition, Nikkei assembled members from the permanent investigative journalism team and the local news section immediately. They gathered reporting from all directions simultaneously, including field interviews, meeting with experts, and contacting the relevant parties.

No matter how good data analysis is, a story that neglects on-the-ground voices and those of the concerned parties cannot be persuasive. It is important to go back and forth between the site and the data many times to reach the truth.

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