Many geophysical technique are being investigated in this domain. You can see some new application like

Underground cavity investigation within the city of Rome (Italy): A multi-disciplinary approach combining geological and geophysical data, by G. Bianchi Fasani et al. 2013

Investigating a damaging buried sinkhole cluster in an urban area (Zaragoza city, NE Spain) integrating multiple techniques: Geomorphological surveys, DInSAR, DEMs, GPR, ERT, and trenching by Domingo Carbonel et al. 2015

And a resistivity map of all the Great Britain has been carried out by British Geological Survey, which is the most huge work I know for instance.

or even my work, in which i tried to detect the underground structure of karstic environment.

Various method we can used for cavity study such as ERT scan, GPR scan.

But it is difficult in Road. So we try to fix a vibration meter in one low moving vehicle to measure four wheels vibration in every meter to continuously record in equipment in premonsoon and post monsoon. 

 Rujun  Hello

Many geophysical methods can be applied for cave delineation (including city conditions).

Some information about the geophysical methods and methodologies applied for the cave recognition and its parameters estimation are described in the following articles presented in the ResearchGate:

Eppelbaum, L.V., Khesin, B.E. and Itkis, S.E., 2001. Prompt magnetic investigations of archaeological remains in areas of infrastructure development: Israeli experience. Archaeological Prospection, 8, No.3, 163-185.

Neishtadt, N., Eppelbaum, L. and Levitski, A., 2006. Application of seismo-electric phenomena in exploration geophysics: Review of Russian and Israeli experience. Geophysics, 71, No. 2, B41-B53.

Eppelbaum, L.V., 2007. Localization of Ring Structures in Earth’s Environments. Jour. of the Archaeological Soc. of the Slovakian Acad. of Sci., Spec. Issue: Arch. Prosp., XLI, 145-148.

Eppelbaum, L.V., Ezersky, M.G., Al-Zoubi, A.S., Goldshmidt, V.I. and Legchenko, A., 2008. Study of the factors affecting the karst volume assessment in the Dead Sea sinkhole problem using microgravity field analysis and 3D modeling. Advances in GeoSciences, 19, 97-115.

Eppelbaum, L.V., 2009. Near-surface temperature survey: An independent tool for buried archaeological targets delineation. Journal of Cultural Heritage, 12, Suppl.1, e93-e103.

Eppelbaum, L.V., 2011. Study of magnetic anomalies over archaeological targets in urban conditions. Physics and Chemistry of the Earth, 36, No. 16, 1318-1330.

Eppelbaum, L.V., 2011. Review of environmental and geological microgravity applications and feasibility of their implementation at archaeological sites in Israel. International Journal of Geophysics, doi: 10.1155/2011/927080, ID 927080, 1-9.

Eppelbaum, L.V. and Mishne, A.R., 2011. Unmanned Airborne Magnetic and VLF investigations: Effective Geophysical Methodology of the Near Future. Positioning, 2, No. 3, 112-133.

Ezersky, M., Eppelbaum, L., Al-Zoubi, A., Keydar, S., Medvidiev, B., Akkawi, E., Abueladas, A.-R. and Al-Ruzouq, R., 2013. Geophysical prediction and following development sinkholes in two Dead Sea areas, Israel and Jordan. Environmental Earth Sciences, doi: 10.1007/s12665-013-2233-2, 1-16.

Al-Zoubi, A., Eppelbaum, L., Abueladas, A., Ezersky, M. and Akkawi, E., 2013. Methods for removing regional trends in microgravity under complex environments: testing on 3D model examples and investigation in the Dead Sea coast. International Journal of Geophysics, Vol. 2013, Article ID 341797, 1-13, http://dx.doi.org/10.1155/2013/341797.

Eppelbaum, L.V., Zheludev, V. and Averbuch, A., 2014. Diffusion maps as a powerful tool for integrated geophysical field analysis to detecting hidden karst terranes. Izv. Acad. Sci. Azerb. Rep., Ser.: Earth Sciences, No. 1-2, 36-46.

Eppelbaum, L.V., 2015. Quantitative interpretation of magnetic anomalies from thick bed, horizontal plate and intermediate models under complex physical-geological environments in archaeological prospection. Archaeological Prospection, 23, No. 2, 1-16.

Eppelbaum, L.V., 2007. Revealing of subterranean karst using modern analysis of potential and quasi-potential fields. Proceed. of the 2007 SAGEEP Conference, 20, Denver, USA, 797-810.

Best regards,

Lev

Resistivity can be investigated by dipole-dipole method.

Dear  Rujun

I am agreed with Lev on use of Microgravity. Disadvantage is only sensitivity of the method to small or deep cavities. I think ERT is a good method, especially its modification  with a land streamer (electrodes located at the moving belt. For a small cavities GPR is most suitable. We scan GPR antenna along parallel lines.

See attached paper    

Hi Chen,

The above responses are great. However, the pictures that you have mentioned about cavity detection seems to be detection of sink holes. In this case, I will recommend to use surface waves testing, specifically, multichannel analysis of surface waves method (MASW). There are limitations of GPR and ERT when tested in clayey soils. MASW method can give you accurate shear wave velocity profile upto depth of 30 meters.

In my PhD studies, I am working on detection of underground cavities using surface waves. You can see the conference paper on my profile.

Dear Hassan

You are right that MASW method can help at proper relationship between lateral resolution depth to cave. Generally it is not subject for discussion. Every geophysical method has its advantages and disadvantages and selection of the method for survey is defined by many factors: ground conditions, depth to cavity and cavity size as well as resolution of the method applied.

Selection of the method is problem of investigator knowing above factors better of all advisers.

kind regards

Michael

Hi!

In urban areas the problem is complex because not always is possible to use an specific method. Using an combination of geophysical methods (MASW, Resistivity, microgravity) you may have good results. I recommend you to have an resistivity profiling using appropriate array (Wenner works good for shallow depth ) and after you locate anomalous areas you can use 2D Electrical tomography (Pseudosection, "Real Section" see my profile) for defining  depth and size of cavity.

GPR is fasten and cheaper. We have great results here detecting cavities from water flow effects in soil.

I would recommend GPR.  You can scan large areas of road very quickly with an antenna towed behind a vehicle.  Depths down to 5m is easily surveyed with GPR and air filled voids make good GPR reflectors.  Many of the other methods mentioned are fine for detailed studies of smaller areas but if you have many kilometers of road to scan then GPR is the best method.

I suggest u must use an integration between 2 or three methods to get highly resolution imsge about the cavity ( MASW, GBR, 2D resistivity)

nderground detection

Detection of buried objectsThe presence of underground objects such as drums, tanks and other buried structures can indicate potential environmental hazards. There are various geophysical methods that can be used in order to detect these objects. Depending on the target and overburden properties, electromagnetic methods (e.g. EM-61, EM-31), ground penetrating radar and earth magnetics can be used to locate such buried objects.

please visit the website

http://www.geophysicsgpr.com/english/services.htm 

Top

Depends upon the properties of the soil and the depth at which these structures are present. Ground Penetration Radar GPR could be one of the better options 

I suggest you to use GPR and ERT for for detecting road cavities. 

I would suggest electromagnetic induction methods (EMI), with several distances between coils and/or frequencies or ground penetration radar because ERT may be difficult to do over asphalt or concrete.

Road cavities are most  common in carbonate rocks where caverns or even caves are likely to form, though some cavities may form in evaporite beds.  These terrains should be given special attention.  Ground penetrating radar at relatively low frequency is the most efficient and  accurate method to detect road cavities, because other electromagnetic methods cannot be easily deployed in urban areas.

Hi, everyone! Thank you very much for your answers. The road cavities are most caused by water leakage of underground pipe in China. Heavy truck, subway construction, building construction, and so on, may cause the leakage of underground pipe. We can find this problem by the images given in the question. Sands and clay are transported to other place by leakage water and a cavity is produced by this transportation.  

 The near surface resistivity of damaging road may be 10 ~ 100 ohm*meter. Water content of the near surface may be high. A developing cavity may be filled by water or air. The depth of cavity range from 2~20 m according to historical data. Based on above condition and strong electromagnetic interference in city area, how to detect such cavity by geophysical methods? 

several geophysical methods may be used to detect the cavities. i would recommend the electrical resistivity method  , seismic,  magnetc and gravity  methods and   subsurface  radar. electomagnetic method may also be used

This a case type of liquifaction and like i mentioned before most geophysical tools is useful but in case of high resolution result must integrate tools

The fastest method for detection would be GPR (or EMI).

As stated, we can have natural cavities (e.g. under carbonate rocks) or artificial cavities developed by water/sewer underground pipe leakage at “loose soil” (e.g. sandstone).

The targets - cavities can be filed (totally or not) with air (high electrical resistivity), sediments or water (mean to low electrical resistivity).

Fortunately the cavities’ s size and depth hazard will follow the penetration deep versus target size of methods’ s detection, i.e. deeper smaller cavities are not detected –

but they are not dangerous. For this reason, GPR would be the fastest and less prone to EM interferences at cities.

GPR method is most effective method for predication cavities also 2D resistivity method can be applied for this 

GPR will indeed work if the waves can penetrate deep enough below the soil or altered bedrock. In case that fails, ert can help and of course microgravimetric measurements. There are several papers in engineering geology where you can find many case studies. 

I hope this helps 

GPR will indeed work if the waves can penetrate deep enough below the soil or altered bedrock. In case that fails, ert can help and of course microgravimetric measurements. There are several papers in engineering geology where you can find many case studies. 

I hope this helps 

GPR will indeed work if the waves can penetrate deep enough below the soil or altered bedrock. In case that fails, ert can help and of course microgravimetric measurements. There are several papers in engineering geology where you can find many case studies. 

I hope this helps 

I searched for cavities here in Brazil many times using GPR and it is very effective. In urban areas and not so deep as I can see in your pictures I use highest frequency shielded antennas (200 - 400MHz). Just to check: all these cavities in your pictures seems to be caused by water leak, a very common origin. Good luck.

The are different geophysical methods such as GPR ,2D and 3D resistivity imaging,Electromagnetic method,Magnetic and Microcgravity methods.But,the GPR ,2D and 3D the most use in cavity detection.In addition there is Bristoss method by using  Pole dipole array,see attach files

I suggest for you to use the ERT and GPR methods, they are very fasts and not costs compared to the other geophysical methods. make sure to use the ERT method in 3D in order to visualize the underground cavities, especially in urban areas.  

As it is shown most of the researches, Ground Penetration Radar methods suits best for the urban city areas

Ground penetration radar method is appropriated in exploration and select frequency for material. 

You can go for electromagnetic methods using coil as transmitter and a sensitive magnetometer as receiver. Go for flux gate magnetometer since you want to sense 5-6 m deep or for SQUID if you want to go deeper. This will work as any NDE method. The resistivity profile when you go from filled places to places with cavity will show variation. You have to optimize the transmitter coil to induce a good amount of primary field.

Dear Rujun,

There are several methods to detect cavities. Some more efficient are GPR (Ground Penetration Test), that is a less expensive method. The other method is Microgravity that is more expensive but more accurate. In some cases geoelectric method is a reliable too (especially when the cavity is wet).

good luck 

Dear Zaman,

Thank you very much! How do you think of buildings in two sides of street. It will effect the measurement of Microgravity.  

With best regards,

Hi Rujan,

How much is the distances?

in any case if the buildings are close to the stations, the correction is necessary.

Dear Zaman,

In city area, many buildings are taller than 50-100 meters. The distance between building and road are about 20 - 50 meters. 

Hi Chen, I think you may try gravity method. OK if you have short track, you can GPR method, but if you want to mapping of the cavities, you may use gravity methods.  Regards.

By using Ground Penetration Radar

I'd jointly use GPR and CCR methods ! I'm dealing with same problems in Rome...i thinks both methods are quite fast and quite appropriate to such kind of targets

The first method is GPR ,and the other are 2D and 3D imaging resistivity imaging.

There are so many geophysical methods to find such cavities.Simple methods like seismic using plate and hammer.Also electromagnetic methods like GPR  can easily detect the susceptibility of air inside the cavity.As resistivity method can also measure the resisitivity of air trapped inside the cavity.As u are talking about accidents that is usual in china its better to go for electromagnetic method or GPR.

First we need to understand the Geology of the study area.

1. what is water flow direction and road cut

2. soil classification

3. low land or high land

4. what it the radial depression formed due to over load from surrounding area

5. Water saturated is increasing in rainy reason

6. GPS is better survey for Cavity but same time we locate and justify which one is the critical condition formed area. After that we conduct the survey in the stressed zone.

Dear Chen,

This is a challenging question and almost all of the answers would be OK in different situations. It is definitely site-dependent since noise levels and environmental situations would extremely affect data quality, and, could prevent the targets to be detected. Totally (with an overall view of all answers), several noticeable points are as follows:

1. In areas where tall buildings exist, microgravity would not work since the effect of cavity (water filled or air filled) with respect to tall building diminishes. That's because of gravity corrections which can not be performed in the desired accuracy. On the other hand, microgravity would delineate cavities in other areas (where tall buildings do not exist), but the operation is slow, and, the price is high. I think that It is not suitable in your desired scale. 

2. As conductive materials present in a very large volume  inside and outside the earth (e.g. metallic objects, conductive soil, etc.) it creates a problem. In the case of EM methods (e.g. GPR), the main problem is the conductive materials outside because the targets are located in depth

Dear Mashhadi,

Thank you very much! Your summary and suggestion are excellent. I will try these methods. 

With best regards

Rujun

Hello Rujun,

how did you succeed with geophysics ?

I did a lot of microgravity for voids/caves and can say, that in your case of very shallow caves it would work easy. The advantage is you do not need any cables (as resistivity surveys), just you need to avoid industrial microseismic noise (I often measure during night) and have a good gravimeter and operator. I suggest to combine with GPR.

Good luck,

Jan Mrlina

Hello Rujun

there are many  geophysical methods to  detect caves in paved  roads , I think the best one is Gpr  ,   microgravity , and capacitive resistivity imaging with towed arrays ...

best wishes 

HI RUJUN  THERE ARE SEVERAL METHODS YOU COULD USED TO   DETECT VOIDS  IN  ROADS.  SUBSURFACE RADAR WOULD BE A GOOD METHOD AS WELL AS ELECTRICAL RESISTIVITY  SURVEYING. THE VOIDS MIGHT NOT PRODUCE A LARGE ENOUGH   ANOMALY TO DETECT WITH THE GRAVITY METHOD. . ELECTRO MAGNETIC METHODS  MIGHT WORK BUT I THINK THE MOST EFFECTIVE METHOD MIGHT BE SUBSURFACE RADAR

The best geophysical methods that use to detect the cavities in the road are seismic refraction method, electrical resistivity method ' microgravity method and ground penetrating radar method.

ERT is very efficient

The best method to detect the cavitie in road is ground penetration radar (GPR).

I believe that the GPR and ERT (2D and 3D) are very useful for that target.

I am totally with Ammar... even if you apply the seismic method it is possible to interpret them. You just need to calibrate parameters. But, by considering your pictures, Ruhun, both the GPR as well ERT (either 2D or 3D) are the accurate to this case.

Hi Chen. I agree with our colleagues and complement with one option: Deep Penetrating Radar (Loza series) that can investigate with great resolution up to 75m, differently from GPR that hardly have good results deeper than 6m, except in very specific conditions .

The best methods are

GPR

Microgravimetry

Electrical Resistivity methods (2D Wenner or Dipole-Dipole)