Assessing the presence, and especially the amount, of ground water from space is notoriously difficult, and in many cases simply impossible by direct measurement, because electromagnetic waves do not penetrate very deep into soils and rocks. So, one critical element you should specify in your question is the range of depths into the ground you are interested in.

However, within that general context, the penetration depth of radiation is dependent on the wavelength. In particular, microwaves can, under specific conditions, penetrate much deeper than solar radiation, for instance.

One researcher who's worked a lot in this area is Prof. Farouk El-Baz. You'll find his CV and a long list of publications at

http://www.bu.edu/remotesensing/faculty/el-baz/

Please refine your question if you are actually looking for information on soil moisture at or near the surface, for which various solutions have been explored, some of which with reasonable success.

I suggest looking to GRACE data that provides information about the total mass variations and, hence, on the total amount of water in the soil.

see, e.g., http://drought.unl.edu/MonitoringTools/NASAGRACEDataAssimilation.aspx

Note the data are freely available (http://grace.jpl.nasa.gov/data/)

You can use the information provided by Luca Brocca (i.e., GRACE-derived terrestrial water storage - TWS) associated with some data such as soil moisture (SM), surface water (SW - e.g., lakes and rivers), snow and ice (if is the case) to compute ground water (GW) anomalies (the mean being removed) as:

GW = TWS - SM - SW -...

It provides an overall information of GW variations over a particular region (the size depends on GRACE's resolution).

Please, see the paper: "Satellite-based estimates of groundwater depletion in India" by Rodell et al. (2009).

Good luck!

As per my best knowledge Remote sensing directly can't help to estimate amount of ground water

GRACE satellite can estimate the GW. Read some paper from Yoichi Fukuda and Keiko Yamamoto (Kyoto University).

From the above discussions, I am wondering, can we treat the moisture of soil or water near the surface as ground water? 

This probably won't help you much just yet but in less than 24 hours of this writing NASA will launch its SMAP (Soil Moisture Active Passive) satellite which will eventually answer your question. Here's the link to NASA's web site for the project:

http://smap.jpl.nasa.gov/

Perhaps you can find links that might help with your query.

Thanks for all ,I appreciate your support and help.

Note that the satellite soil moisture products mentioned by Dr. Manca (http://hsaf.meteoam.it/), and available over Europe for the H-SAF project, are also available on a global scale. Both the satellite product obtained from ASCAT and the modelled+satellite product of ECMWF are freely available.

The precipitation products will be available soon on the full disk of Meteosat-10, therefore also over the whole Africa.

https://smap.jpl.nasa.gov/

Here you can find and download the trends for Total Water Storage (Groundwater storage + surface water storage), derived from GRACE for the globe :

https://ccar.colorado.edu/grace/gsfc.html