We find uniquely strong tube waves observed during the crosswell survey at Nagaoka, Japan. Uniform linear moveouts of tube waves with velocity of 1.4 km/s are generated by second-order scattering at the upper side of the wells. Even the second order, the amplitude of such tube waves is strong because of the distance of source and receiver wells. Based on the physics of such waves, we apply an inversion technique to estimate the location and strength of the scatterers. Scatterers are located at the upper part of the wells, but their locations do not correspond well to the local geology. Hence we consider that they are more related to local fractures and/or the condition of wellbore. Because the tube waves propagate along the source and receiver arrays many times, we can use them to accurately estimate subsurface velocities along the wells. Using these tube waves at different stages of the survey, we monitor the subsurface velocities. The estimation is very stable and the velocities show the general trend of subsurface structure, and we can potentially monitor the time-lapse changes of seismic velocities around the wells.