  We design an event recognition-analysis system that can automatically detect
solar type III radio burst and can mine information of the burst from the
dynamic spectra observed by Nancay Decameter Array (NDA). We investigate the
frequency drift rate of type III bursts and the speed of electron beams
responsible for the generation of the bursts. Several computer vision methods
are used in this automatic analysis system. The Hough transform is performed to
recognize the line segment associated with type III bursts in the dynamic
spectra. A modified active contour model is used to track the backbone of the
burst and estimate the frequency drift rate at different frequency channels. We
run this system on the NDA data from 2012 to 2017, and give a statistical
survey of the event number distribution, the starting and stopping frequencies
of bursts, the frequency dependence of the drift rate, and the exciter speed
using three corona density models. The median value of the average frequency
drift rates is about 6.94MHz/s for 1389 simple well-isolated type III bursts
detected in the frequency range 10--80 MHz of NDA observation. The frequency
drift rate changes with frequency as $df/dt = -0.0672 f^{1.23}$ from a
least-squares fitting. The average exciter speed is about 0.2c based the
density models. We do not find any significant dependence of the drift rate and
the exciter speed on the solar activity cycle.
