In this paper the covering-blanket method widely used to estimate fractal dimension is improved. The D-dimensional area K, which has never been detailed in previous references, is clarified and further extended to a fractal signature as a function of scale and space. After defming two discrepancy measurements of multiscale fractal signature, an algorithm of man-made target detection, which is based on fractal signature change, is presented and tested on looking-forward infrared images collected by long wave infrared camera on sea surfaces. The results of using the D-dimensional area are compared with those of using the fractal dimension, and suggest that the proposed method performs better in detecting ship targets embedded in natural scenes.