Abstract: An image processing apparatus includes an edge detector for detecting whether or not a target pixel of an input image signal is at an edge portion of a discontinuous density image; a first quantizer for generating quantized image data with respect to the target pixel by using a first error spreading filter having a relatively high error spreading effect, such that a density error between the input image signal and an output multilevel image signal is incorporated in "m" neighboring pixels surrounding the target pixel; a second quantizer for generating quantized image data with respect to the target pixel by using a second error spreading filter having a relatively low error spreading effect, such that a density error between the input image signal and the output multilevel image signal is incorporated in "m" neighboring pixels surrounding the target pixel; and a selector for selecting either the quantized image data generated by the first quantizer or the quantized image data generated by the second quantizer

Abstract: An earth probing system uses deep penetration of electromagnetic waves into soil and other media. Advantage is taken of the lower attenuation of radar waves in soil by frequencies of about three megahertz or less. Bursts of electromagnetic energy of various frequencies in this range are consecutively transmitted. The transmitting antenna is continuously tuned, so as to maintain resonance during each burst, allowing large circulating currents and high power output. In a receiving antenna system, a dual antenna arrangement is providing for obtaining improved reception. A corresponding dual antenna circuit employs "spatial notch filtering", automatic adjustment of antenna gain-frequency variations, as well as compensation for transmitter gain variation. The system may be implemented in a totally analog, totally digital, or hybrid manner. Preferably, a signal processing method detects and digitally samples signals reflected from subsurface layers and buried objects.