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Ambient noise imaging (ANI) is the revolutionary idea of detecting undersea objects using the ocean's background noise. In this study, in order to realize an ANI system, sound fields focused by an acoustic lens system constructed with a single biconcave lens were analyzed using the 2D finite difference time domain (FDTD) method. By comparing the image points and -3 dB areas obtained by lenses with various apertures A and radii R of curvature, sufficient resolution, e.g., a beam width of 1 deg at 60 kHz, was observed using a lens of A = R = 2.0 m. In another 2D FDTD analysis, rigid target objects in an acoustic noise field generated by a large number of point sources were successfully detected using the lens determined above. The noise sources were supposed to be not only at front-light but also backlight positions. The difference in spectra levels between the target direction and the others was greater than 5 dB over 40 kHz for front-light or 50 kHz in back-light sources.