Sufficient knowledge about subsurface structures of historical buildings is very important for their structural analysis, maintenance and repair. Due to strict preservation regulations for historical sites, only indirect disturbance-free observation techniques like gravity or magnetic measurements are suitable for data acquisition. For the data analysis as a solution to an inverse problem, the object modelling technique proposed by Rumbaugh et al. [Object-oriented modelling and design. Englewood Cliffs, NJ: Prentice Hall, 1991] was used to create a Windows(TM) program that allows a visual examination of subsurface structures by the built-in multi-tasking and point-and-click capabilities. This approach has many advantages compared to the past without the ease of object-oriented window facilities for both developers and users. The described method was successfully applied to investigate the subsurface structures of Hagia Sophia, one of the oldest sacred monuments in the world, built between 532-537 AD under the reign of Justinian in today's Istanbul. (C) 1998 Elsevier Science Ltd. All rights reserved.