The fate of the historic fortifications at Alcatraz island based on terrestrial laser scans and ground‐penetrating radar interpretations from the recreation yard

Abstract

Alcatraz Island National Historic Landmark is best known as a former high‐security federal penitentiary that once housed many of the United States most notorious gangsters. Today, it is a popular tourist destination adjacent to the Golden Gate Bridge in scenic San Francisco Bay. Alcatraz is less known in its former military role as a 19th‐century coastal fortification protecting the interests of a rapidly westward‐expanding nation during the turbulent era of Manifest Destiny, the 1849 Gold Rush and the Civil War. The fortification, with its underground ammunition magazines and tunnels, is important from a military history perspective, marking the transition to earthen structures from the traditional brick and masonry constructions that characterized earlier 19th‐century coastal defences. In this paper, geophysical interpretations based upon an attribute analysis of ground‐penetrating radar data are combined with terrestrial laser scans, georectifications based on historical documents, maps and photographs to develop an iterative model for locating and determining the extent and integrity of subsurface historical architectural remains beneath the former recreation yard of the Alcatraz penitentiary. Using this approach, remnants of buried structures including a ‘bombproof’ earthwork traverse and its underlying vaulted brick masonry tunnel and ventilation ducts were discovered to run east‐west beneath the recreation yard. This integrative and iterative process permits accurate spatial identification of these and other 19th‐century architectural structures depicted in historical documents. This approach can be applied to subsurface investigations at other important cultural landmarks worldwide.

Unveiling the Hidden Secrets of Bomarzo Cathedral: New Evidence from Last Ground-Penetrating Radar Survey

“…As often happens when using GPR to investigate historical buildings [22][23][24][25][26][27][28][29], the results obtained from this study reveal the presence of an intriguing stratigraphy that holds both historical-cultural and structural significance. Through a careful analysis of the processed data, it is possible to confidently confirm the existence of at least four rectangular chambers beneath the flooring and altar, situated at a depth of approximately 0.5 m from the ground level (Figure 5).…”

The Fortress Beneath: Ground Penetrating Radar Imaging of the Citadel at Alcatraz: 1. A Guide for Interpretation

“…Thompson [19] provides a detailed history of Alcatraz island. A previous GPR survey of Alcatraz civil war-era fortifications located outside the main prison building was reported by de Smet et al [20].…”

“…Thompson [19] provides a detailed history of Alcatraz island. A previous GPR survey of Alcatraz civil war-era fortifications located outside the main prison building was reported by de Smet et al [20]. GPR data were collected on the floor of the cellhouse in nine areas known as Sunrise Alley, Michigan Avenue, Broadway, Park Avenue, Sunset Strip, Library, South ABCblocks, Barbershop and Times Square (Figure 2).…”

Combined Migrations and Time-Depth Conversions in GPR Prospecting: Application to Reinforced Concrete

“…As can be seen, the targets placed before the abscissa at 0.4 m appear more focused making use of the value v=v1 (see the rectangle in the left-hand panel), and the targets displaced after the abscissa at 0.4 m appear more focused making use of the value v=v2 (see the circles in the right-hand panel). In particular, in the left-hand panel, the two small targets at about 0.58 and 1.18 m appear as two frowns [19], which is the typical effect of an underestimation of the propagation velocity. As can be seen from Figure 5, the migrated data show a substantial difference in the propagation velocity before and after the abscissa x = 45 cm.…”

“…It has been imported historically from seismics [10,11] but can also be mathematically derived from Maxwell's equations [12] and in particular from the theory of diffraction tomography [13][14][15]. It has been exploited for and/or adapted to many specific applications [16][17][18][19], and any meaningful list of case histories would be beyond our purposes.…”