Resistivity imaging (electric t o m o g r a p h y ) is a m e t h o d o l o g y for defining lateral variations of resistivity associated with structural anomalies such as caves, water contamination, and fractured zones a m o n g others. Important urban areas in M e x i c o City are currently at high risk of collapse. Cavities and shallow fractures h a v e been created as a result of mining in hilly regions beneath several of today's urbanized n e i g h b o r h o o d s located in the southwestern portion of M e x i c o City. Selected areas h a v e been investigated. C a v e s and tunnels w e r e found for depths ranging b e t w e e n 5 m u p to 15 m. Diameters of such features w e r e b e t w e e n 5 m u p to 30 m. S o m e of these structures are across p a v e d roads and beneath buildings and houses, running for several tens of meters in length. Topographic effects w e r e also studied in o n e profile. It s h o w e d important inaccuracies in resolving the g e o m e t r y of the cave, mainly in terms of depth to the top, w h e r e differences of 2 m to 3 m w e r e encountered. G P R ( G r o u n d Penetrating Radar) w a s also used to confirm the results obtained in a resistivity profile surveyed on a test site. Results are very encouraging, demonstrating that the resistivity i m a g i n g helped to locate and characterize m i n e d areas, jointly with other geophysical m e t h o d s . Unfortunately, true resistivity is not well resolved, b e c a u s e of inherent ambiguity of the inverse m e t h o d used. I n t r o d u c t i o n M e x i c o City constitutes one of the largest concentra tions of h u m a n activities in the world, with a population of about 20 million people. T h e City w a s founded originally on a small island in a lake surrounded by volcanic ranges. T h e confined nature of this intramontane valley directly af fects air quality, water supply and urban development. Therefore, administering services for the inhabitants of this city is a big challenge ( C a m p o s et al, 1997). O n top of these p r o b l e m s , c o m e s the insufficient housing and shelter ing for the population, a p r o b l e m that has increased during the past 4 0 years. During the forties and the fifties, m o s t of the material used for construction purposes w a s obtained from mines excavated in the western Sierras. Such m a t e rials were e m p l o y e d to build houses, apartment and gov ernmental Yam&mgs m d o w n t o w n M e x i c o . W o w e v e t , mos>Y of these sites were exploited illegally. W h e n this industry c a m e to an end at the beginning of the sixties, m o s t of the cavities resulting from m i n i n g were a b a n d o n e d or refilled with debris. S o m e caves w e r e later used as shelters for entire fam ilies, or had other uses by the people in the neighborhood. N o w a d a y s , the location of most of these structures is un k n o w n . L o w -, middle-and even high middle-neighbor hoods have settled on top of m i n e d zones for the past 30 years. M a n y accidents h a v e since occurred, caves...

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Typical 3-D electrical resistivity tomography sampling schemes, which require a grid of electrode lines to be deployed, are limited by physical conditions of the area under study. New array techniques are needed to characterize the subsoil beneath anthropogenic or natural structures to define hazardous zones. Use of multiple L-shaped arrays overcome the need for a grid of electrodes by surrounding an area in a square of electrode lines; however, in some instances, the physical environment does not allow closure of a square of electrodes. An alternative array introduced in this investigation is termed the horseshoe array. The horseshoe array combines the L-shaped arrays with equatorial and minimum coupling arrays to overcome array closure problems. Three synthetic examples were investigated to establish the limitations of the horseshoe array, and to describe the geological conditions of the subsoil, e.g., building foundations and fractures. The first two examples represent two resistive cubes initially located in the southern and northern positions of the array, and then are moved to a diagonal of the array. In both examples, the cube located near the electrode lines was well defined, while the cube located near the line with no electrodes was not detected. On the other hand, a weak signal for the cube located along the diagonal was observed, but only when located near the electrode line. This alternative array revealed a low tri-dimensional resolution zone possessing an inverted triangular-shaped geometry towards the line with no electrodes. A third example consisted of a low resistivity thin fracture embedded in a highly resistive infill. The solution computed demonstrated that the horseshoe array can resolve the infill close to the surface; however, the thin fracture is masked by the infill. The above-mentioned methodology was applied on a residential complex named La Concordia. Several buildings within the residential area suffered strong structural damage caused by fractures and subsidence within the subsurface. The residential complex, consisting of six four-story buildings in an area 33 × 80 m2, is located towards the eastern region of Mexico City. The horseshoe geometry, combined with Wenner-Schlumberger, dipole-dipole, equatorial-dipole, and minimum-coupling arrays, was used to investigate the subsurface beneath the buildings. A maximum depth of 8 m was investigated. A pattern of elongated resistivity anomalies (northwest-southeast direction) were associated with possible fracturing or differential compaction. Such features are caused by intense water extraction of the poorly consolidated clays that cover most of the central portion of the Mexican Basin.

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