Expanding the Panama Canal

The Panama Canal is 77 kilometres long and connects the Atlantic Ocean with the Pacific Ocean. The canal is presently handling more vessel traffic than had ever been envisioned by its builders and the proportion of large ships transiting it is increasing steadily. A project to install new locks and enlarge the canal to increase the capacity and also allow bigger vessels to pass has started with a geophysical survey. This article describes the marine part of it.<P>

The Panama Canal is one of the most important shipping routes in the world, handling an estimated 5% of total world trade. Since its opening in 1914, the canal has provided a swift passage between the Atlantic and Pacific Oceans for ships ranging in size from small pleasure craft to 965-foot long Panamax cargo vessels. An increasing number of ships, however, are too large to fit through the canal, requiring longer and wider locks. Therefore, the Panama Canal Authority (ACP) has proposed the Third Set of Locks Project to install new locks and upgrade the canal to allow passage for post-Panamax vessels. This project is a monumental geotechnical effort that is estimated to require eight years of construction at a cost of USD5.25 billion. On 22 October 2006, the Panamanian people overwhelmingly approved the project in a nationwide referendum.

The Panama Canal cuts through a complex geological setting with numerous faults and a broad range of lithologies. The geology will affect many aspects of the canal expansion, which will include 11km of new cuts through soil and rock, dredging of the existing canal and new engineered structures. As part of the subsurface characterisation for the project, a reconnaissance geophysical investigation was carried out by Technos, Inc. in conjunction with the ACP. The investigation included over 81km of sub-bottom seismic reflection data within the canal to map stratigraphy and to identify faults. In addition to the marine data, over 15km of seismic refraction and multi-channel analysis of surface waves (MASW) data were acquired on land. The results of the geophysical investigation were integrated with existing geological data to better characterise stratigraphy, structure and engineering properties of subsurface materials. The geophysical data are also being used to aid seismic hazard analysis and the field mapping of active faults is being performed by Earth Consultants International.

Third Set of Locks Project
The Panama Canal is 77km long and extends south-east from the Atlantic Ocean, through a system of locks, to the Pacific Ocean. The locks raise ships to an elevation of approximately 26 metres (85 feet) above sea level. The Third Set of Locks Project will add new locks on the Pacific and Atlantic sides of the canal capable of handling longer and wider ships. The new locks will require new navigational channels that will join into the existing canal. The new locks will utilise water recycling basins to minimise fresh-water loss through the lock cycle. The navigable depth of the canal will also be increased to allow for deeper draft ships.

Geological Setting
Panama is located in a young and complex tectonic setting surrounded by four plate boundaries. The Isthmus of Panama is part of a volcanic arc that started developing in the Cretaceous period, with deformation and faulting shaping the landforms through to present time. Bedrock includes intrusive and extrusive volcanic rocks, pyroclastic rocks and sedimentary rocks. The northern canal region is composed of Miocene to Holocene sedimentary sequences deposited on eroded pre-Tertiary volcanic rocks. The southern canal region is composed of Miocene-age sedimentary and volcanic rocks covered by residuum overburden or fill from recent excavations of the canal. Numerous faults and shear zones bisect much of the Panama Canal area, although active seismicity is surprisingly low.

Over 11km of new navigation channels will be cut through the soil and rock as part of the project. A portion of the new channels transect relatively steep hills with hard basalt cores and clay overburden. The variability of the geology is a wonder for geologists and a headache for the geotechnical engineers that will have to design structures through it.

Geophysical Survey
In 2006 and 2007, Technos carried out a geophysical investigation along critical portions of the expansion route. Both land and marine seismic data were acquired. In this article, we discuss the marine portion of the project, which includes seismic reflection data acquired within Gatun and Miraflores Lakes.
Marine seismic reflection (sub-bottom profiling) is a method that uses acoustic energy to penetrate the water bottom and reflect back from geological strata. Reflections may occur at stratigraphic interfaces (changes in porosity and/or lithology), as well as at structural features such as faults and fractures. The reflected signals produce a continuous cross-sectional image of subsurface conditions.

Survey Design
The marine survey was carried out in two phases: a reconnaissance survey using a single-channel boomer system and a follow-up survey using a multi-channel airgun system. In the reconnaissance phase, boomer data were acquired along broadly spaced survey lines to identify possible faults and anomalous features within Gatun and Miraflores Lakes. These features were then re-visited with a multi-channel airgun system run along closely spaced parallel lines to characterise their strike and spatial continuity. Dual-frequency bathymetric data were obtained simultaneously with the sub-bottom data to map the highly variable bottom conditions that ranged in depth from 2 to 33 metres.

Equipment
The ACP provided a modern survey vessel equipped with a differential GPS navigation system and an experienced boat crew. An Applied Acoustics AA-300 boomer system was used as the reconnaissance seismic source with an energy level of 200–350 Joules. An 8-element single-channel hydrophone was towed behind the survey vessel to receive the acoustic signals. The shot (ping) rate was set at 0.5 seconds, which produced a sample at approximately 0.4-metre intervals.

A multi-channel system with an airgun source was chosen to provide deeper data over the faults and anomalies identified in the reconnaissance phase. A Bolt Technology airgun with 1 and 5in3chambers pressurised to 2,000psi was fired every 3 seconds, which produced a sample at approximately 2.5-metre intervals. A 24-channel hydrophone streamer with 3-metre spacing was used to provide a higher signal-to-noise ratio compared with a single-channel system. The data were digitally stored on a Geometrics StrataVisor NZ seismograph.

The seismic data were post-processed using a series of steps to transform the data from raw records to interpreted cross-sections. Multi-channel data were processed using a common mid-point (CMP) approach. Predictive deconvolution, Stolt migration, band-pass filtering and gain corrections were used to improve the quality of the data.

Data Quality
The seismic data quality varied considerably throughout the survey area. Excellent quality was achieved to depths of approximately 100 metres with the boomer system and over 250 metres with the airgun system in the majority of the survey areas. The boomer data provide a high-resolution look at near-bottom features including paleochannels, possible faults, grabens and other geological structures. The multi-channel data acquired along parallel survey lines show how these features extend both spatially and with depth.

In some portions of the survey area, bottom conditions degraded the quality of the data. In the main canal channel, the hard bottom produced high-amplitude multiple reflections that mask deeper data. In shallow (<3 metres) areas, ringy data was problematic. Post-processing tools such as predictive deconvolution helped to attenuate multiples and resolve some of the effects of ringing.

Results
The processed seismic reflection data were reviewed for stratigraphic trends and lateral changes indicative of structural features such as faults and intrusive rock. Where possible, boring data were used to constrain the interpretation. Interpreted grabens in the data suggest that extension and normal faulting has greatly influenced the area in the past. Inferred faults were interpreted from offset strata identified in the data that appear to correlate with regional structural trends identified in an ongoing fault-mapping study. Many of the inferred faults can be traced to within 5 metres of the bottom sediments, providing an indication of recent seismic activity. The anomalous areas will be further investigated in future studies and will aid in the regional assessment of faults through the project area.

The Third Set of Locks Project is expected to be complete and open for traffic in 2015. The first land work, the dry excavation of a wide trench connecting Culebra Cut with the Pacific Coast, began in September 2007.