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Arthur E. Berman

 

On August 29, 2005, Hurricane Katrina devastated coastal areas of Louisiana and Mississippi. New Orleans may be lost.  In that city, levees were breached and pumps failed. Eighty percent of the city is flooded with up to 20 feet of water this morning. 

 

Bad policies by politicians and planners contributed to the city’s vulnerability.  Geologists and coastal scientists have been arguing for decades, unable to agree on the causes or rates of subsidence.   Officials and the public understandably felt justified in making no decisions or temporary decisions because experts could not reach agreement. The scientific community must now accept some responsibility for choosing debate and inaction over collaboration and consensus for the public good in Louisiana. 

 

An editorial in the New York Times stated, “At the same time, there must also be an honest recognition of the fact that no amount of engineering - levees, sea walls, pumping systems, satellite tracking systems - can fully bring nature to heel” (New York Times, 2005).

 

Katrina will be called a natural disaster but subsidence is the root cause of the problem. An event like this was anticipated for decades by geologists and coastal experts because of ongoing subsidence in the northern coastal region of the Gulf of Mexico.  If it had not been Katrina, it would have been some other hurricane; if it had not been this year, it would have been another, probably in our generation. 

 

The latest chapter of the continuing debate over subsidence in coastal Louisiana and Texas was triggered by publication of NOAA TECHNICAL REPORT NOS/NGS 50 (Technical Report 50) a year ago.  The report warned that subsidence rates in southern Louisiana were far greater than many workers believed and that swift height modernization was imperative.   

 

Scientists are clearly divided into camps that support different views of the cause for subsidence.  Some believe that ground water withdrawal is the principal cause for subsidence. Others blame oil and gas extraction and many blame the reclamation and restoration efforts of the Army Corps of Engineers.  Technical Report 50 suggests that none of these causes fully account for subsidence rates in southern Louisiana and that natural, geological processes must be considered.

 

The great American geologist Thomas Crowder Chamberlin gave an address to the Society of Western Naturalists in 1889 about the unfortunate tendency of scientists to promote their ideas and theories often to the exclusion of an integrated and balanced perspective of all possibilities.  There is, I believe, a tragic lesson in the debate over subsidence in the Gulf Coast of Louisiana and Texas that Chamberlin described precisely.

 

 “Our desire,” he said, “to reach an interpretation or explanation commonly leads us to a tentative interpretation that is based on relatively hasty examination of a single example or case. Our tentative explanation, as such, is not a threat to objectivity, but if we then begin to trust it without further testing, we can be blinded to other possibilities that we ignored at first glance. Our premature explanation can become a tentative theory and then a ruling theory, and our research becomes focused on proving that ruling theory. The result is a blindness to evidence that disproves the ruling theory or supports an alternate explanation” (Chamberlin, 1890).

 

The silent disaster that Technical Report 50 co-author Roy Dokka has been describing found its voice this week in Hurricane Katrina.

 

In July 2004, NOAA TECHNICAL REPORT NOS/NGS 50 (Technical Report 50) was published by the National Oceanographic and Atmospheric Administration (NOAA)’s National Ocean Service (NOS) and National Geodetic Survey (NGS).  The subtitle of the report aptly abstracts its content:  rates of vertical displacement at benchmarks in the Lower Mississippi Valley and the Northern Gulf Coast.  Technical Report 50 concludes that rates of subsidence in southern Louisiana are significantly higher than previously thought.  The report’s authors, Kurt Shinkle, National Geodetic Survey, and Roy Dokka, Louisiana State University, estimate that southern Louisiana rates of subsidence are between 200 and 5000% greater than previous estimates.  The mean subsidence rate for southern Louisiana, they say, is 11mm (0.43 inches) per year (Berman, 2005). 

 

The report has ignited a debate of surprising intensity considering its conventional method of analysis and its geologically unremarkable inference that the Gulf of Mexico basin is subsiding at rates greater than can be explained by human efforts to extract fluids from the subsurface.  Dr. Roy Dokka, the report’s second author, has been attacked both for the rates of subsidence cited in Technical Report 50 and for his belief that much of the subsidence is due to natural, geological causes. 

 

“Subsidence is much more widespread and much faster than previously thought,” Dokka says. While the rates (in Technical Report 50) are specific to the past 100 years and may not reflect the current rate of subsidence, Dokka says,  “you can’t extrapolate these rates without looking at the future,” and he “absolutely” thinks the rates are natural and will continue (Sever, 2005).  Natural causes, according to Dokka, include tectonic and depositional processes such as crustal down-warping, sediment loading, compaction, salt movement and gravity slumping, as well as eustatic sea-level rise.

 

Bob Morton, a geologist at the USGS Center for Coastal and Watershed Studies (CCWS), is Dokka’s most vocal critic.  Morton believes that most, if not all, of the subsidence and accompanying land loss in southern Louisiana is due to oil and gas production.  In a recent interview with TexasMonthly , Morton stated that Dokka’s research was “scientifically questionable” and suggested that the LSU scientist may be more interested in grabbing headlines than in scholarly pursuit (Cartwright, 2005).

 

“Terms like sediment loading and gravity sliding made perfect sense millions of years ago but they don’t necessarily apply today,” Morton says. “What Dokka doesn’t tell you is that his data is recalculated from data that is at least ten years old. Maybe it applies today and for the next 100 years and maybe it doesn’t. Withdrawing fluids from the subsurface produces the same results as sediment loading---but it’s induced, not natural” (Cartwright, 2005).

 

Kristy Milliken, a graduate student at Rice University and her advisor, Dr. John Anderson, believe that Holocene subsidence rates in southern Louisiana are much lower than rates published in Technical Report 50.  “It is difficult to reconcile those subsidence rates”, said Anderson (Sever, 2005).  Anderson told me that the work he and his students have done along the Texas and Louisiana coasts indicate about 1 mm per year of subsidence based on radiocarbon marker dates (personal communication). 

 

Jeff Williams, a USGS worker at the Woods Hole Oceanographic Institute, questioned the scientific credibility of interpretations in Technical Report 50.  “This report, and the conclusions drawn from it need to be based on the best interpretations of the data available and I’m not confident that they are.” Part of the problem, Williams says, is that the NGS raw elevation data were not published in the report and are not publicly available for peer review (Sever, 2005).

 

 

Subsidence is the downward displacement of the Earth’s surface relative to a fixed datum.  The datum used in Technical Report 50 is the North American Vertical Datum of 1988 (NAVD 88).  The methods used in Report 50 involved a fundamental geodetic analysis. 

 

Technical Report 50 integrated existing first-order leveling (surveying) data, Global Positioning System (GPS) observations and tide gauge information gathered by the National Geodetic Survey (NGS) over the past 70 years to re-calculate the elevations of more than 2700 benchmarks in southern Louisiana. 

 

The resulting analysis indicates that subsidence has occurred during the past century and is probably still going on throughout the lower Mississippi Valley and adjoining coastal plain.  Subsidence is greatest in coastal regions, especially in the Mississippi River delta plain where displacements of up to -30mm/year were computed. Significant subsidence is occurring across much of southern Louisiana at a mean rate of -11mm per year.

 

The report concludes that a comprehensive program of height modernization is needed. This would involve a new, first-order differential leveling campaign covering at least the major branches of the subsidence network established in Technical Report 50. Differential leveling would have to be complemented by GPS observations in order to maintain a long-term vertical reference system for the region.

 

When I spoke to Roy Dokka about why Technical Report 50 has caused so much controversy, he said, “Report 50 is fundamentally a geodetic report.  It is not a strongly interpretive document and, in fact, only a very general background on the causes of subsidence is laid out. The geodetic method used in Report 50 is as good as it gets.

 

“It is apparent from people’s reaction that they see the implications of these rates (of subsidence).  People see the numbers and that scares them.  It will call into question a huge body of science that was thought to be settled for some time.  An anomaly has been noted.”

 

When I questioned him about lower subsidence rates cited by workers at Rice University, Dokka replied, “Why would you prefer methods that average events of the last several thousand years over fundamental geodetic observations made over the past few decades? 

 

“On the other hand,” he added, “It depends on what kind of question you ask.  If you want to understand how geological systems work, you have to look at more than the last few hundred years.  You have to study the Earth at spatial and temporal scales appropriate to your question.  If the question is, How is the Earth going to behave with respect to subsidence in the next 50 years in southern Louisiana?, then it is much more predictive to look at geodetic data rather than studying events of the past few thousand years.”

 

I asked Dave Zilkoski, Deputy Director of the NGS, about the report and its reliability.  “It is a sound report.  No one disagrees with the heights (elevations).  These are the best rates available,” said Zilkoski. Referring to the use of existing leveling data, he said, “If you had up-to-date data, you would have better information.  The report represents the best science we can do without re-leveling.”

 

When I asked about the charges of “questionable science”, Zilkoski replied, “At the NGS we are geodesists.  We can tell you about the vertical movement of monuments (benchmarks).  We don’t deal with the “why” of it.  We leave that to geologists.  I think (Bob) Morton is concerned with the interpretation of the rates.  There is nothing in Report 50 that discusses the causes of rates.  I think it has been Roy (Dokka)’s discussion of geological causes beyond the report that some are objecting to.” 

 

When I questioned him about allegations that Technical Report 50 was merely a re-calculation of 10-year old data, Zilkoski responded, “Data from the 1930s to 1995 was used and results are consistent with earlier work that was done.  Previous work was accurate in terms of relative movement but the absolute values were off.  That’s what this report corrects.  We need to validate the new elevations with leveling and GPS data.  I think what Morton meant was that we didn’t carry elevations to all stations.”

 

He went on to clarify that Technical Report 50 is an official document of NOAA, NOS  and NGS and is fully supported by those agencies.  All data in the report is owned by NGS, is public, and results went through multiple peer reviews prior to publication.  The part that Zilkoski says is open for discussion is the interpretation of the data and the cause of the subsidence rates.

 

Regarding Jeff Williams’ comments to Geotimes , Zilkoski said, “What Williams is quoted as saying is incorrect.  The data and results are publicly available.  Most of it is on our (NOAA/NGS) Website.  It is stated clearly that all you have to do to get more data is to request it.”

 

 

Most man-made subsidence results from ground water withdrawal but the earliest observation of subsidence resulting from human activity was from oil and gas field production.  The Houston, Texas area has perhaps the best examples in the world of subsidence that results from both ground water and petroleum withdrawal.

 

The first documented instance of land subsidence due to fluid withdrawal was from the Goose Creek oil field near the city of Houston.  In 1917 oil was discovered on the margin of Galveston Bay near the mouth of the present-day Houston Ship Channel.  After production of several million barrels of oil, bay waters began to inundate the oil field. (Figure 1).  Pratt and Johnson (1926) recognized newly formed faults and fissures that resulted from fluid withdrawal (Figure 2).

 

The Houston area has experienced the greatest and best-documented ground water-related subsidence in the United States.  Because the relatively shallow Evangeline and Chicot aquifers are highly productive and predictable, most of Houston’s early water needs were met by drilling water wells.  As much as 6 feet of subsidence occurred in the vicinity of the Houston Ship Channel by the mid-1970s (Figure 3).  



By 1979, the Houston Ship Channel area had subsided as much as 10 feet and over 3200 square miles of the Houston metropolitan area had sunk an average of one foot (Galloway et al, 1999).  Most of Houston’s subsidence is due to compaction of subsurface clays because of withdrawal of ground water from surrounding aquifer beds (Zilkoski et al, 2001).

 

 

Hurricane Katrina has silenced the debate in Louisiana, at least for the moment.  What about Texas?  Are coastal regions of Texas also at heightened risk of flooding because of subsidence?  Are all of Texas’ subsidence issues the result of human activity or is there a geological component that should be considered?

 

The Harris-Galveston Subsidence District (HGSD) was established in 1975 to more accurately monitor and to “end subsidence” in the Houston metropolitan area. Due largely to the efforts of the HGSD, the Houston metropolitan area is converting from ground water to surface water use, principally from Lakes Houston, Livingston and Conroe (Figure 4).    Changes in ground water pumping have resulted in impressive reduction or elimination of subsidence in many areas of Houston, though others remain problematic.  For 2004, total groundwater withdrawal in the HGSD was 245 million gallons per day (5.8 million barrels of water per day) which accounted for 27% of total district water usage (HGSD Web site, http://www.subsidence.org ).

 

I met with Ron Neighbors, General Manager of the HGSD, Tom Michel, Assistant General Manager and Cliff Middleton, NGS Geodetic Advisor to the Subsidence District at their office near Webster in the Clear Lake area. 

 

Neighbors lead off the discussion by saying that he is not happy with Roy Dokka and his claim that Houston has inadequate elevation control.  “The HGSD knows more about heights than anyone else in the greater Houston area.  Roy Dokka is creating an unnecessary political problem. He is talking about subsidence that is caused by factors other than ground water withdrawal.  I don’t doubt that natural geological compaction is a factor, but the Subsidence District is specifically charged with limiting subsidence due to ground water and we have pretty much done that in many areas.  There is still major subsidence in the North and West (north Harris and Fort Bend counties) where they are just beginning to meet the HGSD’s requirements to convert from ground water to surface water.”

 

The most reliable way to update and calibrate the elevation values of benchmarks is to use surveying crews to carry a known elevation from a stable monument outside the region of recognized subsidence relative to the North American Vertical Datum of 1988 (NAVD 88).

 

“Re-levelings conducted as recently as 1987,” Neighbors said, “cost at least a million dollars.”

 

There are over 2500 benchmarks in the Houston metropolitan area, many of which were tied by surveying (or differential leveling) and later adjusted to the NAVD 88 datum. Leveling yields an orthometric height, essentially an elevation relative to sea level (Figure 5).  Sealevel is a dynamic value that is related by geodesists to the Geoid, an equipotential surface of the Earth's gravity field, which mathematically best fits global mean sea level.  In a practical sense, this means tying the survey to a tide gauge.  In the Texas Gulf Coast tide gauges at Galveston Island or Corpus Christi are critical.

 

Because of the high cost of re-leveling, the United States Geological Survey and the Harris-Galveston Subsidence District have established a network of 13 mechanical subsidence measurement devices in Harris and Galveston counties called borehole extensometers designed to monitor subsidence without re-leveling. 

 

Deeply anchored benchmarks are placed in what are believed to be stable strata in boreholes drilled to depths that range from 770 to 3072 feet below the surface. Boreholes are drilled to the Burkeville Confining Unit below the compacting zones that surround the Evangeline and Chicot aquifers.

 

The borehole used in extensometer measurement is lined with flexible casing that can adjust to compacting strata.  An inner pipe is anchored to a concrete plug at the bottom of the borehole and connected to a recording device at the surface (Figure 6). The extensometer provides a continuous measurement of the difference between the elevation of the cement plug at the bottom of the borehole and the land surface surrounding the borehole.  Though less expensive than re-leveling, the cost of drilling and maintaining borehole extensometers—about $800,000 per unit--limits their use and distribution in the Houston area.

 

Mike Turco, Houston Office Chief, USGS Texas Water Science Center, directs the  efforts to understand ground water-related subsidence in the Houston area.  “We have 30 years of extensometer data that gives monthly rates of clay compaction around the Gulf Coast aquifers,” Turco said.

 

The limited distribution of borehole extensometer devices is, in part, remedied by use of the Global Positioning System (GPS) of satellites to measure and reference subsidence to certain extensometer sites.  Three anchored benchmarks record both extensometer measurements as well as GPS elevation data. These locations are referred to as Continuously Operating Reference Stations (CORS) (Figure 7). 

 

GPS measures the 3-dimensional position of a point relative to the center of the earth.  This position is then referenced to the ellipsoid, a mathematical best-fit model of the Earth’s surface (Figure 5), which allows a vertical component to be isolated, known as an ellipsoid height.  Here lies the problem in obtaining millimeter-scale elevations with GPS:  there are many models for calculating an ellipsoid and topographic elevations above or below a hypothetical ellipsoid are very small compared with the distance to the Earth’s center. Once an ellipsoidal height is determined, it must be further calculated relative to sea-level (NAVD 88) in order to be reconciled with orthometric leveling data.

 

I asked Dave Zilkoski about the vertical resolution of GPS considering the millimeter range of subsidence values obtained in Louisiana and assuming that similar or even greater resolution might be required in Texas to obtain reliable vertical displacements.

 

“To get sub-centimeter vertical resolution,” he said, “you must occupy a GPS station for a long time. A 24 hour solution only gives about 1-2 centimeter vertical resolution.  For now, the resolution is not as good as leveling but, at least in Louisiana where subsidence rates are high, it provides a framework.”

 

PAMs (Port-A-Measures) are trailer-mounted GPS devices that rotate among various reference stations at one week intervals including CORS stations.  Seven PAM trailers are used to occupy and monitor 28 sites around the Houston area (Figure 7).  Each PAM site has 3 vectors associated with it:  the Addicks CORS, the Northeast CORS and the Lake Houston CORS.  In addition to the HGSD CORS and PAM sites, there are other elevation stations owned by city, state and federal agencies like the City of Houston, Texas Department of Transportation, and by land surveyors and private industry.  In all, there are about 50 GPS stations in the Houston area.

 

When I pressed Zilkoski on the issue of vertical resolution, he said, “A single 24 hour solution does not give the millimeter per year resolution we need. The HGSD PAMs can provide more accurate solutions because they occupy each GPS station for a week at a time.” 

 

My opinion is that of the more than 2500 benchmarks in the Houston area, the only monuments that are reliable relative to NAVD 88 are the GPS stations.  There is adequate vertical elevation control for northwestern Harris County; the rest of Harris County has less height control. There is even less vertical elevation control for much of Fort Bend and Galveston counties and the HGSD is adding 28 additional PAM sites to improve its monitoring capability in these areas. 

 

 

The Gulf of Mexico basin (Figure 8) contains up to 15 km (~50,000 ft) of sedimentary rocks ranging in age from Late Triassic to Holocene.  Most of the reservoirs that produce oil and gas in the Gulf of Mexico basin were originally deposited at or near sea level as shallow marine, deltaic or coastal plain fluvial sand bodies.  These reservoirs now are encountered at drilling depths of up to 7,000 m (20,000 ft) or greater because of basin subsidence.

 

The Gulf of Mexico basin formed as the Pangaea Super Continent began to break apart beginning about 210 million years ago (Ma) in the late Triassic Period (Figure 9).  By the middle Jurassic Period (~160 Ma), salt was deposited across much of the basin.  Significant clastic deposition in the Gulf of Mexico basin in the late Paleocene Epoch (60 Ma) and continues to the present. 

 

The Gulf of Mexico is an extensional basin whose basement fabric is characterized by normal faults inherited from its rift origin (Figure 10).  Because salt separates faulted basement from overlying sedimentary fill, salt mobilization and evacuation processes interacted with fault-bounded basement blocks to produce a complex and dynamic basin architecture (Figure 11).  Listric-normal (“growth”) faults result from sediment loading above a detachment layer (Figure 12). 





 


In the Gulf of Mexico basin, this layer may be either salt or mobile shale. The principal force behind this process is gravity acting on an inclined depositional surface, similar to the force that drives glaciers (Ewing, 1991).

 

The Houston Embayment is part of the Gulf of Mexico basin (Figure 8) characterized by enhanced subsidence and accompanying sediment accumulation.  Normal faults that sole into a deep detachment surface, and salt withdrawal are the principal mechanisms by which the Houston Embayment subsides and accommodates sedimentary fill.

 

Marc Edwards is a consulting geologist and expert on Tertiary petroleum reservoirs in southeastern Texas. I asked Edwards about subsidence rates for the Texas coastal region. He referred to Wilcox and Frio reservoirs:

 

“I am mostly interested in where subsidence rates are unusually high,” said Edwards.  “In the coastal bend Frio, in the heart of growth fault country at the paleo-shelf margin, we have 4th order cycles that are 2,000' thick and 5th order cycles that are 450' thick.  Depending on the duration of various cycles you get rates of a few feet per century. Those are average rates for that time interval.

 

“Back up on the coastal plain, away from shelf margin growth faults, where there is load-induced subsidence reflecting significant sediment supply, I estimate rates at several hundred feet per 4th order cycle (approximately 100,000 years), which is inches per century” (Edwards, personal communication).

 

Extensive normal faults and salt movement characterize this subsidence both in the deep basin as well as on the surface.  Normal faults are recognized over a belt in the on- and offshore Gulf of Mexico that extends approximately 300 km from the basin margin toward its center and 1000 km along the strike of the basin flank (Figures 8 and 13).  


Salt deformation is observed over a zone approximately 400 km by 1200 km.  Any subsurface analysis that does not take these pervasive features into account is incomplete.

 

Faults have been observed and mapped from their surface expression throughout the Houston area (Figure 14).  




More than 150 historically active faults have been identified in the metropolitan area and fault movement has been calculated at rates of 0.2 to 0.8 inches (0.5-2.0 cm) per year (Shah and Lanning-Rush, 2005). Many surface faults can be traced to considerable depth in the subsurface (McClelland Engineers, 1966).  Faults in the Houston metropolitan area are not merely surface phenomena, but are part of the deep and complex overall geologic structure of the upper Texas Gulf Coast (Verbeek et al, 1979).

 

The Long Point Fault is one of Houston’s better-known normal faults because it passes through residential neighborhoods and has caused serious problems by offsetting foundations of homes and public buildings as well as streets (Figure 15). 


 


The Long Point has approximately 2 ft (0.6 m) of surface offset that has occurred over the past 5-15 years, based on cross-cutting relationships of the area’s streets that have been re-surfaced during that period.  This indicates subsidence rates on the downthrown side of the Long Point fault of at least 0.2 ft/year (4 cm/year). 

 

“Evidence from previous studies indicates that these faults are natural geologic features with histories of movement spanning tens of thousands to millions of years. Present-day scarps reflect only the most recent displacements of faults that were active long before the present land surface of the area was formed.” (Shah and Lanning-Rush, 2005)

 

 

The Harris-Galveston Subsidence District and the National Geodetic Survey have established a network of approximately 28 subsidence monitoring stations from which reasonably reliable elevations and vertical displacements can be obtained  (CORS and PAM sites shown in Figure 7).  Extensometer stations provide an approximation of compaction due to groundwater withdrawal, while CORS GPS and PAM stations provide an approximation of total subsidence.  Unfortunately, only the CORS GPS stations provide both data in the same location (Figure 16).  

 

Publicly available data from Houston’s network of GPS and extensometer sites, only permit direct comparison at the Addicks location.  Addicks data, by my analysis, suggests that 6 mm/year or 15% of total subsidence may be related to normal basin subsidence.

 

Estimates of movement by Shah and Lanning-Rush and on the Long Point Fault yield rates approximately equal to and up to 3.5 times greater than Addicks.  These subsidence rates are consistent with ranges reported in Technical Report 50 for Louisiana.

 

In 2004, the U.S. Geological Survey (USGS), in cooperation with the Harris-Galveston Subsidence District, interpreted newly acquired LiDAR (Light Detection and Ranging) data and updated the locations of principal faults (Figure 18). 




Fault interpretations have not been incorporated into publicly available maps of subsidence or water level changes in Texas aquifers by the HGSD. 


Figure 19 is typical of subsidence and other maps provided by the county and federal agencies responsible for subsidence in the Houston and Texas coastal areas.  This, like most maps provided by the HGSD, NGS and USGS, is contoured without regard for known surface expression of faults, not to mention the higher resolution possible through incorporation of subsurface well control and geophysical data.  

   

Mike Turco of the USGS commented, “It has never been the position of the USGS that fluid withdrawal is the only cause of subsidence.  There is a structural component to subsidence.  Petroleum-induced subsidence should be very localized.  Our charge, however, is to understand compaction at and around the extensometers.”

 

 

I spoke with Gilbert Mitchell, NGS Manager of Geodetic Programs, who coordinates the NOAA/NGS state technical advisor program, including Texas, and is NOAA’s height modernization grants manager.  I asked him about the state of vertical control in the Houston area and in Texas in general.

 

“We have sufficient information that we’re comfortable with heights, but Texas needs better, more accurate heights,” he said. “GPS is not quite the answer—yet.”

 

“The HGSD only monitors part of subsidence,” Mitchell said.  “Coverage is limited and much data has not been published.”

 

I asked him if the current CORS and PAM sites were sufficient for floodplain management and surveying needs in Houston.

 

“It’s not a pretty scene,” he commented.  “It’s not good enough in our eyes for subsidence and floodplain mapping especially if want to know if you need flood insurance or not.  We’re on the edge of 2-3 cm of accuracy. Obviously, improvements can be made.”

 

When we talked about geological causes for subsidence, Mitchell said, “We’re not into the cause.  You geologists can have that part of it. It’s not our thing.  We’re worried about heights, whatever causes them.  We just measure them.”

 

Oil and Gas Production As a Factor in Gulf Coast Subsidence: 

I mentioned at the beginning of this article that Bob Morton of the USGS is perhaps the chief critic of Technical Report 50. I requested an interview with Morton but he replied that everything he has to say about subsidence in the Gulf of Mexico could be found in Open-File Report 2005-1216 (OFR 2005-1216).

 

OFR 2005-1216 examines 5 areas located in distal portions of the Mississippi River Delta that have experienced considerable land loss from subsidence over the past 40 years. The report initially states that “… the rapid subsidence and associated wetland loss were largely induced by extraction of hydrocarbons and associated formation water with some subsidence controlled locally by sulfur mining at a few sites” (Morton et al, 2005, p. 1). 

 

In the body of the report, however, the authors admit that at only one of the locations studied can subsidence be possibly related to oil and gas production.  No substantiation is presented other than geographic coincidence of land loss and petroleum production.

 

Morton and his co-authors have not clearly supported their position that oil and gas production is an important factor in the subsidence debate except perhaps in very local situations. I am frankly perplexed why OFR 2005-1216 was even published since it fails to support its own stated conclusions (In fairness, similar conclusions were reached in Morton’s “Causes of hotspot wetland loss in the Mississippi delta plain” that went through a peer review process.  Apparently even peer reviews find justification for publishing what I understand as unsupported claims (Morton et al, 2003)).

 

Thomas Kuhn explains in The Structure of Scientific Revolutions that science is a pursuit that is seldom directed toward discovery of anomalies and, if fact, tends at first to suppress them.  “Scientific research”, he wrote, is "a strenuous and devoted attempt to force nature into the conceptual boxes supplied by professional education" (Kuhn, 1987).

 

Technical Report 50 has revealed and documented an anomaly, namely that subsidence rates in southern Louisiana are higher than previously believed and that a certain component of that subsidence may be due to normal geological factors. Some in the scientific community have denied or discounted the anomaly by disparaging the report, its methods and its authors.  This, according to Kuhn, is predictable.  It does not, however, diminish the anomaly. 

 

In Texas, many workers say that a similar investigation is unnecessary because Houston has the most advanced technology for monitoring subsidence anywhere in the world.

 

I began this article recalling T. C. Chamberlin’s advice to scientists not to be become attached to simple explanations for phenomena in nature.  The application of his meaning in the current debate is this:  workers made an early and valid correlation between high rates of ground water withdrawal and land subsidence.  The acceptance of this explanation was so complete that it was elevated to ruling theory.  Agencies responsible for subsidence monitoring in the Houston area either do not have geologists on staff or do not have geologists with experience in subsurface basin-scale geology.

 

It is time to return to underlying causes and to abandon defense of previous efforts and explanations.  In Texas, it is time to move beyond the accomplishments of ground water subsidence mitigation and the application of technology to subsidence monitoring. It is time to admitthat there is more to the story than ground water.

 

It is time for all of the agencies involved to take collective responsibility for total subsidence.  I believe this should be a federal responsibility and that perhaps the Department of the Interior should mandate attention to total subsidence and demand collaboration among agencies.

 

The Gulf of Mexico basin is subsiding.  That’s what basins do.  This basin was subsiding long before man appeared on the planet.  Let’s get past acceptance of what is geologically undeniable. Let’s work together to find the resources for the first order leveling and expanded GPS network that is needed so we can plan for the future.

 

The Houston Geological Society and the Engineering, Science and Technology Council of Houston are sponsoring a conference November 3-5 in Houston called "Coastal Subsidence, Sea Level and the Future of the Gulf Coast".  I will be there.  I hope many of you will be there also.

 

 

 

 

Berman, A. E., 2005, Anatomy of a Silent Disaster:  Ongoing Subsidence and Inundation of the Northern Margin of the Gulf of Mexico:  Houston Geological Society Bulletin, v. 47, no. 6, p. 31-47.

 

Cartwright, G., 2005, That Sinking Feeling:  TexasMonthly 09.2005, vol. 33, issue 9, p. 158-168.

 

Chamberlin, T. C., 1890, The method of working hypotheses: Science, v. 15, p. 92-96.

 

Diegel, E. A. , D. C. Schuster, J. F. Karlo, R. C. Shoup and P. R. Tauvers, 1995, Cenozoic Structural Evolution and Tectono-Stratigraphic Framework of the Northern Gulf Coast Continental Margin in M.P.A. Jackson, D.G. Roberts, and S. Snelson, eds., Salt Tectonics: A Global Perspective:  AAPG Memoir 65, p.

 

Doyle, D. R., 2001, State plane coordinates and data transformations:  National Geodetic Survey Workshop Program, Austin, Texas.

 

Ewing, T. E. and R. F. Lopez, 1991, Plate 2. Principal Structural Features, Gulf of Mexico basin in A. Salvador, ed., The Geology of North America, Vol. J, The Gulf of Mexico Basin:  The Geological Society of America, accompanying slipcase.

 

Galloway, D.L., Jones, D.R., and Ingebritsen, S.E., 1999, Land Subsidence in the United States: U.S. Geological Survey Circular 1182.

 

Koop, W.J. and R. Stoneley, 1982, Subsidence history of the Middle East Zagros Basin, Permian to Recent:  Philosophical Transactions of the Royal Society, London, Series A, v. 305, p. 149–168.

 

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