Significant ground movement may accompany the extraction of large quantities of fluids from the subsurface. In Imperial Valley, California, one of the potential hazards of geothermal development is the threat of both subsidence and horizontal movement of the land surface. Regional and local survey nets are being monitored to detect and measure possible ground movement caused by future geothermal developments. Precise measurement of surface and subsurface changes will be required to differentiate man-induced changes from natural processes in this tectonically active region

Lofgren, B. E.

English

NASA Technical Reports Server

MEASURING GROUND MOVEMENT IN GEOTHERMAL AREAS 
O F  IMPERIAL VALLEY, CALIFORNIA 
Ben E. Lofgren 
U. S. Geological Survey 
Sacramento, California 
Significant ground movement may accompany the extraction of 
large quantities of fluids from the subsurface. In Imperial Valley, 
California, one of the potential hazards of geothermal development 
is  the threat of both subsidence and horizontal movement of the 
land surface. Regional and local survey nets a r e  being monitored 
to detect and measure possible ground movernent caused by future 
geothermal developments. Precise  measurement of surface and 
subsurface changes will  be required to differentiate man-induced 
changes from natural processes in this tectonically active region. 
I. INTRODUCTION 
As with extractions and injections in oil-field and ground-water 
reservoirs,  a direct relationship i s  expected in hot-water geothermal systems 
between pressure changes induced in the system and ground deformation that 
might result (Ref. I) .  
indicates that both vertical and horizontal components40f &&formation may occur 
a s  a result of either the compaction of surficial unconsolidated deposits o r  of 
deep-seated tectonic readjustments triggered by s t ress  changes, 
changes of s t r e s s  a t  depth sometimes cause serious surface movements. 
problems undoubtedly will  become more  commonplace and severe as man's 
development of natural resources progresses. 
Experience in many areas  of fluid-gressure change 
Even subtle 
Such 
Land subsidence, caused by the withdrawal of water, oil, and gas, has 
become common in the United States, affecting probably 10,000 mi2 (25, 900 
km2) of intensely developed land in five States (Ref. 2). Maximum subsidence 
is 29 feet (9 m) in the San Joaquin Valley and Long Beach areas  of California, 
13  feet (4 m) in the Santa Clara Valley of California, and is in excess of 8 feet 
(2.  4 m) in the outskirts of Houston, Texas (Ref. 3). Horizontal ground move- 
ment exceeds 12 feet (3. 7 m) in the Wilmington oil field of the Long Beach area,  
and, although largely unmeasured, it probably continues in numerous heavily 
pumped ground-water basins of the west. 
forming on the margins of numerous heavily pumped basins, principally in 
Arizona, and presently threatening land use in several areas ,  a r e  caused by 
significant horizontal shifting of the ground. 
the author to steep hydraulic gradients induced by pumping. 
Extensive earth fissures and cracks 
These features a r e  attributed by 
128 
https://ntrs.nasa.gov/search.jsp?R=19750012770 2020-03-22T23:18:17+00:00Z
One of the potential hazards of geothermal production in Imperial Valley, 
either for power generation or  for  water or  mineral  supply, is the threat of 
land subsidence and lateral  ground movement that might result. Where large 
quantities of geothermal water are extracted or  steep pressure gradients a r e  
induced in the hydrologic system, significant deformation of the land surface 
may occur. At Wairakei, New Zealand (Ref. 4), in one geothermal field, where 
large quantities of fluids have been produced, both horizontal and vertical ground 
movement has been measured. Subsidence affects more  than 25 mi2 (65 ?an2), 
and the maximum subsidence rate is about 1. 3 feet (0 .4  m) per year. 
subsidence exceeds 10  feet ( 3  m), and, significantly, the a rea  of maximum 
subsidence i s  outside the production field. 
"hot-water" geothermal fields. 
a production a rea  and the surrounding region a r e  needed to determine not only 
possible ground movement resulting from geothermal developments but also 
movement related to other geologic processes. 
Total 
Similar effects could occur in other 
Periodic resurveys of bench marks throughout 
Jmperial Valley is a "flat, I t  arid a rea  that is irrigated and intensively 
farmed with water from the Colorado River. 
sediment-filled structural trough on the border between the continental block 
of the western United States and the oceanic block of the eastern Pacific. 
number of major  and minor faults (Ref. 5, Fig. 3) ,  largely masked by the 
alluvial deposits, traverse the structural trough. 
most tectonically active a reas  of the country. 
It occupies part of a deep, 
A 
Currently this i s  one of the 
Thermal gradients a r e  unusually high throughout much of Imperial Valley; 
however, eight known areas  of anomalously high temperatures appear especially 
favorable for geothermal production (Fig. 1 l o  Several of these anomalies 
already have been test-drilled and have active hot-water (which flashes to 
steam) wells ready for production; others a r e  scheduled fo r  test drilling in the 
near future. Wherever deep wells have been tested, hot-water geothermal 
conditions have been observed. This suggests that a s  geothermal development 
progresses, not only will  large quantities of thermal waters be produced, but 
a l so  production wi l l  hav& an intimate effect on the hydrologic regime of the 
overlying ground-water reservoir. The possibilities of land subsidence, lateral  
deformation, and infringements on existing water rights, therefore, a r e  very 
real  in Imperial Valley. 
Figure 1 shows eight of the more  prominent geothermal anomalies 
reported by the U. S. Bureau of Reclamation in 1971 (Ref. 6). Of these, the 
Buttes anomaly abutting Salton Sea southwest of Niland (Fig. 1) and the Heber 
anomalies due south of El Centro have received the most  attention by industry, 
and have production wells awaiting pilot-testing. Also, near the center of the 
East Mesa anomaly 7 miles (11 km) southeast of Holtville, several deep test 
we l l s  by the Bureau of Reclamation have produced hot water and steam and are 
presently undergoing further testing. 
11. MONITORING PROGRAM 
A. Vertical Control 
Figure 1 shows the network of leveling established to measure possible 
vertical changes that might accompany geothermal development in Imperial 
Valley. The lines of f irst-order leveling running east, north, and west from 
El  Centro had been surveyed several times prior to 1971, an$ indicated con- 
siderable tectonic movement was occurring in this portion of the structural 
trough. Tectonism will  undoubtedly continue, and may increase, as geothermal 
production continues, 
During the winter of 1971-1972, the first-order lines of Fig. 1 were 
again leveled by the National Geodetic Survey, and, also, the second-order 
lines were established and surveyed by other agencies under direction of the 
National Geodetic Survey. 
from which subsequent elevation changes can be calculated. 
of 1973-1974, the first-  and second-order networks were again surveyed to 
verify measurements of the 1971 -1972 survey and detect changes that had 
occurred during the ensuing years. Figure 2 shows, in addition to the approxi- 
mate  location of the principal known faults traversing the region, the amount of 
vertical change along the first-order lines during the 2-year interval 1971 -1972 
to 1973-1974. In these computations, the bedrock tie west of El Centro was 
considered stable, and all other points in the net were considered floating. 
Interestingly, bench marks near Calexico and at  the bedrock tie east of El 
Centro showed little o r  no change during this 2-year period, 
subsided roughly 0. 6 inch (1. 5 cm), and a general northward tilt of about 5 
inches (13 cm) was measured in the 53 miles (85 h) from south to north in the 
valley. 
here; however, i t  is  quite apparent that the modest activities in the three 
geothermal anomalies being tested had little o r  no effect. Results of the 
second-order leveling a r e  not available for reporting here. 
State and County ordinance, bench marks a t  each geothermal production site 
will  be resurveyed periodically to reference ties of the first- and second-order 
nets to detect subsidence that m a y  accompany geothermal production. Figure 3 
shows the three locations where local level networks a r e  being monitored by 
developers to determine if changes a r e  occurring in a reas  of geothermal 
development, 
This 1971-1972 reference datum serves as a base 
During the winter 
El Centro 
No explanation for this apparent northward regional tilt is attempted 
As required by 
In order  to monitor possible elevation changes occurring on the southern 
margin of Salton Sea, related to either tectonic readjustments o r  to geothermal 
production, two continuous stage recorders were installed a t  strategic locations 
to correlate fluctuations of the sea with two other stage gages of long record on 
the west shore of the sea. One of these new stage recorders is near the 
southern tip of the sea (Fig. 3), the other within the geothermal anomaly 
southwest of Niland. Each of the stage gages is being tied into the valleywide 
network of vertical control. Differential elevation changes of l ess  than a 
centimeter around the southern margin of the sea should be detectable with 
these recorders. 
these recorders is in progress. 
Interpretation of the f i r s t  year of correlative records from 
130 
B. Horizontal Control 
Two types of horizontal control nets a r e  currently being monitored in 
Imperial Valley: f irst ,  a highly precise regional trilateration network spanning 
the structural trough to measure regional tectonic movement, and, second, 
local a r rays  of precise distance measurements in each of the a reas  of geo- 
thermal development to detect possible ground movement accompanying 
geothermal production. 
Figure 4 shows the regional network being monitored by the Geological 
Survey, using geodolite equipment capable of accuracies of 1 unit in l o 7  units 
of distance. 
lateral  horizontal tectonic movement is occurring in the Obsidian Buttes area 
(Fig. 4) southwest of Niland, along what may be an extension of the Brawley 
fault. Interestingly, this indicated movement i s  in the general a rea  where a 
number of recent geothermal test wel ls  have been drilled and tested. 
tectonic movement, however, predates the drilling of the wells. 
Late data suggest a s  much a s  0.02 inch ( 5  mm/year) of right- 
The 
Figure 5 shows the a r rays  of distance measurements being monitored in 
These the Buttes a rea  southwest of Niland to detect any horizontal movement. 
local nets a r e  being resurveyed by the Geological Survey using electronic 
distance-measuring equipment capable of accuracies of 2 units in l o 6  units of 
distance. 
lines can be detected. Because these distance measurements can be made so 
inexpensively, extra shots have been made in the event they a r e  needed for  
future reference, 
Mesa areas;  however, in these a reas  no elevated reference points a r e  available 
and long line-of- sight controls a r e  more  difficult. 
only across  geothermal a reas  where geothermal changes a r e  anticipated, but 
also across  structural zones where tectonic movement might occur. 
Distance changes of only a few millimeters along these controlled 
Similar arrays a r e  being monitored in the Geber and East 
Control lines extend not 
III. SURFACE AND SUBSURFACE INSTRUMENTATION 
Consider fo r  amoment  a geothermal area in which thermal fluids a r e  
extracted a t  depth a t  one location, and these and other fluids are injected nearby 
to minimize formation-pressure declines and subsidence (Fig. 6). Extractions 
cause a drop in formation pressures, compaction, and subsidence. Injections 
cause a pressure buildup, expansion, and rebound. These vertical changes, 
y1 and y2 in Fig. 6, a r e  significant in understanding the mechanics of the 
geothermal system. In addition, steep pressure gradients may develop between 
the two wells, and possibly horizontal ground movement a s  represented by the 
lower a r r o w s  in Fig. 6. To monitor possible horizontal movement, precise 
distance measurements, x and x2, x3 and x4 in Fig. 6, a r e  needed between the 
extraction and injection weils. Compression would be greatest near the extract- 
ing well, and horizontal tension would tend to develop near the injection well. 
This type of horizontal movement, with significant compression in the a rea  of 
fluid-pressure decline and tension around the perimeter, has been observed in 
numerous s t ressed subsurface reservoirs. 
If the land surface tilts, as shown in Fig. 6, the tilt should be a t  a maxi- 
We have reason to 
mum somewhere midway between the centers of extraction and the injection, 
and could be recorded by a tiltmeter a t  this midpoint. 
believe, however, that in many instances the land surface does not deform as a 
131 
stressed beam. Rather, shear seems to develop along near-vertical planes, 
with individual blocks between the shear planes moving vertically up o r  down 
(Fig. 6). 
surface deformation actually occurs. Also, a t  several locations where shallow 
ground water is being pumped in the vicinity of deep geothermal wells (see 
Fig. 6, right), extensometers a r e  being installed to differentiate deep compac- 
tion caused by deep geothermal extractions from relatively shallow compaction 
due to ground-water production. 
two processes, geothermal developments can be unjustly blamed for subsidence 
caused by ground- wa ter pumping. 
We a r e  installing tiltmeters a t  a few sites to determine what type of 
Without some method of differentiating these 
At East Mesa, 17 miles (27 km) east  of El Centro (Fig. l ) ,  where the 
U. S. Bureau of Reclamation is experimenting with deep extractions and injec- 
tions in the geothermal reservoir, tiltmeters and extensometers a r e  being 
installed on an experimental basis, a s  shown in Fig. 7. Two sensitive tilt- 
meters,  installed in 10-foot (3-m) pits to minimize thermal problems, are 
being positioned between the four extraction wells and the one injection well. 
Also, two mid-depth extensometers a r e  being positioned between the a rea  of 
geothermal development and nearby farmlands to monitor changes in water 
levels and compaction in the upper 1150 and 1400 feet (350 and 430 m), respec- 
tively, of alluvial deposits. Already in this area,  even before significant 
geothermal developments have begun, nurnerous complaints from nearby 
ranchers have charged that geothermal drilling has had adverse effects on their 
water wells. 
IV. CONCLUSIONS 
As in three other geothermal resource a reas  of the west, regional and 
local networks of vertical and horizontal surveys have been established in 
Imperial Valley to monitor ground movement that might accompany geothermal 
developments. 
vertical and horizontal changes that occur between surveys. Also, recording 
tiltmeters, extensometers, and stage gages on Salton Sea a r e  being maintained 
to detect changes as they occur. Various Federal, State, and local agencies 
a r e  involved in. this monitoring program. 
Periodic resurveys of these nets will  be the basis fo r  calculating 
Surface changes may be caused by geothermal extractions, fluid 
injections, induced hydraulic gradients, thermal changes, landslides, and 
tectonism. 
from changes due to other geologic processes is one of the principal challenges 
of this research project. 
required before geothermal effects can be determined and firm conclusions 
reached. 
To differentiate ground movement caused by geothermal extractions 
In most instances, several years of records will  be 
132 
REFERENCES 
1. Lofgren, B. E., "Monitoring Ground Movement in Geothermal Areas, I '  
Am. SOC. Civil Engineers, Hydraulic Division Specialty Conf. Proc., 
Zls t  Annual Meeting, pp. 437-447, 1973. 
2. Poland, J. F., and Davis, G. H., "Land Subsidence Due to Withdrawal 
of Fluids, I '  edited by D. J. Varnes, and G. Kiersch, Reviews in Engineer= 
ing Geology, Vol. 11, pp. 187-269, Geol. SOC. America, Boulder, Colo., 
1969. 
3. Poland, J. F., "Subsidence in United States Due to Ground-Water 
Overdraft-A Review, Proc. Am. Soc. Civil Engineers Specialty Confer- 
-' ence Ft. Collins, Colo. , Aug. 1973. 
4. Hatton, J. W., "Ground Subsidence of a Geothermal Field During Exploi- 
tation, 
Geothermal Resources, Pisa, -Italy, 1970. 
United Nations Symposium on the Development and Utilization of 
5. Dutcher, L. C., Hardt, W. F., and Moyle, W. R. Jr., Preliminary 
Appraisal of Ground W a t e r  in Storage with Reference to Geothermal 
Resources in the Imperial Valley Area, California, U. S. Geological 
Survey Circular 649, 1972. 
6. "Geothermal Re source Inves tiga tions , Imperial Valley, California, 
Status Report, I '  U. S. Bureau of Reclamation, 1971. 
133 
Fig. 1. Location of geothermal anomalies and network of leveling, 
Imper id Valley, California 
ORIGINAL PAGE IS 
OF POOR QUALW 
134 
- ---... 
1st-order leveling Fault 
Dashed where approximate 
-6 
2 - y e a r  change, in centimetres lhere 
Fig. 2. Network of first-order vertical control and 
2-year change in elevation 
Stage gage on sea 
Obsidian buttes Fault 
Local monitoring 
A ---... 
Oashed whllie approximate; 
dotted rhare concealed 
Fig. 3. Geothermal a reas  of local vertical and 
* control monitoring 
. .  . . . . , e  
:, 
135 
horizontal 
Fault Probable extension o f  
Brarley Fault 
A 
Bench Mark 
Dashed where approximate 
dotted where concealed 
Fig. 4. Regional network of horizontal control 
136 ORIGINAL PAGE E3 
OF P m  QUALm 
I 2  1) I 3 E  
48 
I1  
J! 
'r 
I .  
I. E 
I 
- 
I 
Fig. 5. Network of horizontal control in Buttes area 
137 
GROUND MOVEMENT 
Fig.  6 .  Surface deformation in an extract-injection a rea  
, R . 1 6  E . , R . 1 7  E .  
-9 
* 
/ 
/ 
I 
I 
I 
1 
Fig. 7. 
a 
EXPLANAT I ON 
T . 1 6  S .  4 PRODUCTION WELL 
INJECTION WELL 
@ TILTMETER 
EXTENSOMETER 
I I HOR I ZONTAL CONTROL 
I *  
I 
2 MILES 
I 
1 
1 
0 1 . 2  3 K I L l E T R E S  
Instrumentation installed at  East  Mesa geothermal a rea  
1 3 8  


Measuring ground movement in geothermal areas of Imperial Valley, California

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server