35°44'

35°40'

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ESTRELLA RIVER

ure

Paso Robles

S Signature

Paso Robles

NW Signature

Paso Robles

NE Signature

Shandon

Signature

Kettleman Hills

Signature

Atascadero

Signature

Coherent 

regions

Coherent 

regions

RANGE DISPLACEMENT

One fringe, or one color cycle, represents 1.1 inches (actually 28

millimeters) of vertical displacement. Areas where the radar

signals were poorly correlated

are transparent.

IN

C

REASING RELATIVE SUBSIDEN

C

E

Point Sal

Morro

Bay

Figure 3. InSAR imagery of full scene covering most of San Luis Obispo County with focus area outlined (area outlined on figure 1).

0 10 20 30 40 MILES

0 10 20 30 40 KILOMETERS

120°30' 120°00'

120°30' 120°00'

35°30'

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Red Hills

Signature

FOCUS AREA

David W. Valentine, Jill N. Densmore, Devin L. Galloway, and Falk Amelung. Use of InSAR to Identify Land-Surface Displacements

Caused by Aquifer-System Compaction in the Paso Robles Area, San Luis Obispo County, California, March to August 1997

Open File Report 00-447

ESTRELLA RIVER

120°44' 120°40' 120°36' 120°32' 120°28'

35°44'

35°40'

35°44'

35°40'

101

San Miguel

SALINAS RIVER

HOG CANYON

-20

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-60

0

0

-10

-10

NW

Signature

NE

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R scenes imaged on succeeding days. Subtraction 

hange" interferogram that contains range 

ellites ERS-1 and ERS-2 were used. Five-month, 

ues described by Zebker and others (1994a), 

s (1998). Each of the four interferograms (3861 

ignatures. Only the 5-month interferogram 

 signatures that warranted further examination. 

ssed from SAR scenes imaged on December 29 

and 30, 1995. The lack of coherent phase signatures in the 7-, 15-, and 20-month interferograms is due to atmospheric effects on 

the radar signal or temporal decorrelation of the interferograms. Areas where it was not possible to correct for tandem effects are 

excluded from the differential interferograms, and instead, the gray-scale image of the radar amplitude is shown. 

 

 Ground-Water Levels

 Water-level data for 58 wells for both the spring and fall of 1997 in 40 public land survey sections were obtained from San 

Luis Obispo County Flood Control and Water Conservation District (unpublished data, 1999). Although the state well number 

was provided for the wells, the exact well locations are not known. Therefore, all wells in a section were assigned an 

approximate position that corresponds with the center of the section, and thus well locations may be off by as much as 0.7 mile. 

Because the data are sparse, a triangular-irregular mesh was used to construct the contour maps of spring 1997 water-surface 

elevations (Figure 2) and the seasonal ground-water level change (Figure 4). The seasonal ground-water level change was 

calculated by subtracting fall 1997 water levels from spring 1997 water levels.

 Results

 The interferogram for March 28-August 15, 1997, shows four phase signatures in two separate locations within the area of 

primary focus for this study (Figure 4). Three of these signatures are located northeast of Paso Robles and the other is northeast 

of Atascadero (Figure 4). Other coherent phase signals, outside of the area of primary focus, are also apparent on this 

interferogram. 

 Paso Robles

 The interferogram shows three phase signatures about 3 miles northeast of Paso Robles (Figure 4). These three phase 

signatures lie along the trend of an unmapped syncline with the thicknesses of Paso Robles formation sediments exceeding 

4,000 feet (Dibblee, USGS, oral commun., 1999). The southern signature shows a relative change of about two phase signatures 

(2.1 inches or 56 millimeters) of increasing range distance (downward land-surface displacement) in an approximately 75-

square-mile area. The maximum downward displacement in the northwest signature is about 0.8 inches (20 millimeters) and the 

northeast signature is 0.6 inches (about 14 millimeters). The southern and northeastern phase signatures coincide with an area of 

seasonal water-level decline of about 60 feet (Figure 4); downward displacement in these areas may be related to water-level 

declines. The northwestern phase signature also appears to coincide with a depression in the spring 1997 water-surface elevation 

(Figure 2). The southwestern boundary of the southern phase signature is subparallel to an extension of the La Panza Fault 

(Dibblee, 1974) and appears to be bounded by the fault. The northwestern and northeastern phase signatures do not appear to be 

related to any mapped geological structures, but their separation from the southern phase signature may suggest the presence of 

a ground-water boundary or barrier (Figure 4). The separation of the northeast and northwest phase signatures appear to 

coincide with the northeast trending ground-water contours (just north of Hog Canyon in Figure 2), which may indicate the 

presence of a ground-water boundary or barrier. It is also possible that the concentration of pumping in the areas of these three 

phase signatures and the subsequent water-level declines has caused localized ground-surface displacement and that these are 

not barriers or subbasin boundaries. 

 Atascadero area

 The interferogram (Figure 4) shows an areally small phase signature in the Atascadero area east of Highway 101 and the 

Salinas River. The eastern edge of the signature is bordered by the San Marcos and the Rinconada faults. The phase signature 

appears to be controlled, in part, by the ground-water barriers formed by the faults and by the geometry of the basins adjacent to 

the faults. The phase signature shows about 1 to 2 inches (28 to 56 millimeters) of downward ground displacement, which 

coincides with the seasonal water-level declines between spring and fall 1997 of about 54 feet (Figure 4). 

 Other regions

 In addition to the phase signatures identified in the area of primary focus, coherent regions also were identified in seven 

additional regions of the interferogram (Figure 3). In the Kettleman Hills area, as much as 4 inches (110 millimeters) of landsurface displacement was identified in two oil fields using InSAR (Fielding and others, 1998). This displacement probably is 

related to withdrawal of oil from the area and not to the withdrawal of ground water. In the Shandon and Red Hills areas, as 

much 2 inches (56 millimeters) of displacement was identified, which is apparently related to pumping for agricultural use. 

Coherent phase signals also were identified in the Morro Bay, Arroyo Grande/Pismo Beach/Nipomo, Santa Maria Valley area, 

and Point Sal areas.

 Summary and Conclusions

 During the 1990’s, the population of San Luis Obispo County has grown steadily and surface-water supplies have been 

insufficient to meet the growing demand for water. Ground-water pumpage has increased to meet this shortfall, resulting in 

seasonal water-level declines and concern that the water resources of the area may become overstressed. One particular concern 

is whether the larger ground-water basins within the county function as large individual basins or whether subsurface structures 

divide these large basins into smaller subbasins. Interferometric Synthetic Aperture Radar was tested for use as a tool to aid in 

locating subsurface structures in ground-water basins by determining land-surface changes. Owing to the high spatial detail of 

InSAR imagery, the InSAR-derived displacement maps can be used with ground-water-level data to reveal differential aquifersystem compaction related to the presence of geological structures or the distribution of compressible sediment that may define 

subbasins.

 The area of this study includes most of San Luis Obispo County, California, which is located about 160 miles northwest of 

Los Angeles, California. The primary focus of this study is a 400-square-mile area near Paso Robles. This area was selected for 

more intense investigation because a ground-water study for the area has been proposed that would benefit from better definition 

of the extent and continuity of the ground-water basins. The main water-bearing units in the Paso Robles area are Quaternary 

younger and older alluvium and Quaternary/Tertiary continental sediments.

 InSAR is a means of remotely mapping land-surface displacements using paired synthetic-aperture radar images taken from 

earth-orbiting satellites. These images are used to create an interferogram that shows the change in the range between the land 

surface and the radar antenna on the order of millimeters for the paired images. The differences between two signals can be 

determined by observing the phase signatures of the radar echoes. These differences are represented graphically by a color 

fringe, which represents about 1.1 inches (28 millimeters) of range displacement. For this study, raw SAR images taken on 

March 28 and August 15, 1997, were used to create an interferogram. Water levels from 58 wells with both spring and fall 1997 

measurements were used to construct water-level contour map for spring of 1997 and a seasonal water-level-change map for 

spring to fall 1997.

 The interferogram showed three phase signatures about 3 miles northeast of Paso Robles, which indicated ground-surface 

displacements of from 0.6 to 2.1 inches (14 to 58 millimeters); the southern and northeast phase signatures coincide with an area 

of water-level decline of over 60 feet. There appears to be a ground-water barrier between the southern signature and the 

northeast and northwest signatures and also between the northeast and northwest signatures. These may not be actual barriers 

but more related to concentrated ground-water pumpage in the area of these signatures. The interferogram also shows an areally 

small phase signature in the Atascadero area that is bounded on the east by previously mapped faults and coincides with an area 

of seasonal ground-water level change of about 54 feet. The ground deformation in this area is on the order of 1 to 2 inches. In 

addition to those phase signatures in the area of primary focus, seven additional coherent phase signals were identified in other