Determination of Specific Yield and Water-Table Changes Using Temporal Microgravity Surveys Collected During the Second Injection, Storage, and Recovery Test

at Lancaster, Antelope Valley, California, November 1996 through April 1997

Prepared in cooperation with the Los Angeles County Department of Public Works and

the Antelope Valley-East Kern Water Agency

U.S. Geological Survey

Water-Resources Investigations

Report 03-4019
Determination of Specific Yield and Water-Table 

Changes Using Temporal Microgravity Surveys 

Collected During the Second Injection, Storage, and 

Recovery Test at Lancaster, Antelope Valley, 

California, November 1996 through April 1997

By James F. Howle, Steven P. Phillips, Roger P. Denlinger, and Loren F. Metzger

Prepared in cooperation with the

LOS ANGELES COUNTY DEPARTMENT OF PUBLIC WORKS and

ANTELOPE VALLEY–EAST KERN WATER AGENCY

Sacramento, California

2003

Water-Resources Investigations Report 03-4019

U.S. GEOLOGICAL SURVEY 7212-51
For additional information write to:

District Chief

U.S. Geological Survey

Placer Hall, Suite 2912

6000 J Street

Sacramento, California 95819-6129

http://ca.water.usgs.gov

Any use of trade, product, or firm names in this publication is for descriptive purposes only and does 

not imply endorsement by the U.S. Government.

U.S. DEPARTMENT OF THE INTERIOR

GALE A. NORTON, Secretary

U.S. GEOLOGICAL SURVEY

Charles G. Groat, Director
Contents iii

CONTENTS

Abstract ................................................................................................................................................................ 1

Introduction .......................................................................................................................................................... 1

Description of Study Area........................................................................................................................... 2

Description of Injection Wells .................................................................................................................... 6

Previous Microgravity Studies.................................................................................................................... 6

Microgravity Surveys.................................................................................................................................. 6

Determination of Specific Yield .......................................................................................................................... 13

Effect of Injection Mound on the Calculated Specific Yield ............................................................................... 14

Gravity-Derived Water-level Changes................................................................................................................. 18

Comparison of Gravity-Derived Accumulated Mass and Injected Volume ........................................................ 24

Discussion of General Applicability and Limitations of Temporal Microgravity Surveys ................................. 25

Summary and Conclusions................................................................................................................................... 25

References Cited .................................................................................................................................................. 27
iv Figures

FIGURES

Figure 1. Location of study area and generalized surficial geology of Antelope Valley, California................ 3

Figure 2. Map showing location of Quartz Hill bedrock reference station and land-surface elevations in

and near Lancaster, Antelope Valley, California............................................................................... 4

Figure 3. Generalized geologic section showing relation of lacustrine deposits to younger and older 

alluvium and aquifers, Antelope Valley, California .......................................................................... 5

Figure 4. Map showing location of gravity stations, monitoring wells, and injection wells and of 

south-to-north and west-to-east profiles, Lancaster, Antelope Valley, California ............................ 7

Figure 5. Lithologic log and well-construction diagram for injection well 7N/12W-27P2 in Lancaster, 

Antelope Valley, California............................................................................................................... 8

Figure 6. Lithologic log and well-construction diagram for injection well 7N/12W-27P3 in Lancaster, 

Antelope Valley, California............................................................................................................... 9

Figure 7. South-to-north profile showing simulated injection mound geometries and corresponding 

hydraulic conductivity, and the simulated two-dimensional gravity response to various injection 

mound geometries, and measured gravity changes at the near-completion-of-injection survey

at Lancaster, Antelope Valley, California.......................................................................................... 15

Figure 8. Model grid and simulated injection mound contours and areal extent of the gravity station 

network in Lancaster, Antelope Valley, California ........................................................................... 17

Figure 9. Graph showing relation among gravity change, specific yield, and water-level change assuming 

a slab geometry of infinite extent....................................................................................................... 19

Figure 10. Profile showing gravity-derived and measured water levels compared to simulated injection 

mounds for a range of hydraulic conductivities along the south-to-north profile, Lancaster, 

Antelope Valley, California............................................................................................................... 20

Figure 11. Profile showing gravity-derived and measured water levels compared to simulated injection 

mounds for a range of hydraulic conductivities along the west-to-east profile, Lancaster, 

Antelope Valley, California............................................................................................................... 21

Figure 12. Profile showing change in gravity at gravity stations along the west-to-east profile, Lancaster, 

Antelope Valley, California............................................................................................................... 22

Figure 13. Profile showing change in gravity at gravity stations along the south-to-north profile, Lancaster,

Antelope Valley, California............................................................................................................... 23

Figure 14. Graph showing accumulated mass at the pre-injection, mid-injection, and near-completion-ofinjection surveys, Lancaster, Antelope Valley, California ................................................................ 24
Tables vi

TABLES

Table 1. Data from pre-injection, mid-injection, and near-completion-of-injection surveys for gravity 

stations at Lancaster, Antelope Valley, California, November 1996 through April 1997................. 10

Table 2. Selected ground-water-level measurements for the pre-injection and near-completion-ofinjection surveys at Lancaster, Antelope Valley, California, November1996 through 

April 1997 .......................................................................................................................................... 16
vii Conversion Factors, Vertical Datum and Abbreviations

CONVERSION FACTORS, VERTICAL DATUM, AND ABBREVIATIONS

CONVERSION FACTORS

VERICAL DATUM

Sea level: In this report, "sea level" refers to the National Geodetic Vertical Datum of 1929 (NGVD of

1929)—a geodetic datum derived from a general adjustment of the first-order level nets of both the

United States and Canada, formerly called Sea Level Datum of 1929.

ABBREVIATIONS

g/cm3 gram per cubic centimeter

m meter

mGal milligal

mm millimeter

μGal microgal

μGal/m microgal per meter

AVEK Antelope Valley–East Kern Water Agency

InSAR Interferometric Synthetic Aperture Radar

LACDPW Los Angeles County Department of Public Works

SWP State Water Project

USGS U.S. Geological Survey

ABBREVIATED GRAVITY UNITS

Milligal (mGal) is defined as 10−3 centimeter per second squared and is equal to 3.281 × 10−5 feet per

second squared. A microgal (μGal) is defined as 10−6 centimeter per second squared and is equal to

3.281 × 10−8 feet per second squared. Gram per cubic centimeter is a measure of density.

Multiply By To obtain

acre-foot (acre-ft) 0.001233 cubic hectometer 

foot (ft) 0.3048 meter

foot per day (ft/d) 0.3048 meter per day

foot per mile (ft/mi) 0.1894 meter per kilometer

 foot squared (ft2) 0.09290 meter squared

gallon per minute (gal/min) 0.06309 liter per second

inch 2.54 centimeter

mile (mi) 1.609 kilometer

square mile (mi2) 259.0 hectare
Well-Numbering System vii

WELL-NUMBERING SYSTEM

Wells are identified and numbered according to their location in the rectangular system for the subdivision of public 

lands. Identification consists of the township number, north or south; the range number, east or west; and the section number. 

Each section is divided into sixteen 40-acre tracts lettered consecutively (except I and O), beginning with “A” in the northeast 

corner of the section and progressing in a sinusoidal manner to “R” in the southeast corner. Within the 40-acre tract, wells are 

sequentially numbered in the order they are inventoried. The final letter refers to the base line and meridian. In California, 

there are three base lines and meridians; Humbolt (H), Mount Diablo (M), and San Bernardino (S). All wells in the study area 

are referred to the San Bernardino base line and meridian (S). Well numbers consist of 15 characters and follow the format 

007N012W34B001S. In this report, well numbers are abbreviated and written 7N/12W-34B1. The following diagram shows 

how the number for well 7N/12W-34B1 is derived.

R13W R12W R11W

Meridian

San Bernardino

 T8N

 T7N

 T6N

RANGE

TOWNSHIP

 R1W R1E

27

R12W

T7N

Well-numbering diagram

6 5 4 3 2 1

7

18 13

8 9 10 11 12

17 16 15 14

19 24

30 25

31 36

20 21 22 23

29 28 26

32 33 34 35

SECTION 34

D C A

EFGH

M L K J

N P Q R

B

7N/12W-34B1
Introduction 1

Determination of Specific Yield and Water-Table 

Changes Using Temporal Microgravity Surveys Collected 

During the Second Injection, Storage, and Recovery Test 

at Lancaster, Antelope Valley, California, November 1996 

through April 1997

By James F. Howle, Steven P. Phillips, Roger P. Denlinger, and Loren F. Metzger

ABSTRACT

To evaluate the feasibility of artificially 

recharging the ground-water system in the 

Lancaster area of the Antelope Valley, California, 

the U.S. Geological Survey, in cooperation with 

the Los Angeles County Department of Public 

Works and the Antelope Valley–East Kern Water 

Agency, conducted a series of injection, storage, 

and recovery tests between September 1995 and 

September 1998. A key component of this study 

was to measure the response of the water table to 

injection, which was difficult because the water 

table averaged 300 feet below land surface. Rather 

than install many expensive piezometers, 

microgravity surveys were conducted to determine 

specific yield and to measure the development of a 

ground-water mound during the injection of about 

1,050 acre-feet of fresh water into an alluvialaquifer system. The surveys were done prior to, 

during, and near the end of a 5-month injection 

period (November 12, 1996, to April 17, 1997). 

Results of the surveys indicate increases in gravity 

of as much as 66 microgals between a bedrock 

reference station and 20 gravity stations within a 

1-square-mile area surrounding the injection site. 

The changes were assumed to have been caused by 

changes in the ground-water elevation.

Gravity and ground-water levels were 

measured simultaneously at an existing well 

(7N/12W-34B1). The coupled measurements were 

used to calculate a specific yield of 0.13 for the 

alluvial aquifer near the well. To determine the 

gravitational effect of the injection mound on the 

gravity measurements made near well 

7N/12W-34B1, a two-dimensional gravity model 

was used. Results of the model simulation show 

that the effect on gravity associated with the mass 

of the injection mound was minor and thus had a 

negligible effect on the calculation of specific 

yield. The specific yield of 0.13, therefore, was 

used to infer water-level changes at other gravity 

stations within the study area. The gravity-derived 

water-level changes were compared with 

simulated water-table changes.

Gravity changes determined from the 

temporal microgravity surveys were analyzed to 

obtain the accumulated mass within the 

unconfined aquifer. The accumulated mass was 

reduced to a gravity-derived injection rate and 

compared with the measured injection rate to 

determine if the gravity changes reflect the 

volumetric response to injection.

INTRODUCTION

Historically, ground-water withdrawals from the 

alluvial-aquifer system in the Lancaster area of the 

Antelope Valley in southern California (fig.1) have 

exceeded natural replenishment, resulting in overdraft 

and land subsidence. Since the 1920s, ground-water 

levels have declined as much as 200 ft in the study area, 
2 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

and land subsidence has exceeded 6 ft (Ikehara and 

Phillips, 1994). Reliance on ground water eased 

somewhat in the 1970s because of the importation of 

surface water from northern California by way of the 

State Water Project (SWP) and the California 

Aqueduct. However, rapid population growth and the 

resulting demand for water has increased ground-water 

withdrawals and renewed concerns about overdraft and 

subsidence.

From September 1995 through April 1998, the 

U.S. Geological Survey (USGS), in cooperation with 

the Los Angeles County Department of Public Works 

(LACDPW) and the Antelope Valley–East Kern Water 

Agency (AVEK), conducted research and monitoring 

during three cycles of injection, storage, and recovery 

in the Lancaster area of the Antelope Valley, California, 

to evaluate the feasibility of artificially recharging the 

ground-water system. A cycle consists of three periods: 

an injection period during which water is injected into 

the aquifer through a well, a storage period during 

which the well is idle, and a recovery period during 

which water is extracted from the aquifer by pumping 

from the same well. The objectives of the study were to 

develop a better understanding of the alluvial aquifer 

system; to assess the effects of injection, storage, and 

recovery on the aquifer system; and to develop tools to 

help plan and manage a larger injection program. The 

role of the USGS in this study was to collect and 

analyze hydraulic and aquifer-system deformation 

data, to develop a simulation/optimization model for 

use in designing and managing a larger scale injection 

program, and to determine the factors controlling the 

formation and fate of trihalomethanes (disinfection byproducts) in the aquifer system.

This report presents the determination of specific 

yield and water-table changes using temporal 

microgravity surveys made during the second injection, 

storage and recovery test, November 1996 to April 

1997. Microgravity data were collected during both the 

second and third cycles of the injection, storage, and 

recovery tests (Metzger and others, 2002); however, 

only data from the second cycle were analyzed for this 

report. Data from the third cycle could not be analyzed 

because of a 2-month delay in the start of the injection 

after the pre-injection gravity survey was completed, a 

week long interruption in the injection, and a 

significantly reduced injection rate for a shorter period 

of time than that for cycle 2.

The microgravity surveys were done as an 

alternative to installing many monitoring wells to 

measure water-level changes resulting from the 

injection test. Because of the depth of water in the 

study area, which averaged 300 ft below land surface, 

the cost to install the number of wells needed to define 

the shape of ground-water mounding near the injection 

site was prohibitive. The microgravity surveys measure 

changes in mass beneath gravity stations resulting from 

the freshwater injection. One of the gravity stations 

was located near an observation well, which allowed 

gravity changes to be correlated with water-level 

changes to estimate a specific yield for the alluvial 

aquifer. Using the gravity-derived specific yield, watertable changes in the vicinity of the injection wells were 

estimated on the basis of the measured gravity changes. 

This report presents the results of those surveys.

A companion report by Metzger and others 

(2002) presents the data collected during injection, 

storage, and recovery tests between September 1995 

and September 1998. Analytical methods and data 

collected for the investigation of the formation and fate 

of trihalomethanes during the third cycle of the 

injection, storage, and recovery test are described in a 

report by Fram and others (2002). Subsequent reports 

describe the processes affecting the trihalomethane 

concentrations associated with the third injection, 

storage, and recovery test (Fram and others, 2003) and 

the development of a simulation/optimization model 

for use in designing and managing a regional scale 

injection program (Phillips and others, 2003). 

Description of Study Area

The study area encompasses about 1 mi2 just 

south of the city of Lancaster, Antelope Valley, 

California (figs. 1 and 2). Lancaster is in the southcentral part of the valley in the western part of the 

Mojave Desert and is about 50 mi north of Los 

Angeles. The study area is on an alluvial fan that slopes 

gently northwestward at a gradient of about 60 ft/mi 

and ranges in elevation from about 2,480 ft above sea 

level on the southern side of the study area to about 

2,440 ft on the northern side at Avenue K. Annual 

rainfall at Lancaster averaged about 8.0 inches for 

1974–98 (Western Regional Climate Center, accessed 

July 10, 1999). Amargosa Creek, an ephemeral 

channel, trends north and then northwest through the 

study area (fig. 2) and generally flows only after 

periods of intense rainfall. 
Introduction 3

Figure 1. Location of study area and generalized surficial geology of Antelope Valley, California. (Modified from Londquist and others, 1993, figure 2). 

Boron

Mojave

34°30'

35°00'

118°30' 118°00'

San Gabriel Mountains

Tehachapi Mountains

Rosamond

Lake (dry)

Rogers

Lake

(dry)

Littlerock

Redman

Mojave

Desert

ANTELOPE

VALLE

Palmdale Y

Los

Angeles

San

Francisco

0 5 10 15 20 25 Kilometers

0 5 10 15 20 25 Miles

117°30'

Map area

Main

Base

CALIFORNIA

GARLOCK FAULT

SAN ANDREAS FAULT

138

58

14

14

138

Rosamond

Area of figure 2

LOS ANGELES COUNTY

KERN COUNTY

SAN BERNARDINO COUNTY

LOS ANGELES COUNTY

MOJAVE DESERT

Quartz

Hill Lancaster

BUTTES

PEERLESS

PEARLAND

NEENACH

LANCASTER

FINGER BUTTES

OAK CREEK

WEST ANTELOPE

WILLOW

SPRINGS

GLOSTER

CHAFFE

NORTH MUROC

Fremont

Valley

Mojave River

ground-water

basin

Califor nia Aqueduct

A

A'

Study area

(figure 4)

Bedrock

Alluvium

Antelope Valley drainage basin boundary

Antelope Valley ground-water basin (Carlson and others, 1998)

Playa surface

NEENACH Ground-water subbasin boundary and name (Bloyd, 1967)

A A' Line of geologic section (figure 3)

EXPLANATION
4 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

Figure 2. Location of Quartz Hill bedrock reference station and land-surface elevations in and near Lancaster, Antelope Valley, California.

T. 7 N.

R. 12 W. R. 12 W. R. 11 W.

T. 6 N.

Lancaster

0

0 1 2 Kilometers

1 2 Miles

QTZ

34°40'

Base from U.S. Geological Survey

Lancaster, 1:100,000, 1981

118°07'30"

Amargosa

Creek

Quartz Hill

AVENUE K

AVENUE L

AIR FORCE

PLANT 42

SIERRA

HIGH

WA

Y

ANTELOPE

VALLEY

FREE

WA

Y

2,600

2,640

2,640

2,600

2,480

2,520

2,520

2,560

2,560

2,400

2,400

2,400

2,440

2,440

2,320

2,360

2,360

QTZ

14

14

Study area (figure 4)

Quartz Hill bedrock reference station

Urbanized area

Land-surface elevation—Contour interval 40 feet.

Datum is sea level

EXPLANATION

2,560



I I I I I I I I I I I I III I I I I I I I I I I

IIII I III I I I III I I I I I I

I I I IIII I I I I I I I I I I I I I I

UNION

PACIFIC RAILROAD
Introduction 5

The study area is in the Lancaster subbasin of the 

Antelope Valley (fig. 1), which is filled with alluvial 

and lacustrine deposits that are locally as much as 

5,000 ft thick (Brenda and others, 1960; Mabey, 1960) 

(fig. 3). The alluvial deposits consist of interbedded 

heterogeneous mixtures of silt, sand, and gravel 

(Dutcher and Worts, 1963; Bloyd, 1967); the lacustrine 

deposits primarily consist of thick layers of clay, 

interbedded with thinner sand and silty sand layers 

(Dibblee, 1967). Stratigraphic, hydrologic, and waterquality data were used to divide the deposits into three 

aquifers: an upper, a middle, and a lower aquifer 

(Leighton and Phillips, 2003). At the injection, storage, 

and recovery site, the upper aquifer extends from the 

water table to a depth of about 510 ft below land 

surface, the middle aquifer extends from about 510 to 

about 730 ft below land surface, and the lower aquifer 

extends from about 870 ft below land surface to the 

bedrock (fig. 3). Ground-water flow in the upper 

aquifer is unconfined, flow in the middle aquifer is 

unconfined to partially confined at depth, and flow in 

the lower aquifer is confined by the lacustrine deposit 

that separates the middle and lower aquifers.

As much as 2 ft of land subsidence has occurred 

in or near the study area from 1930 to 1992 as a result 

of declining ground-water levels and associated 

aquifer-system compaction (Ikehara and Phillips, 1994; 

Galloway and others, 1998b). Measurements of land 

subsidence for 1993–95, made using interferometric 

synthetic aperture radar (InSAR) (Galloway and others, 

1998c), and measurements of aquifer-system 

compaction at a borehole extensometer for 1990–97 

(Sneed and Galloway, 2000) show that subsidence 

continued in Antelope Valley, including the study area, 

during the 1990s. The subsidence is a result of the 

lowered hydraulic heads and increasing effective stress 

in the confining unit (lacustrine clay) and the 

interbedded clay units or aquitards (Carlson and 

Phillips, 1998).

Figure 3. Generalized geologic section showing relation of lacustrine deposits to younger and older alluvium and aquifers, 

Antelope Valley, California. (Modified from Metzger and others, 2002). Line of section is shown on figure 1.

5,000

4,000

3,000

3,000

2,000

2,000

1,000

1,000

Upper aquifer

Land surface

Lower aquifer

Middle aquifer

FEET

Younger alluvium (upper aquifer)

Older alluvium (middle aquifer)

Continental deposits (lower aquifer)

Lacustrine clay deposits

Bedrock

A A'

Sea

level

EXPLANATION

0

0

10 Miles

10 Kilometers

Interbedded

aquitards

Palmdale

Redman

Rogers Lake

Vertical scale greatly exaggerated

Datum is sea level

Highway 58

Lacustrine clay
6 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

Description of Injection Wells

For all three cycles of the injection, storage, and 

recovery tests, the water used for injection into the 

wells was imported from the SWP. For the cycle 2 

injection test, about 1,050 acre-ft of SWP water was 

injected at a rate of 750 gal/min into each of two 

existing production (injection) wells between 

November 12, 1996, and April 17, 1997. These two 

wells (7N/12W-27P2 and 27P3), located just north of 

Avenue L and about 0.5 mi west of Sierra Highway 

(fig. 4), penetrate the upper and middle aquifers and are 

screened from 282 to 717 ft and 280 to 710 ft below 

land surface, respectively (figs. 5 and 6). A well-bore 

velocity log made for well 7N/12W-27P2 under 

pumping conditions indicates that about 90 percent of 

the water pumped (1,350 gal/min) was from the upper 

aquifer and about 10 percent (150 gal/min) was from 

the middle aquifer.

Previous Microgravity Studies

Microgravity techniques were used during 

previous investigations to estimate specific yield and 

water-level changes, although not for an injection 

scenario. Montgomery (1971) estimated the specific 

yield for an unconfined aquifer by correlating gravity 

and water-level variations. Pool and Hatch (1991) 

measured gravity changes caused by the mounding of 

ground water beneath an artificial recharge pond; their 

study most resembles this investigation. More recently, 

Pool and Eychaner (1995) used mircogravity surveys to 

determine aquifer-storage change and specific yield. 

Lines (1996) used microgravity surveys and waterlevel changes to estimate the specific yield of the floodplain aquifer at ten sites along the Mojave River.

Microgravity Surveys

The gravity-station network consisted of 20 

permanent gravity stations within 1 mi of the injection 

site (fig. 4). The gravity stations were areally 

distributed to measure the anticipated shape of the 

ground-water mounding around the injection wells. 

Temporal, or time-series, microgravity surveys were 

conducted at the gravity-station network to measure 

small changes in gravitational acceleration (also 

referred to as gravity) caused by subsurface changes in 

mass. In an injection scenario, mass, in the form of 

water, is added to the aquifer and the associated change 

in gravity is measured with a portable gravity meter. A 

microgravity survey was conducted prior to injection to 

establish baseline gravity values for the gravity-station 

network. Subsequent surveys were conducted to 

monitor the accumulation of mass and determine the 

areal extent of the anomalous mass with time.

In an unconfined aquifer, injected water that 

resaturates the alluvium causes a net increase in mass 

proportional to the volume of water that fills previously 

unsaturated pore spaces. As injection continues, a 

mound of water, henceforth referred to as the “mound” 

or “injection mound,” forms in the aquifer. 

Conceptually, this mound is a mirror image of the cone 

of depression that would form in the same aquifer 

material under the same rate of withdrawal as that of 

the injection. The mound is highest beneath the 

injection wells and flattens exponentially with 

increasing radial distance. The girth and the height of 

the mound increase with sustained injection until a 

regional static equilibrium is achieved. 

In a confined aquifer, injected water will result in 

an increase in hydraulic head over a large area because 

of the low storativity typical of a saturated confined 

aquifer (0.005 to 0.0005; Freeze and Cherry, 1979). 

Temporal microgravity surveys cannot be used to 

monitor the change in hydraulic head in a confined 

aquifer because even a large increase in hydraulic head 

represents only a slight increase in mass beneath an 

individual gravity station due to the low storativity. The 

increases in hydraulic head, or pore fluid pressure, can 

cause some expansion of the aquifer system owing to 

the compressibility of the granular skeleton of the 

aquifer, and this expansion results in millimeter-tocentimeter-scale increases in land-surface elevation 

that can be detected by microgravity measurements and 

differential leveling.
Introduction 7

Figure 4. Location of gravity stations, monitoring wells, and injection wells and of south-to-north and west-to-east profiles, Lancaster, Antelope Valley, 

California.

G6E

GNE

G2N

G1N

7N/12W-34B1

7N/12W-34B1

7N/12W-27F8

7N/12W-27P3

7N/12W-27P2

7N/12W-27P2

G4S

G5S

G4E

G4W

G3N

G3W

G2W

G1W G3S G2S

G1S

G2E

G3E

GZERO

G5N GR

0

0 0.5 Kilometer

0.5 Mile

G4S Gravity station and identifier

Line of profile section (shown in figures 7, 10, 11, 12, and 13)

Monitoring well and identifier

Injection well and identifier

W

W E

N

E

EXPLANATION

AVENUE L

SIERRA HIGH

WAY

IIIIIIIIII

IIIII

I I

IIIIIII

IIIII

IIIIIIIIIIIII

I I I IIIIIII

UNION

PACIFIC RAILROAD

Amargosa

Creek

S

R. 12 W. 118°07'30" R. 12 W.

T. 7 N.

T. 7 N.
8 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

Figure 5. Lithologic log and well-construction diagram for injection well 7N/12W-27P2 in Lancaster, Antelope Valley, California.

0

3

48

62

168

182

272

370

118

18

100

200

300

400

DEPTH BELOW LAND SURFACE, IN FEET

Depth

interval Lithologic log

Injection Well 7N/12W-27P2

Well construction

Coarse to medium sand

Surface soil

Coarse to medium sand

Brown clay and boulders

Coarse to medium sand

and brown clay

Coarse to fine sand

Coarse to fine sand

Coarse to fine sand

and brown clay

Medium to fine brown clay

Coarse to medium sand

and boulders

Land surface, 2,463 feet above sea level

40-inch annulus

40-inch gravel pack

Screen interval 282 to 717 feet

below land surface

Pre-injection water level

Total well depth 727 feet

Near-completion-of-injection

water level

Concrete

34-inch casing to 100 feet

below land surface

16-inch casing to bottom of hole

at 727 feet below land surface

Not to scale
Introduction 9

Figure 6. Lithologic log and well-construction diagram for injection well 7N/12W-27P3 in Lancaster, Antelope Valley, California.

0

5

160

190

210

270

290

310

330

365

375

120

140

30

100

200

300

400

DEPTH BELOW LAND SURFACE, IN FEET

Depth

interval Lithologic log

Injection Well 7N/12W-27P3

Well construction

Coarse to medium sand

and gravel

Coarse to medium sand

and gravel

Sand, gravel, and clay

Brown clay

Brown clay

Brown clay and sand

Brown clay

Sand and clay

Sand and gravel

Sand, gravel, and clay

Medium sand and gravel

Coarse to medium sand

and gravel

Coarse to medium sand

and gravel

Surface soil

Land surface, 2,462 feet above sea level

30-inch gravel pack

Screen interval 280 to 710 feet

below land surface

Near-completion-of-injection

water level

Total well depth 720 feet

Concrete

40-inch annulus

36-inch casing to 152 feet

below land surface

16-inch casing to bottom of hole

at 720 feet below land surface

Pre-injection water-level

Not to scale
10 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA Table 1. Data from pre-injection, mid-injection, and near-completion-of-injection surveys for gravity stations at Lancaster, Antelope Valley, California, November 1996 through April 1997 [Latitude and longitude are referenced to the North American Datum of 1983 (NAD83) determined using global positioning system (GPS) in 1996–97; mean difference in gravity relative to Quartz Hill (3.6 miles from injection site); elevations referenced to the National Geodetic Vertical Datum of 1929 (NGVD29) and measured by leveling to second-order standards of accuracy; elevations for the pre- injection survey measured November 4–13, 1996, and for the near-completion-of-injection, on April 9, 1997, unless indicated otherwise (see footnotes); gravity differences corrected for station elevation changes are rounded to the nearest microgal. mGal, milligal; μGal, microgal; ft, foot; m, meter; mm, millimeter; x, mean; <, less than; —, no data] See footnotes at end of table. Gravity station Latitude Longitude Pre-injection survey (November 5–9, 1996) Mid-injection survey (January 29–February 1, 1997) Change in gravity from previous survey

(μGal)

Mean difference

in gravity

(mGal)

Standard

deviation

(μGal)

Elevation Mean difference

in gravity

(mGal)

Standard

deviation

(μGal) (ft) (m)

GR 34°40′05′′ 118°08′09′′ −6.098 5.3 2,445.648 745.4336 −6.097 1.8 1G5N 34°40′05′′ 118°08′26′′ −4.774 7.3 2,441.606 744.2014 −4.770 1.0 4G3N 34°39′47′′ 118°08′20′′ −4.684 9.0 2,459.698 749.7158 −4.659 9.3 25G2N 34°39′45′′ 118°08′20′′ −4.633 1.5 2,461.429 750.2436 −4.606 2.6 27G1N 34°39′44′′ 118°08′20′′ −4.611 4.5 2,462.123 750.4551 −4.580 2.6 31G4W 34°39′43′′ 118°08′36′′ −2.609 4.6 2,454.687 1748.1885 −2.577 6.5 32G3W 34°39′43′′ 118°08′28′′ −3.648 4.3 2,459.406 749.6268 −3.610 5.0 38G2W 34°39′43′′ 118°08′25′′ −4.014 4.5 2,460.469 749.9511 −3.980 2.1 34G1W 34°39′43′′ 118°08′24′′ −4.196 1.4 2,461.487 750.2612 −4.162 7.5 34GZERO 34°39′43′′ 118°08′22′′ −4.298 3.8 2,461.622 750.3025 −4.266 3.6 32G5S 34°39′32′′ 118°08′20′′ −4.799 3.1 2,473.290 753.8587 −4.779 2.6 20G4S 34°39′35′′ 118°08′20′′ −4.677 3.2 2,469.980 752.8501 −4.658 0.7 19G3S 34°39′39′′ 118°08′21′′ −4.540 3.2 2,465.881 751.6004 −4.514 2.2 26G2S 34°39′41′′ 118°08′21′′ −4.607 2.6 2,464.709 751.2433 −4.572 1.9 35G1S 34°39′42′′ 118°08′21′′ −4.573 4.1 2,463.882 750.9911 −4.518 6.1 55G6E 34°39′44′′ 118°07′27′′ −9.528 2.0 2,467.744 1752.1683

—

—

—

G4E 34°39′43′′ 118°08′16′′ −5.250 1.3 2,465.964 751.6257 −5.237 1.9 13G3E 34°39′43′′ 118°08′19′′ −4.927 3.5 2,464.277 751.1117 −4.906 2.1 21G2E 34°39′43′′ 118°08′20′′ −4.753 7.6 2,463.550 750.8900 −4.716 6.0 37GNE 34°39′51′′ 118°08′07′′ −6.086 6.4 2,460.296 1749.8981

—

—

—

× = 4.2 × = 3.6

QTZ2 34°38′43′′ 118°12′01′′ 2,638.7 3804.3
Introduction 11

Table 1. Data from pre-injection, mid-injection, and near-completion-of injection surveys for gravity stations at Lancaster, Antelope Valley, California, November 1996 through April 1997—Continued

1Elevation measured March 26–28, 1996.2Bedrock promontory used as a stable gravity reference.3Determined by GPS in 1996.4Where vertical control is missing, the free air effect was assumed to be zero. 

Gravity 

station

Near-completion-of-injection 

(April 5–12, 1997)

Change in 

gravity from 

previous

survey

(μGal)

Change from pre-injection Change in

elevation

effect

(μGal)

Gravity 

difference 

corrected 

for station 

elevation change

(μGal)

Mean difference

in gravity

(mGal)

Standard

deviation

(μGal)

Elevation

Microgravity

(μGal)

Elevation

(mm) (ft) (m)

GR −6.095 7.4 — — 2 3 — (4) 3G5N −4.767 4.3 2,441.614 744.2038 3 7 2.4 1 8G3N −4.646 1.3 2,459.700 749.7166 13 38 .8 <1 38G2N −4.592 4.1 2,461.430 750.2440 14 41 .4 <1 41G1N −4.562 2.5 2,462.124 750.4554 18 49 .3 <1 49G4W −2.572 3.5 — — 5 37 — (4) 37G3W −3.599 4.9 2,459.407 749.6272 11 49 .4 <1 49G2W −3.982 4.6 2,460.467 749.9504 -2 32 −.7 <1 32G1W −4.157 1.8 2,461.484 750.2602 5 39 −1.0 <1 39GZERO −4.254 3.5 2,461.619 750.3014 12 44 −1.1 <1 44G5S −4.769 3.1 — — 10 30 — (4) 30G4S −4.653 4.8 2,469.983 752.8509 5 24 .8 <1 24G3S −4.510 2.5 2,465.879 751.6000 4 30 −.4 <1 30G2S −4.563 4.7 2,464.708 751.2431 9 44 −.2 <1 44G1S −4.507 2.5 2,463.881 750.9908 11 66 −.3 <1 66G6E −9.519 1.4 — — — 9 — (4) 9G4E −5.218 1.9 2,465.966 751.6265 19 32 .8 <1 32G3E −4.887 0.6 2,464.279 751.1123 19 40 .6 <1 40G2E −4.715 5.0 2,463.550 750.8901 1 38 .1 <1 38GNE −6.077 3.8 — — — 9 — (4) 9

× = 3.4
12 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

A microgravity survey consists of two loops of 

measurements that begin and end at a reference station. 

For this study, two reference stations were 

established—one at Quartz Hill (QTZ) about 3.5 mi 

west-southwest of the injection site and one (GR) 

within the study area (figs. 2 and 4). Quartz Hill, a 

crystalline bedrock promontory, was used as a stable 

gravity reference. The GR reference station was 

established to eliminate the travel time between the 

QTZ reference station and the study area. GR is located 

about 0.5 mi upgradient from the injection site where 

water-table change and associated changes in mass 

were expected to be minimal.

Three microgravity surveys (pre-injection, midinjection, and near-completion-of-injection) were done 

for the cycle 2 injection test (table 1). Gravity was 

measured three times at the GR reference station 

during each survey to evaluate instrument drift during 

the survey and twice at each gravity station to assess 

the repeatability and accuracy of the measured 

differences in gravity. The mean difference in gravity 

and the standard deviation for each station (table 1) 

were calculated from two measurements made within 2 

to 3 hours of each other. A thorough discussion of the 

methods of data collection and of the sources of survey 

error is given by Metzger and others (2002, appendixes 

A, B, and C). All values of gravity are relative to the 

QTZ bedrock reference station.

For each of the surveys, the first step consisted of 

determining the difference in gravity between the 

QTZ reference station, where mass changes were 

expected to be negligible, and the GR reference station. 

The variation in the surveyed mean difference in 

gravity for the GR reference station was 3 μGal, 

ranging from –6.098 mGal in the pre-injection survey 

to –6.095 mGal in the near-completion survey (table 1). 

Once the mean difference in gravity between the QTZ 

and GR reference stations was established for each 

survey period, surveys relative to the GR station were 

made. The total difference in gravity between the QTZ 

reference station and the gravity stations in the study 

area was determined by adding the difference in gravity 

between the QTZ and the GR reference stations to the 

difference between the GR station and other stations in 

the study area. Absolute values of gravity were not 

determined, but absolute values of gravity for this part 

of Antelope Valley are about 979,500 mGal (Hanna and 

Sikora, 1973).

Changes in elevation at a gravity station can 

affect a measurement of gravity because of the strong 

vertical gradient of gravity, 308.6 μGal/m (Dobrin, 

1960, p. 189). Because gravity measurements are 

reported to an accuracy of 1 μGal, gravity-station 

elevation changes greater than 1.6 mm were corrected 

for a change in the elevation of the gravity station. For 

example, an increase in elevation of 0.0016 m 

(1.6 mm) at a station will decrease the measured 

gravity value at that station by about 0.5 μGal and a 

decrease in elevation of 0.0016 m will increase the 

measured gravity value at the gravity station by about 

0.5 μGal. For this study, differential leveling done to 

second-order results (Bossler, 1984) was used to detect 

the vertical changes at the gravity stations (table 1). 

With the exception of gravity station G5N, all the 

gravity stations had elevation changes less than or 

equal to 1.6 mm and no correction to the measured 

difference in gravity was required (table 1).The 

maximum elevation change was detected at gravity 

station G5N (+2.4 mm) between the pre-injection and 

the near-completion-of-injection surveys. A change of 

0.0024 m in elevation at the G5N station resulted in a 

change in gravity of 0.7 μGal. The positive elevation 

change means that the gravity station is further from 

the injection mound mass and, therefore, the measured 

difference in gravity is deficient (Nettleton, 1976, p. 

19). To correct the measured difference in gravity 

(7 μGal) for the positive elevation change, 0.7 μGal 

was added to the measured difference, giving a change 

in gravity of 7.7 μGal for station G5N. Because only 

one gravity station required an elevation-change 

correction, the measured changes in gravity during the 

injection cycle cannot be the result of gravity station 

elevation changes.
Determination of Specific Yield 13

The maximum positive elevation change 

between the pre-injection and the near-completion-ofinjection surveys was expected to be near the injection 

wells, where aquifer deformation owing to increased 

pore-fluid pressure in the confined units may cause 

expansion of the granular skeleton of the aquifer. 

However, the maximum positive elevation change was 

at the G5N gravity station, which is about 2,300 ft 

north of the injection wells. This spatial discrepancy 

may be related to the variability in the poroelastic 

properties of the aquifer material. Time-series leveling 

data for the study area (Metzger and others, 2002; 

Phillips and others, 2003) and InSAR data (Galloway 

and others, 1998c) indicate that the elastic skeletal 

specific storage (the component of storage associated 

with the elastic deformation of aquifer materials) is 

greater at the northern end of the study area than in the 

area of the injection wells, which may account for the 

larger elevation changes at G5N.

DETERMINATION OF SPECIFIC YIELD

Gravity and ground-water levels were measured 

simultaneously at monitoring well 7N/12W-34B1 

(fig. 4) to calculate specific yield for the unconfined 

(water table) aquifer for the vertical interval of waterlevel change. Specific yield is the volume of water that 

drains by gravity or that resaturates under hydrostatic 

conditions for a unit volume of aquifer material and is 

expressed as a dimensionless fraction or percentage.

A change in gravity is related to a change in 

mass, which is attributed to the change in the volume of 

water occupying pore space in the unconfined aquifer 

and can be calculated by

, (1)

where

Specific yield (Sy) was determined by dividing the 

change in mass by the measured water-level change 

(∆ water level)

Sy = ∆ mass/∆ water level (Pool and Eychaner, 1995). (2)

Once the specific yield is known and the change 

in mass is known, the change in water level can be 

calculated by solving for ∆ water level using equation 

2.

For a 3-week period prior to cycle 2 injection, 

wells operated by the Los Angeles County Department 

of Public Works within 1 mi of the injection site were 

not pumped to allow water levels to approach static 

equilibrium, or recover (Metzger and others, 2002). 

During this period, water levels were monitored at well 

7N/12W-34B1, which is 30 ft south of gravity station 

G5S (fig. 4). Near the end of this recovery period 

(November 7, 1996), the mean difference in gravity at 

G5S relative to the QTZ reference station was –4.799 

mGal (1 standard deviation = 3.1 μGal) (table 1) and 

the water level at well 7N/12W-34B1 was 351.8 ft 

below land surface (table 2). Measurements were 

repeated on April 10 and 11, 1997, near the end of the 

injection period. The mean gravity difference was 

–4.769 mGal (1 standard deviation = 3.1 μGal) and the 

ground-water level was 334.0 ft below land surface, 

yielding changes of 30 μGal and 17.8 ft, respectively. 

Substituting 30 μGal for the measured change in 

gravity in equation 1 yields an increase in mass 

equivalent to 2.35 feet of water at well 7N/12W-34B1. 

Substituting 2.35 ft of water for change in mass and 

17.8 ft for water-level change in equation 2 results in a 

specific yield of 0.13.

The calculated specific yield of 0.13 for the 

aquifer in the study area is in general agreement with 

the values of 0.10 to 0.15 estimated by Durbin (1978) 

for this part of the Antelope Valley. Durbin estimated 

the specific yield using lithologic well logs and then 

correlated the lithologic data with data from laboratory 

tests by Bloyd (1967), which were done on similar 

materials collected from Antelope Valley. The specific 

yield using microgravity measurements (0.13) was 

used in two models being developed for the study area, 

a regional-scale (Leighton and Phillips, 2003) and a 

subregional-scale numerical model (Phillips and 

others, 2003).

∆g is the measured change in gravity, in 

microgals,

12.77 is the mass equivalent of 1 ft of water, 

assuming a slab geometry of infinite 

extent, in microgals, and

∆ mass is the change in mass at a gravity 

station, in equivalent feet of water.

∆g ⁄ 12.77 = ∆mass (Dobrin, 1960)
14 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

EFFECT OF INJECTION MOUND ON THE 

CALCULATED SPECIFIC YIELD

The irregular geometry of the injection mound 

may affect the measured vertical component of gravity 

at the G5S station and hence the calculated specific 

yield. The value for the mass equivalent of 1 foot of 

water, 12.77 μGal (equation 1) (Dobrin, 1960, p. 175), 

assumes a slab geometry of infinite extent: however, 

the geometry of the injection mound is better depicted 

as a series of stacked disks of finite diameter, each 

having a smaller diameter than the disk below it. The 

discrepancy between the slab geometry and the 

geometry of the injection mound casts doubts as to 

whether equation 1 can be used to estimate the change 

in mass in equivalent feet of water.

A commercially available two-dimensional 

gravity model (GravModeler) was used to assess the 

gravitational effect of the injection mound on the 

gravity measurement at the G5S station.The model is 

based on the line integral approach of Talwani and 

others (1959), who derived expressions for the vertical 

and horizontal components of the gravitational 

acceleration for a two-dimensional polygon of arbitrary 

shape. GravModeler computes the gravity response at 

the earth’s surface across the model width due to a twodimensional buried mass or polygon. The polygon is 

defined in terms of density, depth, and cross sectional 

geometry. The two-dimensional computation means 

that the user-defined polygons are assumed to be 

infinite in extent into and out of the model profile. 

Polygons that intersect the right and left model 

boundaries also are assumed to be infinite in extent. 

GravModeler computes the gravity response based on 

the density contrast between the polygon in question 

(injection mound) and the background density (aquifer 

material). The modeled density contrast between 

saturated and unsaturated aquifer material is 

proportional to the porosity or specific yield (Nettleton, 

1976, p. 245). Because the density of water is 1 g/cm3

and the calculated specific yield was 0.13, the density 

contrast of the injected mound was simulated as 0.13 

g/cm3. The depth to the bottom of the injection mound 

was based on the pre-injection water-level elevation 

measured in three wells along the south to north profile 

(fig. 7A; table 2). The model width (5,000 ft) was 

chosen such that the infinitely projected polygons at the 

model boundaries would have a relatively small 

thickness. Therefore, the computed gravity response 

resulting from this boundary assumption would be 

negligible near G5S. The cross sectional geometries of 

the various injection mounds (fig. 7A) were entered 

into GravModeler using a graphical user interface.

The injection mound geometries were 

determined using the USGS three-dimensional groundwater flow model MODFLOW developed by 

McDonald and Harbaugh (1988). The ground-water 

flow model was used to simulate the upper part of the 

aquifer system in the study area during the injection 

test. The simulation was done for transient conditions 

using 15 stress periods of 10 days each. The lateral 

model boundaries were about 7,500 ft from the 

injection wells (fig. 8). A constant southward hydraulic 

gradient of 0.0017 was specified on the basis of 

measured water levels. Cell sizes were varied laterally, 

increasing from 20 ft2 at the injection wells to more 

than 300 ft2 at the outer margins of the modeled area. 

Cell thickness initially was 200 ft for a single-layer 

model, but varied with water-table change. The 

thickness of the cells corresponds to that determined 

from regional and local investigations of the aquifer 

system (Leighton and Phillips, 2003) and is considered 

to represent the unconfined part of the aquifer system. 

The only stress represented in the ground-water flow 

model was injection, which was specified at a constant 

rate of 675 gal/min (90 percent of the total injection 

rate) for each of the two injection wells (7N/12W-27P2 

and 27P3). The model domain was assumed to have a 

specific yield of 0.13 (the gravity-derived value). Five 

model simulations were run with different hydraulic 

conductivity values (8, 10, 12, 15, and 18 ft/d), which 

are within the range of the values estimated from 

results of aquifer-test analyses and of the simulations 

of the study area. The hydraulic conductivity value was 

assumed to be constant over the model area for each 

simulation (figs. 7 and 8). 
Effect of Injection Mound on the Calculated Specific Yield 15

Figure 7. South-to-north profile showing (A) simulated injection mound geometries and corresponding hydraulic conductivity, and (B) the simulated twodimensional gravity response to various injection mound geometries, and measured gravity changes at the near-completion-of-injection survey at Lancaster, 

Antelope Valley, California.

0

80

60

40

20

GRAVITY, IN MICROGALS





Near-completion-of-injection water level

Pre-injection water-level elevation and projected water table

Measured gravity change near-completion-of-injection and station identifier

EXPLANATION

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500

K=8

K=10

K=12

K=15

K=18

Simulated injection mounds

and corresponding hydraulic

conductivity (K) in feet per day

Simulated gravity response

and corresponding hydraulic

conductivity (K) in feet per day

G5S

G5S

G4S

G3S

G2S

G1S

G1N

G2N

G3N

G5N

B

A

DISTANCE, IN FEET

(Horizontal exaggeration X1.5 relative to line of section in figure 4)

32 microgals

K=8

K=10

K=12 K=15

K=18

2,250

2,100

2,150

2,200

ELEVATION, IN FEET ABOVE SEA LEVEL

SOUTH

7N/12W-34B1

7N/12W-27P2 (projected)

7N/12W-27F8

NORTH

Vertical exaggeration X10.5

Bend in section



  
16 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA Table 2. Selected ground-water-level measurements for the pre-injection and near-completion-of-injection surveys at Lancaster, Antelope Valley, California, November 1996 through April 1997 Injection period 7N/12W-34B1 7N/12W-27P2 7N/12W-27P3 7N/12W-27F8 Date

Waterlevel

depth 

below land 

surface 

(feet)

Waterlevel

elevation

(feet)

Date

Waterlevel

depth 

below land 

surface 

(feet)

Waterlevel

elevation

(feet)

Date

Waterlevel

depth 

below land 

surface 

(feet)

Waterlevel

elevation

(feet)

Date

Waterlevel

depth 

below land 

surface 

(feet)

Waterlevel

elevation

(feet)

Pre-injection water-level measurements 11/07/96 351.8 2,123 11/12/96 337.3 2,126 11/12/96 337.3 2,125 11/09/96 311.4 2,130.2Near-completion water-level measurements 04/10/97 334.0 2,141 04/14/97 238.7 2,224 04/17/97 236.1 2,226 04/16/97 300.6 2,141.0
Effect of Injection Mound on the Calculated Specific Yield 17

Figure 8. Model grid and simulated injection mound contours and areal extent of the gravity station network in Lancaster, Antelope Valley, California.

0

0 500

5,000 Feet

1,000 Meters

N

+2 f

+4

eet

+6

feet

+8 feet

feet

+10 feet

Enlarged inset

Gravity station

Injection well

EXPLANATION

Simulated injection mound

contours for a hydraulic

conductivity of 15 feet per day

Areal extent of gravity network

G5N GR

G4W

G5S

G4E

+10

+12

+14

+16

Water-level change, in feet

0

0 500

5,000 Feet

1,000 Meters

N

+2 f

+4

eet

+6

feet

+8 feet

feet

+10 feet

Enlarged inset

Gravity station

Injection well

EXPLANATION

Simulated injection mound

contours for a hydraulic

conductivity of 15 feet per day

Areal extent of gravity network

G5N GR

G4W

G5S

G4E

+10

+12

+14

+16

Water-level change, in feet
18 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

Figure 7B shows the gravitational response at the 

G5S gravity station to various simulated injection 

mound geometries. Results of the simulations show 

that lower hydraulic conductivity values resulted in a 

higher injection mound than did the higher hydraulic 

conductivity values (fig. 7). Thus, the injection mound 

simulated with the lowest hydraulic conductivity value 

(8 ft/d) yielded the greatest gravitational response 

(32 μGal) at the G5S gravity station, 2 μGal greater 

than the measured change of 30 μGal at the 

G5S gravity station for the near-completion-ofinjection survey (table 1). 

As previously mentioned, the injection mound 

can be visualized as a stack of disks, each having a 

smaller diameter than the disk below it, or as a flattened 

cone. The two-dimensional gravity model, however, 

calculated the volume of the mound as an infinite ridge, 

rather than as a cone, with a cross-sectional geometry 

equivalent to that shown on the south-to-north profiles 

in figure 7A. The larger volume resulting from the 

infinite ridge caused an overestimation of the gravity 

response, which was greatest at the ridge crest and 

decayed with distance from the injection wells. Even if 

the 2-μGal gravitational effect of the largest simulated 

injection mound is not reduced to compensate for the 

overestimated volume of the infinite ridge, it is small 

relative to the measurement error of 6.2 μGal at the 

G5S gravity station (the sum of the standard deviations 

of the measured mean difference in gravity from the 

pre-injection and near-completion-of-injection surveys; 

(table 1). Therefore, the irregular injection mound 

geometry is considered to have had a negligible effect 

on the measured gravity at the G5S station and the 

specific yield calculation. If the injection mound had 

been much larger or if the gravity station had been 

closer to the injection mound, the mass of the injection 

mound would have had a more significant effect on the 

measured gravity.

GRAVITY-DERIVED WATER-LEVEL 

CHANGES

Ground-water-level changes were estimated at 

each gravity station using the gravity-derived specific 

yield (0.13) and gravity changes measured during the 

near-completion-of-injection survey (table 1). The 

relation among water-level change, specific yield, and 

change in gravity is shown in figure 9.

The gravity-derived water-level changes and the 

simulated water levels along the south-to-north and the 

west-to-east profiles are shown in figures 10 and 11. 

The gravity-derived water levels reasonably match the 

simulated injection mounds along the south-to-north 

profile for a range of hydraulic conductivities 

(8 to 18 ft/d; fig. 10), except near the injection wells. 

The difference between the measured water-level 

change at well 7N/12W-34B1 and the simulated waterlevel changes varies from 0.3 ft for a hydraulic 

conductivity of 8 ft/d to −5.6 ft for a hydraulic 

conductivity of 18 ft/d (fig. 10).
Gravity-Derived Water-level Changes 19

Figure 9. Relation among gravity change, specific yield, and water-level change assuming a slab geometry of infinite extent. WATER-LEVEL CHANGE, IN FEET

CHANGE IN GRAVITY, IN MICROGALS

80

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70 80

Sy = 0.07

Sy = 0.09

Sy = 0.11

Sy = 0.13

Sy = 0.15

Sy = 0.17

Sy = 0.19

Sy = 0.21

Sy = 0.23

WATER-LEVEL CHANGE, IN FEET

CHANGE IN GRAVITY, IN MICROGALS

80

70

60

50

40

30

20

10

0

0 10 20 30 40 50 60 70 80

Sy = 0.07

Sy = 0.09

Sy = 0.11

Sy = 0.13

Sy = 0.15

Sy = 0.17

Sy = 0.19

Sy = 0.21

Sy = 0.23
20 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

Figure 10. Gravity-derived and measured water levels compared to simulated injection mounds for a range of hydraulic conductivities along the south-tonorth profile, Lancaster, Antelope Valley, California. Location of profile shown in figure 4.

Simulated injection mounds for a range of hydraulic conductivities (K) in feet per day

Gravity-derived water level based on measured change in gravity and standard

deviation





Near-completion-of-injection water level

Pre-injection water level and projected water table

EXPLANATION

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500

K=8

K=10

K=12 K=15

K=18

2,250

2,100

2,150

2,200

ELEVATION, IN FEET ABOVE SEA LEVEL

SOUTH

7N/12W-34B1

7N/12W-27P2 (projected)

7N/12W-27F8

NORTH

  



Vertical exaggeration X10.5

Bend in section

G5SG4SG3SG2SG1SG1NG2NG5N

G3N

DISTANCE, IN FEET

(Horizontal exaggeration X1.5 relative to line of section in figure 4)



Gravity-Derived Water-level Changes 21

The gravity-derived water levels along the westto-east profile were within the range of the simulated 

water levels farthest east of the injection wells for 

hydraulic conductivities 15 to 18 ft/d (fig. 11), but the 

gravity-derived water levels at gravity stations G2E, 

GZERO, G1W, and G2W, which are near the injection 

wells, were lower than the simulated water levels. 

Because of the proximity of these stations to the 

injection wells, the changes in water levels and gravity 

were expected to be large. A subsequent injection test 

at this same site (Metzger and others, 2002) produced 

similar results with a distinct gravity low at the G2W 

station and a corresponding gravity high at the G3W 

station. This suggests that there are intrinsic differences 

in the aquifer material being resaturated beneath these 

stations. Possible explanations for the discrepancy 

between the simulated and gravity-derived water levels 

near the injection site may be the variability of the 

aquifer properties (hydraulic conductivity and specific 

yield) and (or) interference from environmental factors 

associated with the injection site (vibration from the 

injection wells, electromagnetic fields created from the 

high-voltage power supply for the injection wells, and 

buried water-supply pipes) that may affect gravity 

measurements. Additional data on the aquifer 

properties and the effect of these environmental factors 

on gravity measurements are needed to further explain 

these discrepancies.

Figure 11. Gravity-derived and measured water levels compared to simulated injection mounds for a range 

of hydraulic conductivities along the west-to-east profile, Lancaster, Antelope Valley, California. Location of 

profile shown in figure 4.

0 500 1,000 1,500 2,000

2,250

2,100

2,150

2,200

ELEVATION, IN FEET ABOVE SEA LEVEL

7N/12W-27P3

7N/12W-27P2

WEST EAST

K=10

K=8

K=12

K=15

K=18

Simulated injection mounds for a range of hydraulic conductivities (K)

in feet per day

Gravity-derived water level based on measured change in gravity and

standard deviation

Pre-injection water level and projected water table

EXPLANATION

Vertical exaggeration X7.4 G4W

G3WG2WGZEROG2EG3EG4E

G1W

DISTANCE, IN FEET

(Horizontal exaggeration X1.7 relative to line of section in figure 4)







Near-completion-of-injection water level



22 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

The changes in gravity (pre-injection to the midinjection to the near-completion-of-injection) at the 

gravity stations along the west-to-east and south-tonorth profiles are shown in figures 12 and 13. Most of 

the gravity change at the stations along the two profiles 

occurred by midway through the cycle 2 injection. On 

day 81 of 157 days of injection, 77 and 70 percent of 

the total gravity change along the west-to-east and 

south-to-north profiles, respectively, had occurred. 

Because the rate of injection was constant for the 157-

day period, the large percentage of the gravity change 

by mid-injection suggests that the growth of the 

injection mound was slowing (approaching static 

equilibrium) and that the hydraulic response to the 

injection was spreading at the periphery of the mound 

beyond the areal extent of the gravity network (fig. 8).

Figure 12. Change in gravity at gravity stations along the west-to-east profile, Lancaster, Antelope Valley, 

California. Location of profile shown in figure 4.

Injection well 7N/12W-27P2 Injection well 7N/12W-27P3

pre-injection

= one standard deviation

WEST EAST G4W G3W G2W G1W GZERO G2E G3E G4E

0

-10

10

20

30

40

50

60

CHANGE IN GRAVITY, IN MICROGALS

WEST-TO-EAST DISTANCE FROM GRAVITY STATION GZERO, IN FEET

2,000 1,500 1,000 1,000 2,000 500 500 0 1,500

mid-injection

near-completion

of injection
Gravity-Derived Water-level Changes 23

Figure 13. Change in gravity at gravity stations along the south-to-north profile, Lancaster, Antelope Valley, California. 

Location of profile shown in figure 4.

near-completion

of injection

mid-injection

pre-injection

= one standard deviation

SOUTH NORTH G1S G1N G2N G3N G5N GZERO G4S

AG3SG2S

VENUE L

G5S

70

80

0

10

-10

20

30

40

50

60

CHANGE IN GRAVITY, IN MICROGALS

SOUTH-TO-NORTH DISTANCE FROM GRAVITY STATION GZERO, IN FEET

1,000 1,000 2,000 500 500 0 1,500

Injection wells

(one on each side of

GZERO, projected)

Bend in section
24 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

COMPARISON OF GRAVITY-DERIVED 

ACCUMULATED MASS AND INJECTED 

VOLUME

Gravity changes determined from the temporal 

microgravity surveys were analyzed to obtain the 

accumulated mass within the unconfined aquifer during 

the cycle 2 injection period. The accumulated mass was 

reduced to a gravity-derived injection rate and 

compared with the measured injection rate to 

determine if the gravity changes reflect the volumetric 

response to injection.

As outlined by Telford and others (1976, 

p. 85–87), it is possible to determine the total mass of 

any gravity anomaly using Gauss’ theorem. The 

expression for the anomalous mass (M) is given by

, (3)

where

The total sum of gravity change or accumulated 

mass can be obtained by integrating the entire gravity 

anomaly for a region in which it is observed. For this 

study, this was done by triangulating the observation 

points to determine an irregular triangular mesh and 

then integrating the gravity for each triangle by 

assuming a linear variation of gravity between 

measured values on the vertices of each triangle. The 

result of this integration provided the total accumulated 

mass produced by the anomaly. 

γ is the universal gravitational constant,

∆g (x, y) is the gravity change for the surface 

area, and

dxdy is an infinitesimally small surface area.

M 1

2πγ

--------- ∆g x( ) , y

area

∫ ∫ = dxdy

Figure 14. Accumulated mass at the pre-injection, mid-injection, and near-completion-of-injection surveys, Lancaster, Antelope Valley, California.

0

100,000

200,000

300,000

400,000

500,000

600,000

700,000

0 20 40 60 80 100 120 140 160 180

TIME, IN DAYS

ACCUMULATED MASS, IN SHORT TONS

mid-injection

near-completion of injection

Average rate of accumulated mass is 3,437 tons

per day or 570 gallons per minute

Accumulated mass

pre-injection
Discussion of General Applicability and Limitations of Temporal Microgravity Surveys 25

The accumulated mass in terms of short tons of 

water is plotted with respect to time on figure 14. The 

average injection rate derived from the accumulated 

mass over the injection period (157 days) is 3,437 tons 

per day, or 570 gal/min; this is 42 percent of the 

injection rate for the upper aquifer (about 1,350 

gal/min). The difference between the injection rate 

derived from the accumulated mass and the measured 

injection rate suggests that most of the injection mound 

was beyond the areal extent of the gravity network. The 

ground-water flow model results also showed that most 

of the area over which the water-levels changed was 

outside the area of the gravity network (fig.8).

DISCUSSION OF GENERAL APPLICABILITY 

AND LIMITATIONS OF TEMPORAL 

MICROGRAVITY SURVEYS

In an injection scenario, time-series microgravity 

surveys may be effective for determining aquifer 

specific yield, estimating water-table changes, and 

elucidating the areal extent of an injection mound. 

Listed below are some limitations of this geophysical 

technique and considerations for its successful 

application.

(1) At specific yield of 0.10, a 1 mm change in 

gravity-station elevation is the gravimetric equivalent 

of about 73 mm of water-level change (Pool and 

Eychaner, 1995). Vertical control accurate to within 1 

mm is required to rule out, or compensate for, changes 

in elevation.

(2) Microgravity surveys cannot distinguish 

between water-table rise and water added to the 

unsaturated zone above the water table.

(3) Gravity stations used for determining specific 

yield should be sufficiently distant from the injection 

mound to minimize the effect of the irregular injection 

mound geometry on the measured gravity and hence 

the specific yield. Precursory modeling of the hydraulic 

and gravity responses to injection can greatly aid in 

determining the potential for such error, and in 

designing well and gravity networks if the potential 

error is significant.

(4) The depth to the water-table relative to the 

lateral extent of the water-table change can limit the 

application of measured gravity changes for estimating 

water-level changes or accumulated mass. If the depth 

is large relative to the lateral extent of the injection 

mound, the gravity signal would be distributed over a 

broad area of the land surface, which would cause the 

shape of the injection mound to be muted or 

indistinguishable. The ratio of the depth to the mound 

to the lateral extent of the mound should be much less 

than 1. For this study, the ratio of the depth to the 

mound to the simulated radius of influence is about 

0.02. This ratio was adequate for using the gravity 

measurements to detect the shape of the injection 

mound.

(5) The areal extent of gravity measurements for 

determining water-table changes is limited by the 

resolution of the gravity meter used and by the aquifer 

properties. For example, if the minimum resolution of a 

particular meter is 10 μGal and the specific yield of the 

aquifer is 0.13, the minimum detectable water-level 

change is 6 ft (fig. 9). Consequently, the tapering edges 

of an injection mound less than 6 ft thick would be 

undetectable by the gravity meter.

(6) Environmental factors such as proximity to 

sources of vibrations (trains, trucks, and earth moving 

equipment), electromagnetic fields (high-voltage 

power lines, radio broadcasting facilities, and cell 

phones), buried water pipes, and earthquakes can 

render a gravity meter useless.

SUMMARY AND CONCLUSIONS

A preliminary aquifer injection, storage and 

recovery program at Lancaster, California, was 

monitored by the U.S. Geological Survey to evaluate 

the feasibility of artificially recharging the groundwater system through existing production wells. One 

component of this study was to measure the response 

of the water table to injection, which was difficult 

because the water table averaged 300 feet below land 

surface. Rather than install many expensive monitoring 

wells, temporal microgravity surveys were used to 

monitor the water-table response to injection.
26 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

A gravity-station network, consisting of 20 

permanent gravity stations within 1 mile of the 

injection site, was developed to measure the anticipated 

shape of the ground-water mounding around the 

injection wells. Temporal, or time-series, microgravity 

surveys were conducted at the gravity-station network 

to measure small changes in gravitational acceleration 

caused by subsurface changes in mass. In an injection 

scenario, mass, in the form of water, is added to the 

aquifer, and the associated change in gravity is 

measured with a portable gravity meter. A microgravity 

survey was conducted prior to injection to establish 

baseline gravity values for the gravity-station network. 

Subsequent surveys were conducted to monitor the 

accumulation of mass and determine the areal extent of 

the anomalous mass with time. Differential leveling 

was used to assess whether vertical aquifer-system 

deformation contributed to the measured gravity 

changes. Only one gravity station required an 

elevation-change correction, less than 1 microgal, 

showing that the gravity station elevation changes 

cannot account for the measured changes in gravity.

Specific yield was estimated to be 0.13 using 

coupled measurements of gravity and water-level 

change at an existing monitoring well. The gravityderived value of specific yield is consistent with the 

values for this part of the Antelope Valley estimated in 

previous investigations using lithologic well logs and 

laboratory tests of similar materials. The calculated 

specific yield was used to convert the measured 

changes in gravity for the other locations to water-table 

changes.

The gravitational effect of an irregular injection 

mound geometry needs to be considered because the 

non-slab geometry of the injection mound not directly 

beneath a gravity station may contribute to the 

measured vertical component of gravity. To assess the 

gravitational effect of the injection mound on the 

gravity measurements used to calculate specific yield, a 

two-dimensional gravity model was used. The results 

of the gravity simulations showed that the subjacent 

mass of the injection mound had a negligible effect on 

the vertical component of gravity at well 7N/12W34B1 and, hence, on the specific-yield calculation.

Ground-water-level changes were estimated 

using the gravity-derived specific yield and measured 

gravity changes. A simple one-layer, steady-state 

simulation of ground-water flow was used to predict 

the shape of an injection mound assuming the gravityderived specific yield and a range of hydraulic 

conductivities. The gravity-derived water levels 

reasonably match the simulated injection mounds 

along the south-to-north profile for hydraulic 

conductivities 8 to 18 feet per day. Gravity-derived 

water levels for the stations farthest east of the injection 

wells, on the west-to-east profile, were within the range 

of simulated injection mounds for hydraulic 

conductivities 15 to 18 feet per day, but the gravityderived water levels for stations near the injection wells 

(G2E, GZERO, G1W, and G2W) were lower than 

simulated water levels. Possible explanation for the 

discrepancy between the simulated and gravity-derived 

water levels near the injection site may be the 

variability of the aquifer properties (hydraulic 

conductivity and specific yield) and interference from 

environmental factors associated with the injection site 

(vibration from the injection wells, electromagnetic 

fields created from the high-voltage power supply for 

the injection wells, and buried water-supply pipes) that 

may affect gravity measurements. Additional data on 

the aquifer properties and the effect of these 

environmental factors on gravity measurements are 

needed to further explain these discrepancies. Ideally, 

coupled measurements of gravity and water-level 

change would be made at enough locations to 

adequately define the variability in specific yield.

The accumulated mass of the injection mound 

beneath the gravity network was determined using 

Gauss’ theorem. The average injection rate derived 

from the accumulated mass over the injection period is 

3,437 tons per day, or 570 gallon/minute; this is 42 

percent of the injection rate into the upper aquifer 

(about 1,350 gallon/minute). The difference between 

the injection rate derived from the accumulated mass 

and the measured injection rate suggests that most of 

the injection mound was beyond the areal extent of the 

gravity network. The ground-water flow model results 

also showed that most of the area over which the waterlevels changed was outside the area of the gravity 

network.
References Cited 27

REFERENCES CITED

Brenda, W.K., Erd, R.C., and Smith, W.C., 1960, Core logs 

from five test holes near Kramer, California: U.S. 

Geological Survey Bulletin 1045-F, p. 319–393.

Bloyd, R.M., 1967, Water resources of the Antelope Valley–

East Kern Water Agency area, California: U.S. 

Geological Survey Open-File Report 67-20, 73 p.

Bossler, J.D., 1984, Standards and Specifications for 

Geodetic Control Networks: NOAA, National Geodetic 

Information Branch, 31 p.

Carlson, C.S., and Phillips, S.P., 1998, Water-level changes 

(1975–98) in the Antelope Valley ground-water basin, 

California: U.S. Geological Survey Open-File Report 

98-561, 2 sheets.

Dibblee, T.W., Jr., 1967, Areal geology of the western 

Mojave Desert, California: U.S. Geological Survey 

Professional Paper 522, 153 p.

Dobrin, M.B., 1960, Introduction to Geophysical 

Prospecting. Second Edition: New York, McGraw-Hill, 

446 p.

Durbin, T.J., 1978, Calibration of a mathematical model of 

the Antelope Valley ground-water basin, California: 

U.S. Geological Survey Water-Supply Paper 2046, 51 p.

Dutcher, L.C., and Worts, G.F., 1963, Geology, hydrology, 

and water supply of Edwards Air Force Base, Kern 

County, California: U.S. Geological Survey Open-File 

Report 43-05, 225 p.

Fram, M.S., Bergamaschi, B.A., Goodwin, K.D., Fujii, 

Roger, and Clark, J.F., 2003, Processes affecting the 

trihalomethane concentrations associated with the third 

injection, storage, and recovery test at Lancaster, 

Antelope Valley, California, March 1998 through April 

1999: U.S. Geological Survey Water-Resources 

Investigations Report 03-4062, 72 p.

Fram, M.S., Berghouse, J.K., Bergamaschi, B.A., Fujii, 

Roger, Goodwin, K.D. and Clark, J.F., 2002, Waterquality monitoring and studies of the formation and fate 

of trihalomethanes during the third injection, storage, 

and recovery test at Lancaster, Antelope Valley, 

California, March 1998 through April 1999: U.S. 

Geological Survey Open-File Report, 02-102, 48 p.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: 

Englewood Cliffs, N.J., Prentice-Hall, 604 p.

Galloway, D.L., Phillips, S.P., and Ikehara, M.E., 1998a, 

Land subsidence and its relation to past and future 

water supplies in Antelope Valley, California. Land 

Subsidence Case Studies and Current Research, 

Proceedings of the Dr. Joseph F. Poland Symposium, 

Special Publication No. 8, p. 529–539, ISBN:0-89863-

197-1.

———1998b, Land subsidence and its relation to past and 

future water supplies in Antelope Valley, California in

Borchers, J.W., ed., Land Subsidence (1995): Belmont, 

Calif., Association of Engineering Geologists, Special 

Publication, no. 8, p. 529–539.

Galloway, D.L., Hudnut, K.W., Ingebritsen, S.E., Phillips, 

S.P., Peltzer, G., Rogez, F., and Rosen, P.A., 1998c, 

Detection of aquifer system compaction and land 

subsidence using interferometric synthetic aperture 

radar, Antelope Valley, Mojave Desert, California: 

Water Resources Research, vol. 34, no. 10, p. 2573–

2585.

Hanna, W.F., and Sikora, R.F., 1973, Principle facts for 1220 

gravity stations in the Los Angeles 1 degree by 2 degree 

Quadrangle, California: U.S. Geological Survey OpenFile Report 73-099, 31 p.

Ikehara, M.E., and Phillips, S.P., 1994, Determination of 

land subsidence related to ground-water-level declines 

using Global Positioning System and leveling surveys 

in Antelope Valley, Los Angeles and Kern Counties, 

California, 1992: U.S. Geological Survey WaterResources Investigations Report 94-4184, 101 p.

Leighton, D.A., and Phillips, S.P., 2003, Simulation of 

ground-water flow and land subsidence in Antelope 

Valley, California: U.S. Geological Survey WaterResources Investigations Report 03-4016. 107 p.

Lines, G.C., 1996, Ground-water and surface-water relations 

along the Mojave River, Southern California: U.S. 

Geological Survey Water-Resources Investigations 

Report 95-4189, 43 p.

Londquist, C.J., Rewis, D.L., Galloway, D.L., and 

McCaffrey, W.E., 1993, Hydrogeology and land 

subsidence, Edwards Air Force Base, Antelope Valley, 

California, January 1989–December 1991: U.S. 

Geological Survey Water-Resources Investigations 

Report 93-4114, 71 p,

Mabey, D.R., 1960, Gravity survey of the western Mojave 

Desert, California: U.S. Geological Survey Professional 

Paper 316-D, p. 51–73.

McDonald, M.G., and Harbaugh, A.W., 1988, A modular 

three-dimensional finite difference ground-water flow 

model: U.S. Geological Survey Techniques of WaterResources Investigations, book 6, chap. A1., 484 p.

Metzger, L.F., Ikehara, M.E., and Howle, J.F., 2002, Verticaldeformation, water-level, microgravity, geodetic, waterchemistry, and flow rate data collected during injection, 

storage, and recovery tests at Lancaster, Antelope 

Valley, California, September 1995 through September 

1998: U.S. Geological Survey Open-File Report 01-

414, 149 p.

Montgomery, E.L., 1971, Determination of coefficient of 

storage by use of gravity measurements: Ph.D. 

dissertation, University of Arizona, Tucson, 144 p.

Nettleton, L.L., 1976, Gravity and magnetics in oil 

prospecting: New York, McGraw-Hill, 464 p.
28 Determination of Specific Yield and Water-Table Changes Collected During the Second Injection Storage and Recovery Test at Lancaster, CA

Phillips, S.P., Carlson, C.S., Metzger, L.F., Howle, J.F., 

Galloway, D.L., Sneed, Michelle, Ikehara, M.E., 

Hudnut, K.W., Behr, J.A., King, N.E., and Lewis, A.S., 

2003, Analysis of tests of subsurface injection, storage, 

and recovery of freshwater in Lancaster, Antelope 

Valley, California: U.S. Geological Survey WaterResources Investigations Report 03-4061.

Pool, D.R., and Eychaner, J.H., 1995, Measurements of 

aquifer-storage change and specific yield using gravity 

surveys: Ground Water, vol. 33, no. 3, p. 425–432.

Pool, D.R., and Hatch, M., 1991, Gravity response to storage 

change in the vicinity of infiltration basins: Fifth 

Biennial Symposium on Artificial Recharge of 

Groundwater, Tucson, Arizona, May 29–31, 1991, p. 

171.

Sneed, M., and Galloway, D.L., 2000, Aquifer-system 

compaction and land subsidence: Measurements and 

simulations—the Holly site, Edwards Air Force Base, 

Antelope Valley, California: U.S. Geological Survey 

Water-Resources Investigations Report 00-4015, 65 p.

Talwani, M., Worzel, J.L., and Landisman, M., 1959, Rapid 

gravity computations for two-dimensional bodies with 

application to the Mendocino submarine fracture zone: 

Journal Geophysical Research, v. 64, no. 1, p. 49–59.

Telford, W.M., Geldart, L.P., Sheriff, R.E., and Keys, D.A., 

1976, Applied Geophysics: Cambridge University 

Press, 860 p.

Western Regional Climate Center, Southern California 

Climate Summaries, accessed July 10, 1999, at 

<http://www.wrcc.dri.edu/summary/climsmsca.html>.