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October, 1986 Blacksburg, Virginia

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Page 1: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

October, 1986

Blacksburg, Virginia

Page 2: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

I

TIDAL GRAVITY ANOMALIES IN SOUTHEASTERN NORTH AMERICA

by

Dwight Allen Holland

Dr. Edwin S. Robinson, Chairman

(ABSTRACT)

Tidal variations of gravity were measured at fourteen sites in

southeastern North America for periods of between 40 and 199 days.

These measurements were used to obtain tidal gravity anomalies that

indicate the geologic effect of the earth on tidal gravity. The tidal

gravity anomaly is a vector quantity representing the difference between

measured tidal gravity and the theoretical tidal gravity on al

spherically symmetrical earth model subject to ocean tidal loading. The

real part of the anomaly vectors include 8 values in the range of :0.5

microgals, 4 values in the range of 0.5 to 1.5 microgals, 1 value of 1.5

to 2.5 microgals, and 1 other value in the range of -0.5 to -1.5

microgals, This grouping is consistent with a worldwide distribution of

values from regions where the asthenosphere is at intermediate depth,

the stress conditions are not excessive, and geothermal heat flow isE

approximately 60 mw/mz.l

Page 3: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

iii”

ACKNOWLEDGMENTS

Dr. Edwin S. Robinson has given much professional direction and

guidance to this project and his expertise was willingly shared. His

role as advisor and friend has positively shaped my graduate experience.

Ors. Gilbert A. Bollinger and John K. Costain provided critical

review of the manuscript. at the Virginia Division of

Mineral Resources helped a great deal with the data acquisition at the

Charlottesville, Virginia station.

data from Morgantown, West Virginia and Columbia, South Carolina for

analysis.

was a valuable source of help in the drafting of

several figures.

The computer time for this research was paid for by the Department

of Geological Sciences, Virginia Polytechnic Institute and State

University.

iii

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iv

This thesis effort is dedicated

to my grandparents,

along with of Emory and Henry College

with all my thanks.

iv

Page 5: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

TABLE OF CONTENTS

E12ABSTRACT

ACKNOWLEDGMENTS .............................................. iii

LIST OF TABLES ............................................... V1

usr or Fxsumzs .............................................. *11INTRODUCTION ................................................. 1

THE TIDAL GRAVIMETRIC FACTOR ................................. 4

TIDAL GRAVITY MEASUREMENTS ................................... 8

HARMONIC ANALYSIS .....;...................................... 15

THE WORLD OCEAN TIDE ......................................... _ 21

LOADING RESPONSE FUNCTIONS ................................... 27

OCEAN TIDAL LOADING VECTORS .................................. 31

TIDAL GRAVITY ANOMALIES ...................................... 34

DISCUSSION OF RESULTS ........................................ 40

CONCLUSION ................................................... 45

BIBLIOGRAPHY ................................................. 46

APPENDIX ..................................................... 48

VITA ......................................................... 78

v

Page 6: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

LIST OF TABLES

läs Ess1 General Description of Stations .................... 9

2 Gravimeter Calibration for Three Instruments ....... 14

3 Principal Harmonic Constituents .................... 17

4 Observed M2 Parameters ............................. gg

5 Gravimetric and Vertical Displacement Response‘ Functions .......................................... 3Q

6 Theoretical M2 Tidal Loading Vectors ............... 33

7 Tidal Gravity Anomalies ............................ gg

vi

Page 7: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

LIST OF FIGURESl

Lime has1 Geographic Station Positions .................. 3

2 Typical 1 month Tidal Gravimeter Record ....... 11

3 Schwiderski M2 Cotidal-Corange Chart .......... 264 Gravimetric Response Function ................. 29

5 Vector Relationships Used for Analysis ........ 38

6 Real Y vs. Heat Flow for Europe and the world.. 42

7 Heat Flow Map Based upon Silica Geothermo-metry ......................................... 43

8 Three Histograms of Real H*in Different Ranges. 44

vii

Page 8: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

‘INTRODUCTION

Recent tidal gravity studies have suggested that geotectonic

structures differ in their response to the gravitational attraction of

the sun and moon (Yanshin and others, in press). These differences are

indicated by tidal gravity anomalies which may reveal information about

broad areas of lateral inhomogeneity in the earth's asthenosphere and

lithosphere. A tidal gravity anomaly is the vector difference between

tidal gravity measured at a point on the earth and the theoretical tidal

gravity calculated for a spherically symmetrical earth model subject to

ocean tidal loading. .

This study examined the variation of the tidal gravity anomalies

obtained from the lunar semi-diurnal tidal harmonic (M2) in southeastern

North America. Tidal gravimeter measurements were made at the fourteen

sites shown in Figure 1. The M2 tidal harmonic constituent has been

selected for analysis because it is the strongest and best separated

,wave in the tidal spectrum, and the corresponding M2 constituent of the

world ocean tide is thought to be reasonably well understood. The M2

wave is that harmonic constituent of the total tide which would be

produced by an object with a mass equal to the moon traveling in a

circular orbit within the plane of the earth's equator.

Determination of tidal gravity anomalies requires knowledge of the

world ocean tide, the capability to calculate the elastic deformation of

the earth caused by the ocean tide, and careful analysis of tidal

gravity records. A discussion of each of these topics will be presented

1

Page 9: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

· 2

as well as some aspects of the tidal gravity record acquisition and

analysis.

In recent studies of tidal gravity anomalies in Eurasia and Africa,

Yanshin and others (in press) and Melchior and Deßecker (1983)

recognized patterns of variation that could be correlated with geologic

and heat flow provinces. The present study is the first attempt to

identify similar patterns in North America. The results appear to

confirm those found in Eurasia and Africa.

Page 10: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

I I 2

5;; T so' es' 6o° 1s'1o°I

‘~°¤

IL-—--3{ { ·

•%?E

~•

E }" T "‘ Ä ‘°

-..- . W°IWY¢·'i I{ I \N

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'T * "' ‘ °7.¤"“’ KNX}//____ ., M,Ä.„---§-·‘·I Cf.I I I .II1--~

.1 1•T 5 .° I I EG

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' miles

I I

Figure 3. Tide] Gravity Station sites in the southeastern UnitedStates. Letter designations are identified in Tab1e°1.

Page 11: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

THE TIDAL GRAVIMETRIC FACTOR

A gravimeter operated continuously at a site on the earth's surface

detects a cyclic fluctuation which is the tidal variation of gravity.

It is produced by the masses and motions of the moon and sun, and the

elastic response of the earth to the luni-solar tidal force. A measure

l of this elastic response is given by a term called the tidal gravimetric

jagtgg. This term is the ratio of the measured tidal variation of

gravity at a point on the earth and the corresponding theoretical

variation on a rigid globe predicted from the astronomy of the

earth-moon-sun system.

It is conventional to express the cyclic tidal variation of gravity

at some location as a series of harmonic constituents. Methods of

harmonic analysis for calculating the amplitudes and phase angles of

these constituents from a tidal gravity record are discussed later. The

common practice in tidal gravity studies is to determine values of the

tidal gravimetric factor separately from the amplitudes of different·

harmonic constituents.

The physical significance of the tidal gravity factor can be

described as follows. A point on the earth experiences four effects

that contribute to the measured·tidal variation of gravity. These

effects are:

1. The direct gravitational attraction of the sun and moon;

2. Deformation of the solid earth caused by the lunisolar

gravity;

4 .

Page 12: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

5

3. The vertical displacement of a point due to the change in

the shape of the earth; and

4. Ocean tidal loading perturbations.

Neglecting for the moment the perturbation induced by ocean

loading, the tidal gravimetric potential (W) at a point on the earth's

surface may be written (Garland, 1971):

H = Ht + wm - gdN

where Ht is the lunisolar potential, Wm is the potential related to the

tidal change in the shape of the earth, gdN is the potential related to

a vertical displacement dN at the point, and g is the gravitational

attraction at a point due to the earth's mass. Wm is the potential of

the mass of the tidal bulge produced in response to the tide-producing

forces of the sun and moon. The quantity gdN is the potential change

·due to the variation in the vertical position of the observation point

also known as the free air effect. The potentials Hm and gdN are

assumed to be proportional to Ht:

Wm = kwt and

gdN = hHt

where the constants of proportionality h and k are Love‘s numbers

(Melchior, 1966). Therefore, the tidal potential can be written in

Page 13: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

,

6 T

terms of Love numbers as‘

w¤wt(1+k-h)

The tidal variation of gravity (dg) in terms of Love numbers is then

obtained by differentiating this potential:

dg. ä‘;-%.1@.¤BR 3R BR

where R is the radius of the earth.

The tidal potential caused by a body of mass m can be represented

(Melchior, 1966) as ·

w = GMR2(3 cosz 6 - 1)’¢ TT2r

.where 6 is the time varying angular position of the heavenly body

relative to the zenith of the observer, with r as the distance between

the center of the earth and the heavenly body, and m and R are the

earth's mass and radius. Differentiation shows that:

awt awtER- =° T °

The term ggü is the attraction of mass redistribution needed to changethe earth's shape. It can be represented by a surface density function

Page 14: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

7 .

given by a surface harmonic series. By differentiating this series termw 3w• •

m = -m

•by term, it is found that ER. ER. . The vertical gradient ofgravity is, gg. = - ä., as can be shown by differentiating Newton's

Universal Law of Gravitation. Making these substitutions, tidal gravityZw 3W, _ __; m _ 2 dN =at a point on Sue earth is found to be dg R + T .9R.

(1-3/2 k + h) ääi . But on a perfectly rigid body h = k = 0 so that theBW

tidal gravity would be dgt =.äRE. Therefore, the gravimetric factor is

6= dg/d9t=1-3/2k+h.

For an idealized model consisting of concentric spherical shells

with elastic properties corresponding to those of the principal zones of

the earth the values of the Love numbers_are close to k = 0.61 and h =

0.30. Therefore, the tidal gravimetric factor is close to 1.16 (Alsop

and Kuo, 1964). Differences between this and measured values of the

tidal gravity factor can be attributed to effects of ocean tidal

loading, which are quite well understood, and to inhomogenieties

presumably in the lithosphere and asthenosphere whose effects are poorly

understood. The purpose of this study is to detect evidence of these

latter effects. To accomplish this, adjustments must first be made to

account for the former effects. -

Page 15: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

TIDAL GRAVITY MEASUREMENTS

Tidal gravity measurements have been made at 14 stations in the

southeastern United States with 5 different Geodynamics gravity meters.

Ten of the sites included in this study were described by Robinson

(1974). This initial tidal gravity survey has been extended by more

recent measurements of approximately three and six months duration in

Morgantown, West Virginia, and Charlottesville, Virginia, respectively.

Additional six-month records were obtained in Blacksburg, Virginia and

Columbia, South Carolina. Also, a three-month record obtained by Wilson

(1978) at Bay St. Louis, Mississippi and a two·month record from

Knoxville, Tennessee are included in this study. Station locations,

instruments and record lengths are given in Table 1. Typical tidal

gravimeter records are reproduced in Figure 2.

The geodynamics tidal gravimeter is a modified North American

exploration gravimeter secured in an air-tight aluminum pressure case.

This instrument is designed for long-term recording with low drift

rates. The air-tight pressure case is enclosed inside an insulated

wooden case with the manufacturer stating that temperature control on

the functional part of the instrument is within 0.005 degrees C near an‘

optimum ambient temperature of 36°C. Internal temperature control,

important for meter stability, is maintained by an AC thermister bridge

directing the inner and outer heating coils. ~Gravitational field changes are sensed by monitoring the beamv

position in the gravimeter by a differential capacitor transducer. This

8

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Page 19: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

12

signal is converted to a voltage which is proportional to the beam

position. The voltage is then filtered and transmitted to the chart

recorder. The analog chart is then digitized at 1 hour intervals for

subsequent analysis. The relative accuracy of values read from the

chart is generally better than 1 1 microgal.

Each Geodynamics tidal gravimeter is equipped with an internal

calibration system. It consists of a plate mounted near the beam, and a

circuit for applying a precisely controlled voltage to this plate. when

activated, it deflects the beam by an increment corresponding to a fixed

change in gravity. The value of this equivalent change in gravity can

be determined by operating the tidal gravimeter at a location where the

gravity field is varying by a known magnitude.

In some continental regions it can be argued that ocean loading and

anomalous geologic effects are small enough to neglect for purposes of

instrument calibration. By methods presented later, it can be shown

that the ocean loading effect on the lunar diurnal harmonic constituent

01 is very small in Blacksburg, Virginia. Because this site is in a

region of normal heat flow, the anomalous geologic effect should also be

quite small. Therefore, the tidal gravity factor found from thel

01 harmonic constituent should be 1.160 1 .005 at the Blacksburg,

Virginia observation site. The product of this value and the 01

constituent at the same location on a rigid earth determined from the

formulas given by Longman (1959) is the basis for the tidal gravimeter

calibration used in this study. The value of this product is 34.7

microgals for Blacksburg, Virginia.

Page 20: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

13

All of the tidal gravimeters were operated at the Blacksburg,

Virginia site for calibration purposes. Using the record from each

instrment the amplitude of the internal calibration signal on the

analog chart was compared with the amplitude of the 01 constituent found

from the harmonic analysis. By adjusting the 01 constituent amplitude

to the known value of 34.7 microgals, the change in gravity equivalent

to the internal calibration signal was determined. Results are given in

Table 2 for three of the five gravimeters utilized in this study.

Weekly calibration signals were produced on each record for

detecting and adjusting any changes in instrument response. These

changes can result from different factors such as the slow tilting of

‘ the instrument from level orientation.

The North American Geodynamics gravimeters used for this study are

known to exhibit very complex characteristics resulting in a small

non-zero phase response to a given driving constituent. The simplest

rheological model that one might choose for Geodynamics gravimeters is a

two-parameter model. This model consists of the combination of a Hooke

and Kelvin body with one associated parameter to be determined for each

body. Ducarme (1975a) determined these parameters for 8 Geodynamics

gravimeters. Phase lags from 0.74 to 2.05 degrees for the M2A

constituent were measured with an average for all instruments of 1.28

degrees. Instruments used in this study are similar to those tested by

Ducarme. For lack of more direct evidence the phase shifting properties

of these instruments were assmed to be in the range given above.

Page 21: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

14

TABLE 2

_ GRAVIMETER CALIBRATION PULSE GRAVITY CHANGE EQUIVALENTS ‘

Instrument Calibration Pulse Eguivalent

826 82.2 ugals

735 133.0 ugals

VPI-17 126.0 ugals

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HARMONIC ANALYSIS

. The tidal gravity spectrum consists of more than 1,000 harmonic

constituents with periods known from the astronomy of the earth·moon-sun

system. Fewer than 12 of these constituents account for more than 95

percent of the strength of the tidal gravity field. Amplitudes and

phase angles of the harmonic constituents in Table 3 are of principal

interest for most studies of tidal gravity.

The method of Venedikov (1967) was used for harmonic analysis of

the tidal gravity records included in this study. This is the standard

method which has been adopted by the International Center for Earth

Tides, Brussels, Belgium. FORTRAN programs for implementing this

procedure are described by Venedikov and Paquet (1967) and Ducarme

(1975b). These programs compute amplitudes and phase angles of the

principal tidal harmonic constituents, and the corresponding tidal

gravity factor.

The Venedikov method begins with the separation of diurnal,

semi·diurnal, and ter-diurnal tidal constituents, and the elimination of

instrumental drift and very long-period constituents. For a given

· Species of tides, even and odd filters are applied on a sequence of 48

hourly readings. The diurnal filters have amplification factors that

are designed to accentuate the diurnal waves and eliminate the

semi-diurnal ones. The reverse is true for the amplification of the

semi—diurnal waves (Melchior, 1983).

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The filters are constructed mathematically in such a way that they

eliminate the instrumental drift as an arbitrary combination of

orthogonal polynomials of order three. The filters themselves are

derived from matrix considerations involving the evaluation of

observational errors and particular constituent amplification needs.

The filters are applied to every continuous 48 hour record segment and

then the analysis is shifted to the next 48 hours, etc., thus ensuring

independent results and non-superposition of calculation effects. If a

gap occurs in the record, the filters are re-evaluated at the next 48V

hours data segment.

Next, the Venedikov filter acts on the 48 hour sequence of obser-

vations, producing a filtered number which is further processed as an_

observed quantity. A system of harmonic equations is then generated

such that through a least square process the unknown tidal parameters of

amplitude and phase are found from the solution of the system. The

system of equations to be solved has the mathematical representation of

. the observational equations as well as the theoretical equations needed

to represent the lunisolar tide on a rigid body. Instrumental drift is

eliminated by the Venedikov filters because the drift is considered

noise with a frequency lower than that of the diurnal tidal band.

In the process of solving the observational equations root mean

square (rms) errors are determined for each harmonic constituent. These

values are combined to obtain the rms errors for the combined diurnal

.constituents and the combined semi-diurnal constituents.‘ These rms

error estimates provide a basis for evaluating the record quality.

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17

TABLE 3

PRINCIPAL TIDAL HARMONIC CONSTITUENTS -(Schureman, 1958)

Symbol Name Period Qhr)

M2 - Principal Lunar 12.421

S2‘

Principal Solar 12.000

N2 Lunar ellipticity 12.658

K2 « Lunisolar 11.967 _K1 Lunisolar 24.934

01 Lunar declination 25.819

P1 Solar declination 24.066

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18

The original FORTRAN program of Venedikov and Paquet (1967)

included 79 constituents in the harmonic series used to represent the

luni-solar field. The program has since been upgraded by Ducarme

(1975b) who introduced the harmonic development of Cartwright. _. .

and Edden (1973) which consists of 347 constituents. Harmonic analysis

of the new tidal gravity records presented in this study, and thosek

obtained by Robinson (1974) were done with the upgraded program as well

as with the older program. Differences in the results of the two

programs are small enough to neglect for the purpose of this study.

Harmonic analysis of the individual records proceeded in the

following way. First, a moving window analysis was done to assess·

variation in record quality. This involved separate harmonic analysis

of one·month segments, advancing at weekly intervals through the record.

The rms statistics provided the basis for judging record quality.

Record segments of superior quality are indicated by combined rms error

values of less than 5 microgals for the diurnal or semi·diurnal groups

of constituents. Values between 5 and 10 microgals are considered

acceptable but those above 10 microgals are judged to indicate record

segments of poor quality.

By means of moving window analysis it was possible to locate short .

° segments of record, less than a week long, of poor record quality which

otherwise degraded superior or acceptable records. These segments were

then deleted, and the harmonic analysis was done on the remaining record.

Final results from the records edited in this way are in Appendix I.

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Factors contributing to poor record quality include earthquake

disturbances and unusually high microseismicity, power failures which

lead to internal temperature change in the gravimeter, unexplained

responses of the instrument to sudden changes in atmospheric pressure,

and building or ground disturbances leading to instrument level changes.

Often, these factors may not be noted at the time of occurrence, and

come to attention later during record processing.

Although 15 constituents are tested in the harmonic analysis, only

the results for the lunar semi-diurnal constituents M2 were used in thisstudy. In the study area, this constituent has the largest amplitude,

and is the best separated from the other constituents of the harmonic

series.

M2 amplitudes, phase angles and corresponding tidal gravity factors

for the fourteen observation sites are given in Table 4. The measured

tidal gravity factors are seen to differ from a value of 1.16, indi-

cating that the combined effect of geology and ocean tidal loading on

tidal gravity can be measured.

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. 20

TABLE 4

OBSERVEO M2 TIOAL GRAVIMETRIC FACTORS (Ä)

Phase (degree)Station Ampliitude Amplitude (not corrected +

Designation City, State (ugals) (6) 1.3 degrees forinstrument response)

*A Atlanta, GA 60.35 2 .24 1.160 2 .005 -4.01 2 .20

B · Blacksburg, VA 56.68 2 .09 1.1906 2 .0019 -2.074 2 .092

*BSL Bay St. Louis, MS 65.83 2 .17 1.184 2 .0035 -1.823 2 ?

C Columbia, SC 61.87 2 .24 1.1990 2 .0047 -1.559 2 .223

**0 Durham, NC 56.74 2 .32 1.920 2 .0067 -3.01 2 .32

**H Huntington, HV 52.60 2 .17 1.847 2 .0036 -1.57 2 .18

*J Jackson, MS 61.88 2 .48 1.160 2 .010 -0.2 2 .41

*KNX Knoxville, TN 55.29 2 .29 1.1668 2 .005 -1.67 2 .25

*M Memphis, TN 58.13 2 .29 1.163 2 .006 -0.2 2 .25

*R Ruston, LA 61.23 2 .29 1.153 2 .006 -3.4 2 .04

**T Tuscaloosa, AL 59.71 2 .39 1.1686 2 .0083 -0.28 2 .41

UVA Charlottesvil1e, VA 55.52 2 .18 1.1920 2 .0039 -1.18 2 .188

*N wooster, OH 50.03 2 .39 1.169 2 .0037 --·--

UVA Morgantown, HV 55.29 2 .24 1.2098 2 .0083 -1.208 2 .389

t A Phase angle of zero was assumed when no phase angle information was available.

* Older Robinson data and earlier Venedikov software.

** Older Robinson data processed by more recent Venedikov/Oucarme software.

Stations not labeled by an asterisk include new data and the latest Venedikov/Oucarme software.

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THE WORLD OCEAN TIDE

The world ocean tide constitutes a temporally and spatially varying

load on the solid surface of the earth. This fluctuating load deforms

the elastic earth and perturbs the tidal variation of gravity. The

perturbation of the gravity field due to ocean tidal loading has long

been known to extend over continental areas (Kuo, 1970). If the

geologic effect is to be evaluated at a tidal gravity observation site,

it is first necessary to determine the effect of ocean tidal loading.

This can be accomplished by knowledge of:

1. The form of the ocean tide, and ·'

2. The elastic response of the earth to a surface load.

The form of the ocean tide can be described in terms of tidal

variations of sea level at points on a grid that extends over the entire

ocean. At each grid point the water level fluctuation can be resolved

into the harmonic constituents previously discussed. Direct measurement

_of the ocean tide is restricted mostly to coastal areas where tide

gauges can be operated. Very few deep water measurements have been made

because of the high cost and difficulties in operating the special

instruments required for this purpose. Knowledge of the ocean tide

comes largely from numerical solutions of a set of three partial

differential equations known as the Laplace Tidal Equations.

The basic Laplace Tidal Equations express the tidal motion in an

ocean of homogeneous, frictionless, incompressible water contained in a

rigid ocean basin with fixed boundaries. They relate the height of the

-21

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22

tide (h) and the east (u) and north (v) tidal current velocity

components to ocean depth (z) which varies with colatitude (6) and

· longitude (A), the earth's angular rotation velocity (Q), radius R, and

gravity (9).

These equations can be written (Doodson, 1958) in the form:

%%·2s°zv cos9=%%

9V -· .....1 .9*L "ZQU C°S 6 °

_é1Slnö 81

1 .9. - 9 .-ün--....6 [3. <=··=‘··@>+ QT <=v>] Bt

where h' = h - h and h is the equilibrium tide elevations. Because h, u,

and v vary periodically, they can be expressed as

h = H cos hat - wh)

u = U cos hat - wu)

v =_V cos (ct - wv)

where ¤ and W are the angular frequency and phase angles of a tidal

harmonic constituent and H, U and V are the constituent amplitudes.

Efforts to solve the Laplace Tidal Equations to determine h, Wh, u

Wu, v, and WV are summarized by Schwiderski (1980). Early work con-

sisted of obtaining analytica] solutions for ocean basins of idealized

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23

form. Application to the real ocean required the high speed computer

and bathymetric data that were not available until the 1960's.

Beginning then, finite difference methods and other schemes were

developed to solve first the basic equations given above, and then to

modify these equations taking into consideration friction, compressi-

bility, and non-fixed boundaries for the world ocean. Solutions

obtained by various workers revealed the main features of the ocean

tide. However, the ocean loading effects calculated by Robinson (1974)

from these various solutions were sufficiently different from one

another to preclude their use in adjusting tidal gravity records for the

purpose of subsequent identification of a geologic effect.

During the 1970's, improvements in the numerical technique and

supplementary bathymetric data made it possible to improve the accuracy

of solutions to the Laplace Tidal Equations. There is now a growing

consensus that results obtained by Schwiderski (1979) for the M2 consti-

tuent of the ocean tide are accurate enough to make the adjustments

required to begin the search for the geologic effect. These results

consist of amplitude and phase angles, relative to the Greenwich meri-

dian, specified at points on a 1 degree by 1 degree latitude and longi—

tude grid extending over the world ocean. These data are available on" digital magnetic tape from the Naval Surface Weapons Center, Dahlgren,

Virginia.

Numerical solutions to the Laplace Tidal Equations can be displayed

graphically by means of cotidal-coamplitude charts. The cotidal-coampli—

tude chart of Schwiderski (1980) shown in Figure 3 illustrates the time

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24

and spatial variations of the M2 ocean tide. Cotidal and coamplitude

lines shown on the maps contain the equal phase values and tidal

amplitudes, respectively. By convention, the time of high tidal

occurrence is measured with reference to the Greenwich meridian. A

cotidal line of 3 hours connects those grid points where high tide

occurs three hours after the moon has passed directly above the

Greenwich meridian. Cotidal lines can be expressed in terms of degrees,

since for example, the tidal crest of a semi-diurnal wave moves by

approximately 30° per hour.

The tide can be viewed as a very long wave moving on the ocean.

Cotidal lines show the position of the wave crest at different times.

The points around which the tidal movement is centered are called

amphidromic points. Amphidromic points have a tidal amplitude of kero,

and exhibit a tidal crest rotating about them generally counterclockwise

in the northern hemisphere with clockwise flow rotation in the southern

hemisphere.

The tide model in Figure 3 does not include landlocked bodies of

water such as the Great Lakes and the Black and Dead Seas. Other

gridwise disconnected bordering waters not included are such bodies of

water as the Baltic, Irish, Mediterranean, Red, and Japan Seas. Gulfs,

such as those of California and Persia, as well as the Hudson and Korean

Bays are likewise not included. Similarly, many shallow (less than 5

meters in depth) and/or narrow waters such as the entire Barrier Reef

area, the Gironde Estuary, and the Fjords of Norway could not be

modeled. The combined effect of ocean tides in these waters has a _

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25 _

negligible effect on tidal variations of gravity except at nearby

observations sites.

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Ä:.g

äg

ää

a

—··

EI!&

g‘E

i§§

§--

S;

I‘·

Qämüflä

—~

I

:2

\gi

gw

l Avééääläää”'g?’mag%§@%m‘§g

gggg

Äf

Ö

EE

gigég„ä

ää

:

äéwaä

ä;

•= I

E :6g

8GJ 5;

66

’2 E

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LOADING RESPONSE FUNCTIONS

Calculations of the effect of the ocean tide on tidal gravity

requires knowledge about how the earth responds to surface loads. It is

well known that a unit load acting at a point on the surface of a solid

body produces elastic deformation throughout the body. For the case of

a semi-infinite homogeneous body the elastic displacement Ü (r, 2) of a

point at distance r and depth 2 relative to the load is expressed in the

equilibrium equation

++ + .,, + -> _

(>„+2u)VV°S—pVxVxS=0

where A and u are Lame's constants. This is the classic Boussinesq

problem which has the solution (Farrell, 1972)7

u(rz)=—l—•4nuR 7.+},1 E2

2_ 1 2 A + r Z .1+ R- +liligf.

+ 2where u and v are the radial and vertical components of S, and R2 = r +

22. These expressions indicate that the elastic response diminishes in

approximate proportion with inverse distance from the load.

T 27

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28

A much more difficult problem to solve is the response of a

self·gravitating sphere consisting of concentric homogeneous elastic

shells to a unit point load. Farrell (1972) reviews this problem in -

some detail, and presents series solutions for the radial and vertical

components of displacement and strain in the body. He combines these

results to obtain a Green's function that expresses the perturbation of

gravity produced at any point on the sphere by a load consisting of a

unit mass acting on a unit surface area. Such a Green's function

accounts for:

1. The gravitational attraction of the load,

2. The change in gravity resulting from elastic deformation

in the shape of the original sphere, andu

3. The change in gravity related to vertical displacement '

at any point on the sphere.

A gravimetric Green's function suitable for the purposes of this

study was calculated by Farrell (1972) using the elastic proportion of

the idealized earth model number 8734/06/06/68 which was obtained by

Backus and Gilbert (1970). This function is illustrated in Figure 4,

and numerical values for different geocentric distances are listed in

Table 5.

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° 29

10-22

ZQ .E 10-2*< .¤¤izDAEäu.: ¤¤

{ Negative

O 10'zS Positive

Negative

1 2 5 10 20 50 100 200

· GEOCENTRIC ANGLE(degrees)

Figure 4: Gravimetric response function for solid earth model 8734/06/06/08 (Backus and Gilbert, 1970) calculated by N. E.Farrell (1972). Amplitude indicates gravity perturbationresulting from application of a 1 gram load.

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TABLE 5

GRAVIMETRIC AND VERTICAL DISPLACEMENT RESPONSE FUNCTIONS(Farrell, 1972)

Geocentric Gravimetric Response Vertical DisplacementDistance Function Response Function(degrees) (gals) (cm)

1.0 -2.698 E-23 -1.243 E-171.2 ·-2.092 E-23 -9.573 E-181.6 -1.381 E-23 -6.246 E-182.0 -9.098 E-24 -4.451 E-182.5 -7.023 E-24 T -3.145 E-183.0 -5.227 E-24 -2.342 E-184.0 -3.165 E-24 — -1.425 E-185.0 -2.083 E-24 -9.486 E-196.0 -1.463 E-24 -6.789 E-197.0 -1.086 E-24 -5.168 E-198.0 -8.424 E-25 -4.130 E-199.0 -6.755 E-25 -3.421 E-19

10.0 -5.549 E-25 -2.907 E-1912.0 -3.927 E-25 -2.200 E-1916.0 -2.128 E-25 -1.365 E-1920.0 -1.149 E-25 -8.621 E-2025.0 -4.298 E-26 -4.484 E-2030.0 1.148 E-27 -1.167 E-20

- 40.0 4.518 E-26 1.937 E-2050.0 5.911 E-26 3.677 E-2060.0 5.504 E-26 4.164 E-2070.0 4.088 E-26 3.802 E-2080.0 2.147 E-26 2.899 E-2090.0 -1.240 E-29 1.694 E-20

100.0 -2.141 E-26 3.594 E-21110.0 -4.156 E-26 -9.825 E-21120.0 -5.970 E-26 -2.250 E-20130.0 -7.546 E-26 -3.385 E-20140.0 -8.849 E-26 -4.347 E-20150.0 -9.863 E-26 -5.110 E-20160.0 -1.057 E-25 -5.654 E-20170.0 -1.097 E-25 -5.968 E-20180.0 -1.105 E-25 -6.045 E-20

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OCEAN TIDAL LOADING VECTORS

The perturbation of tidal gravity caused by the M2 harmonic

constituent of the ocean tide was calculated for each of the 14

observational sites. The M2 constituent of the ocean tide wasrepresented by the amplitude and phase angles obtained by Schwiderski

(1978) for points on a grid covering the world ocean. Each pair of

amplitude and phase angle values describes a vector which represents a

time varying load acting within a particular grid cell. For purposes of

computation, the real and imaginary parts of these vectors were

calculated from the corresponding amplitudes and phase angles.

Real and imaginary parts of the perturbation of tidal gravity at an

observation site were then obtained by convolution of the gravimetric

Green's function, described in Figure 4 and Table 5 and the real and

imaginary arrays representing the global M2 ocean tidal constituent.

More specifically, each grid cell of an array, bounded by latitude and

longitude coordinates occupies the area increment

dA = R2 smeaean

where R is the earth radius and d6, dp represent the 1° x 1° spacing of

latitude and longitude lines on the worldwide grid, with 6 as the

latitude of the center of the area increment. The load acting on dA of

the earth's surface is the weight of water in a volume equal to

the amplitude of the tidal constituent in that grid cell multiplied by

31

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32

the area increment. The weight of water for each area increment on the

world ocean was calculated and assumed to be acting at a point on the

center of each of the 1° x 1° grid cells.

The individual perturbation to the earth's gravity field at an

observation site caused by a specific load in a grid cell was found by

multiplying real and imaginary parts of the load and the Green's

function value corresponding to the geocentric distance of the cell from

the site being considered. Summation separately of all of the real and

imaginary parts from the world ocean grid yields the total ocean tide

induced gravity perturbation vector at a given site, amplitude and phase

h angle of this vector were then found from the final summation of the

real and imaginary parts. The results of these computations are listed

in Table 6 for each of the 14 stations occupied.

In Table 6 two phase angles are given for each site. Because the

phase angles given by Schwiderski (1978) for the M2 ocean tidal

constituent are referenced to the Greenwich meridian, convolution of the

M2 ocean tidal arrays with the gravimetric Green's function yields phase

angles for the ocean loading perturbations of tidal gravity that are

also referenced to this prime meridian. To obtain the local phase angle

referenced to the longitude of each site the value of 2A was added to

° the Greenwich phase angle corresponding to that site. The factor 2 is

used because the M2 constituent is semi-diurnal.

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TABLE 6

THEORETICAL M2 TIDAL LOADING

VECTORSGreenwich Amplitude LocalStation Amplitude Phase (gravity Phase

Designation City, State (gal) (degrees) factor) (degrees)

A Atlanta. GA 0.636x10°6 160° 1.1705 - .31

*8 Blacksburg, VA 0.993x10°6 165° 1.1773 · .57

BSL Bay St. Louis, MS 0.239x10'6 111° 1.1615 · .20

*C Columbia, SC 1.287x10°6 170° 1.1820 · .57*0 " Durham, NC 1.468x10°6 169° 1.1858 · .82

*H Huntington, HV 0.701x10°6 160° 1.1726 - .43

J Jackson, MS 0.245x10°6 121° 1.1624 359.81

*KNX Knoxville, TN 0.471x10°6 152° 1.1681 · .29

M Memphis, TN 0.275x10°6 133° 1.1637 359.80

R RuSt0n, LA 0.202x10°6 104° 1.1612 359.82

*T Tuscaloosa, AL 0.352x10°6 143° 1.1652 - .23

*UVA Charlottesville, VA 1.261x10°6 167° 1.1821 · .76

*UVA Morgantown, HV 0.889x10°6 164° 1.1763 - .57

* Utilizes updated Melchior potential development.

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TIDAL GRAVITY ANOMALIES

The object of this study is to find indications of a geologic

effect on the tidal variation of gravity. Following Melchior and others

(1981) the search for such indications involves comparison of measured

tidal gravity with theoretical tidal gravity on an idealized earth model

subjected to ocean tidal loading. The resulting discrepancies must be

related to differences between the earth and the idealized model.

At a tidal gravity observation site the discrepancy ;Ibetween

measured tidal gravity ÄIand the theoretical tidal gravity T is:

+ + ->X=A-T „

where the theoretical value is found from the tidal gravity Ü on an

idealized earth model and the perturbation Ü produced by ocean tidal

loading:

The relationship between the vectors is illustrated in Figure 5.

Tidal gravity on an idealized elastic model is given by the product

of the luni-solar tidal force FIand the tidal gravimetric factor of the

model: I+ ->°

·. R=6F

34

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35

For the M2 harmonic constituent the tidal force is given to a

first-order approximation (Melchior, 1966) by the expression:

FM2 Q 74.702 cosz W microgals

where W is the latitude. From a review of the properties of spherically

symmetrical models most closely representing the earth, Melchior and

DeBecker (1983) concluded that the tidal gravimetric factor corres-

ponding to the M2 constituent must be close to 1.160, and that the

response can be treated as being in phase with the tidal force (wahr,

1982). _

Therefore, the real part of the vector RM2 can be expressed to the

first order as:

real RM = 1.160 x 74.702 cosz W2

.and the imaginary part of RM is

imaginary RM2 = 0

Using these expressions together with values from Table 4 for the

vector El and values from Table 6 for the vector K, components of the

anomaly vector were calculated for all the observation sites. The

results are in Table 7. These vectors represent the tidal gravity

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36

anomaly at each site. They should provide some indication of a geologic

effect on tidal gravity, if such an effect can be detected.

Phase shifts introduced by the tidal gravimeters were initially not

taken into consideration in the calculations. The non-phase corrected

values are designated ;' in Table 7. Based upon tests of similar

instruments reported by Ducarme (1975) an instrument phase shift of

-1.3° was arbitrarily assumed for the five gravimeters used in this

study, with an estimated uncertainty of :0.7°. This assumed instrument

phase shift was then subtracted from the tidal gravity phase angles

given in Table 4 to obtain values subsequently used to calculate a phase

_ corrected tidal gravity anomaly Y for each site. These results are also

included in Table 7.

The accuracy of the values found for Y depends upon the precision

of K, corrected for instrument phase shift, and the precision of Ü.

Differences in tidal gravity measurements made with the four gravimeters

at the same location in Blacksburg, Virginia indicate that the amplitude

of Ä is accurate to within : 1 microgal. Judging from these measure-

ments and those reported for other instruments by Ducarme (1975), the

phase angles adjusted for instrumental effects are accurate within

:0.7°. No attempt was made to assess the absolute accuracy of Ü, which

was calculated from the ocean tidal data of Schwiderski (1978). Because

the same data were used to determine E for each site, relative errors

between sites should be quite small and systematic, and should not

significantly distort any regional patterns of variation in Y within the

study area. The uncertainties in values of ifcan be estimated from the

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37

error ranges for the amplitudes and instrument corrected phase angles

of

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38

* BT{

a'r or T

A L

F

Ä;observed Mz vector }A: amplitude, az phase}.

lätidal vector for an elastic non-viscous earth with liquid core but oceanless }R: amplitude, zerophase}.

L-;ocean attraction and loading vector }L: amplitude, 7: phase}.

Tl'•=theoretical tidal gravity with ocean loading effects added }T: amplitude, 7:phase}.

geologic effect |X: amplitude, X: phase}.

Z=error envelope of observed M, vector.

The correct scale of this figure for the southeastern North America M: wave is approximately:

_l€·:Ä~?¤ 50-67 microgal Y= 0-3.5 microgal g

Ta 0.2-1.5 microgal Z = ; 1°; ;>_ 1.0 microgal

rr = 0-4° phase lag

Figure 5: Definitions and notati ons for the construction *0f the tidalgravi ty anomaly vector.

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39

TABLE 7

TIDAL GRAVITY ANOMALIES (X)

X ( al, degree) r (ugal)-

XcosxPhase (Q) No Phase (X‘) Phase (r) No Phase (r')

Station City, State Corrected Correction Corrected Correction

A Atlanta, GAE

2.37; 104° 3.61; 100° - .57 -.63

*8 Blacksburg, VA .67; 18° 1.61; 68° .64 .60

BSL Bay St. Louis, MS 1.14; 17° 1.90; 57° 1.09 1.03

*C Columbia, sc. .88; -21° 1.27; 51° .82 .80A

*0 Durham, NC .93; 73° 2.23; 84° .27 .23

*H Huntington, HV .59; -15° 1.25; 64° .57 .55

J Jackson, MS 1.31; 259° .42; 107° .25 .12

*KNX Knoxville, TN 1.34; 94° .01; 129° .09 -.006E

M Memphis, TN 1.23; 268° .03; 180° -.04 -.03

R Ruston, LA 2.23; 101° 3.47; 98° -.43 _ .48

*T Tuscaloosa, AL 1.22; -83° .17; 16° .15 .16

*UVA Charlottesville, VA .79; -53° .62; 42° .48 .46

H Hooster, OH 1.71; 264° .47; 249° -.18 -.17

*HVA Morgantown, WV 1.62; -22° 1.71; 23° 1.59 1.57

* Stations with potential derived empirically for Melchior results and inputedin ocean-loading program.

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‘ DISCUSSION OF RESULTS

Regional geologic effects on tidal gravity have only recently been

recognized. Yanshin and others (in press) and Melchior and Deßecker

(1983) suggest patterns of regional variation that are evident from

their analyses of tidal gravity measurement at 178 sites in Eurasia and

Africa. At most locations they found the tidal gravity anomaly

amplitudes to be on the order of one microgal, as are the results in

Table 7 for this study. This is close to the measuring precision of

most tidal gravimeters and the precision to which ocean tidal loading

effects can be determined. Therefore, they suggest the most practical

way to establish meaningful correlations is by means of grouping and

averaging values from many sites rather than attempting to interpret

individual values.l

Yanshin and others (in press) obtained their clearest correlations

between tidal gravity anomalies and geotectonic environment from

.analysis of the real part of the vector, real Y. They recognized a

tendency for relatively large positive values of real Y to occur in

geotectonic environments characterized by a shallow asthenosphere and a

tensile stress state. Here the earth could be viewed as more mobile and

responsive to the tidal gravity force. Further, they noted that large

negative values of real Z tend to occur in geotectonic environments

where the asthenosphere is relatively deep and a compressive state of

stress exists. Small values of real Zloccur in environments inter-

mediate to these two extremes.”_

40 -7

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41

The clearest confirmation of this global pattern found by Yanshin

and others (in press) is the correlation between real ; and heat flow.

Values of real ; from sites where reliable heat flow data were available

are plotted in Figure 6. The results indicate a relationship between

the tidal gravity anomaly and heat flow.

The fourteen sites available for this study are too few to justify

an attempt at recognizing tidal gravity anomaly patterns within the

study area. However, treated as a single group, the values in Table 7

are found to be consistent with the global pattern of Yanshin and others

(in press). Regional heat flow in the area, estimated by the method of

silica geothermometry (Sass and others, 1981), is consistent with

conventional heat flow measurement patterns shown in Figure 7. Judging

from the contours over the study area, the heat flow at all of the sites

appears to be in the range of 1.0 to 1.6 HFU, or 42 to 67 E; . The

squares in Figure 6 encompass this range, showing that the relationship

between real Z and heat flow in the study area falls within the global

pattern.

Histograms in Figure 8 show the distribution of real { values in

different ranges. The distribution without regard to sign, lreal ; I,

is similar to that of Yanshin and others (in press). The distribution _

with regard to sign indicates a weak positive value for real I in this

area.

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Tap-YansNn,M¢ld1io1,a¢•doth¢rs1-5 vcwhs cl r values vs. Heat flcw for•

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43

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Page 51: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

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f'=iCOSXV

‘l’op—Hls¤ogran• ol lvl values in microgals.

110 5 1 1.5 2.0 rnicrogals

l‘ lreal flMiddle-Histogram ol r' values with positive and negatlve microgal values.

1 111 In 1

-1.5 -1 -0.5 0 0.5 1 1.5 2 micogals

real YBottom-Different histograrnatic represenution ol r' rnicrogal values.

ll 1-1 0 1 2 microgals

real ll-

All above histograms contain phase•cotre¤ed r' values.

figureI:_

Page 52: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

CONCLUSION ‘A geologic effect on tidal gravity has been detected in south-

eastern North America. Here the measured tidal gravity anomalies on the

order of one microgal are consistent with global patterns reported by

Yanshin and others (in press) and Melchior and DeBecker (1983). The

amplitudes and directions of the tidal gravity anomaly vectors group in

the range expected for a geotectonic environment in which the

asthenosphere is at an intermediate depth and existing stress is not

excessive. The accuracy of the tidal gravity measurements used in this

study is between 0.5 and 1.0 microgal, and similar accuracy is estimated

for calculated gravity perturbations produced by the ocean tide. In' view of these limits of accuracy the tidal gravity anomalies are too

small to support further speculation about the geotectonic environment.’

The important results of this study is that the existence of such

anomalies has been confirmed.

l 45

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BIBLIOGRAPHY

Alsop, L. E., and J. T. Kuo, The characteridicnumbers of semi-diurnalearth tidal components for various earth models. Ann.‘Geophysics, 20(3), 286-300, 1964.

Backus, G., and F. Gilbert, Uniqueness in the Inversion of InaccurateGross Earth Data, Phil. Trans. Roy. Soc. London, Ser. A, 266,123, 1970.

Cartwright, 0. E. and A. C. Edden, Corrected tables of tidal harmonics,Geophy. J. Roy, Astro. Soc., Vol. 33, pp. 253-264, 1973.

Doodson, A. T., Oceanic Tides, Advances in Geophysics, 5, 117-152, 1958.

Ducarme, B., A fundamental station for trans-world tidal gravityprofiles, Physics of the Earth and Planetary Interiors, 11,119-127, 1975a.

Ducarme, B., The Computation procedures at the International Centre forEarth Tides (ICET), Marees Terr. Bull. Inform., No. 72,4156-4181, 1975b.

Farrell, N. E., Deformation of the Earth by Surface Loads, Rev. Geophys.'and Space Phys., 10(3), 761-797, 1972.

Garland, G. D., Introduction to Geophysics-Mantle, Core, and Crust, 420pp., N. B. Saunders Co., Philadelphia, 1971.

Kuo, J. T., Jachens, R. C., white, G., and M. Ewing, Transcontinentalgravity profile across the United States, Science, 168,

_ 968-971, 1970.

Lonman, I. M., Formulas for computing the tidal accelerations due tothe moon and sun, J. Geophysical Research, 64(12), 2351-2356,1959.

Melchior, P., and M. Deßecker, A discussion of world-wide measurementsof tidal gravity with respect to oceanic interactions,lithosphere heterogeneities, earth's flattening and inertialforces, Physics of the Earth and Planetary Interiors, 31,27-53, 1983.

Melchior, P., The Tides of the Planet Earth, Pergamon Press, New York,1983.

46

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47

Melchior, P., Moens, M., Ducarme, B., and M. Van Ruymbeke, Tidal LoadingAlong a Profile Europe - East Africa - South Asia - Australiaand the Pacific Ocean, Physics of the Earth and Planetary' Interiors, 25, 71-106, 1981.

Melchior, P., The Earth Tides. Pergamon Press, New York, 1966.

. Robinson, E. S., A Reconnaissance of Tidal Gravity in SoutheasternUnited States, Journal of Geophy. Res., 79(29), 4418-4424,1974.

Sass, J. H., Blackwell, D. D., Chapman, D. S., Costain, J. K., Decker,E. R., Lawver, L. A. and C. A. Swanberg with contributions byBlackstone, D. L., Brott, C. A., Heasler, H. P., Lachenbruch,A. H., Marshall, B. V., Morgan, P., Munroe, R. J., and J. L.Steele, Heat Flow from the Crust of the United States, Chap.·13, Properties of Rocks and Minerals, Ho, Editor, 1981.

Schureman, P., Manual of Harmonic Analysis and Prediction of Tides, U.S.. Coast and Geodetic Survey, Special Publication No. 98, 1958.

Schwiderski, E. W., Global Ocean Tides, II, The semi-diurnal principallunar (M2) report, Naval Surface Weapons Center, Dahlgren,Virginia, 1979.

Schwiderski, E. W. On Charting Global Ocean Tides, Rev. of Geophy. andSpace Phys., 18(1), 243-268, 1980.

Schwiderski, E. W., Global Ocean Tides, Part I: A Detailed Hydro- .dynamical Interpolation Model NSWC/DL TR-3866, Naval SurfaceWeapons Center, Dahlgren, Virginia, 1978.

Venedikov, A. P., and P. Paquet, Sur l' application d'une methode pourl' analyse des marees terrestres a partir d'enregistraementsde longueurs arbitraires, Marees Terr. Bull. Inform. 48, pp.2090-2114, Observ. Roy. de Belg., Brussels, 1967.

Wahr, J. M., Body tides on an elliptical rotating elastic and oceanlessearth, Roy. Astron. Soc. Geophy. Jour. 64, 677-703, 1981.

Wilson, D. C., Ocean Tidal Loading in Southeastern United States, M. S.Thesis, Virginia Polytechnic Institute and State University,1978.

Yanshin, A. L., Melchior, P., Keilis-Borok, V. I., Deßecker, M.,Ducarme, B., and A. M. Sadovsky, Global distribution of tidal

. gravity anomalies and an attempt of its geotectonicinterpretation, Proceedings of the Tenth InternationalSymposium on Earth Tides, Madrid, in press.

Page 55: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

APPENDIX I”

RESULTS OF TIDAL GRAVITY HARMONIC ANALYSIS

FOR BLACKSBURG, VA, COLUMBIA, SC,

MORGANTOHN, NV AND CHARLOTTESVILLE, VA

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Page 85: October, 1986...exploration gravimeter secured in an air-tight aluminum pressure case. This instrument is designed for long-term recording with low drift rates. The air-tight pressure

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