General Field Operations for Absolute Gravity

8 July 98 version, Dan Winester (many pages of details)

A. Introduction and Background:

This is an introductory manual on conducting absolute gravity

field operations. It is intended for visiting scientists (here-

after referred to as Users), who will accompany a NOAA-supplied,

field operator during field campaigns involving the NSF-owned, ab-

solute gravity meter or will be otherwise handling field logistics.

This manual will cover general gravity meter operations, logistics

planning and site selection. Some items will be brief since it is

expected that Users will get hands-on training in meter operations

and data processing by personnel from the U.S. Department of

Commerce's National Oceanic and Atmospheric Administration (NOAA)

Table Mountain Gravity Observatory (TMGO) in the Boulder/Longmont,

CO, area. Alternatively Users can arrange for NOAA operator to

handle all logistics and processing. For this document, it is

assumed that User has already obtained funding and reserved field

time for the gravity meter and is now planning field logistics.

An absolute gravity meter observes the acceleration of gravity

directly by means of observing the free-fall of a reflective corner

cube in a vacuum. This is in contrast to a spring-based relative

meter which observes the gravity difference(s) between sites. The

current FG5 meter (Photo 1) is the fifth in a series of designs of

absolute meters developed with the guidance of Dr. James Faller of

JILA (a joint research facility of the University of Colorado and

the National Institute of Standards and Technology (NIST)). In

1994 the National Science Foundation (NSF) through the University

of Colorado/CIRES purchased an absolute gravity meter (FG5-111)

with an Iodine-stabilized laser.

The NSF meter is available to anyone in the U.S. university

geophysics community and, to a lesser extent, U.S. government

agencies, physics groups and others. A Steering Committee has been

formed to coordinate the meter's usage. In 1994 NSF made a con-

tract with NOAA to maintain the gravity meter, provide a field

operator, provide a relative gravity meter for gradient determina-

tions and do standard data processing. This was approved for

renewal by NSF in 1998. The NOAA Coordinator, the Steering

Committee Chair or their designate will certify to the meter's

field readiness before each campaign. They have the authority to

reschedule a campaign if the meter has problems.

The FG5 absolute gravity meter has an estimated, instrumental

precision and accuracy of 1.1 MICROGal (= Ò1.1 x 10-8 meters/second2).

Environmental noise can degrade the accuracy of the field measure-

ment, although this can be somewhat overcome by increasing the num-

ber of drops or sets of drops. An acceleration value is determined

for a point about 1.3 meters above the floor (or ground). Precise

relative meter measurements (Photo 2) are then made to determine a

vertical gravity gradient and transfer the gravity value to the

floor or to a height of 1.0 meters, which is the current, consen-

sus, reference standard. Routine corrections are made for lunar

and solar attraction, ocean loading, solid earth response, polar

motion, atmospheric pressure and system/floor response. Correc-

tions can be made for soil moisture, snow cover, lake levels and

ground water variations, if known.

Absolute gravity measurements are most commonly used in geode-

tic studies which look at temporal changes in the acceleration of

gravity. For example, an observed increase in gravity of about 3

MICROGal corresponds to a decrease in elevation of about 1 centimeter.

Comparisons can be made to apparent elevation changes seen in GPS,

VLBI, tide gauges and/or levelling. Absolute gravity can observe

changes in nearby masses (either density or position) such as

ground water, atmospheric fronts and magma. Absolute meters can

provide values for gravity base stations, particularly in remote

areas where control from relative meters is not reliable. Absolute

meters are used in conjunction with metrology work. The latter two

applications may not require the full precision of the FG5 meter.

NOAA has run its own absolute gravity observing program since 1986.

Figures 1 and 2 are maps of NOAA and NSF-meter absolute gravity

observations. World-wide deployment is possible. A forthcoming

NOAA web page will show observations sites of other agencies.

Figure 3 displays sites where absolute sites (of any type of meter)

have been observed and where reconnaisance has been done.

For more information, contact:

Roger Bilham,

Geological Sciences Department, Campus Box 399

University of Colorado, Boulder, CO 80309-0399

1-303-492-6189

fax 1-303-492-2606

e-mail: bilham@stripe.colorado.edu

NOAA - TMGO; 1-303-497-7405

B. Trip Logistics:

Planning a field campaign for the absolute gravity meter

consists of mostly common sense management of funds and logistics.

The User should first coordinate a project with NOAA Coordinator or

the Steering Committee Chair. At this time scheduling and costs

can be estimated. See Section B.3. 5 and 6 and Table 1. The User

will need to set up payment procedures for per diem, transportation

of personnel and equipment and miscellaneous field purchases of

supplies; in a way which will satisfy User's, NOAA's and NSF's

rules. Funds can be administered through User's accounting system

or they can be transferred to NOAA's system.

User will need to provide (proof of) insurance for gravity

meter during transport and usage. User is responsible for any

damage occuring campaign. Fully equipped, the gravity meter and

peripherals are worth about US$400,000. A project-specific list of

pieces and values will be provided for those pieces needed in any

one campaign. This excludes the NOAA van, which is insured by the

government. User may ride or drive a NOAA van and be covered under

U.S. Torts claim (self-insurance) if a government employee is

present. User must obtain his own visa(s) and inoculations. NOAA

will provide these for its employee.

Federal travelers working under federal Travel Orders receive

a Meals and Incidental Expenses Rate and reimbursement up to a Max-

imum Lodging Rate and reimbursement for miscellaneous expenses.

Incidental Expenses include things like laundry. It is preferable

that these be the Federal rates, however, arrangements can be made

to use User's Agency's rates. Federal travellers must be paid per

diem by a federal agency, not by a non-federal User. NOAA can then

bill User for amount of per diem. However, for NSF-sponsored pro-

jects, NSF rules dictate that federal agencies may not bill their

projects. In these cases, NOAA operator will file for per diem

with the User, sign the check over to the government and file for

reimbursement from NOAA. Alternatively, User may pay for NOAA

operator's lodging, meals and miscellaneous in the field direct to

suppliers. Since operator will be travelling under government

travel orders, he will get travel advances and workman's comp

coverage from government.

There are two modes of field campaign travel -- by van/truck

and by airplane. Generally campaigns will start from TMGO in Col-

orado. Trips by van are generally easier and cheaper.

1. Field Access by Ground Transportation:

NOAA can provide a van and, if necessary, a trailer (subject

to availability) (Photo 3). NOAA will pay for gasoline and other

vehicle operations costs. A one-ton capacity van with a cargo bed

of 2.6m x 1.8m can carry the gravity meter, all peripherals, one or

two operators and luggage. The trailer is generally used for

transport of air shipping boxes. See Tables 2 and 3 for dimensions

of equipment. The operators and equipment can be driven to any-

where in the continental United States, Central America and south-

ern Canada. Putting a van on a train or ferry, such as to islands,

the Alaska Inland Passage or northern Canada, has proved success-

ful. User may provide a vehicle after guaranteeing that it has

sufficient floor space and carrying capacity. NOAA personnel

should be allowed to drive the vehicle and be covered under insur-

ance. Slight modifications to the vehicle may be necessary to se-

cure equipment and bring 12 V DC power to the cargo area.

The advantages of ground travel are that the equipment can

remain on power, scheduling is quite flexible, the operators can

protect the equipment and rental vehicles may not be needed. It is

possible on a ground trip for a NOAA operator to do the field work

alone (i.e. without User accompanying) if 1) local help is availa-

ble (hireable) to help carry boxes; 2) sites are indoors and 3)

User entrusts NOAA operator to do site reconnaissance. Work will

slow down by about 10%, but up to 45% of per diem will be saved.

For vehicular travel in Mexico, a driving permit and vehicle

insurance must be obtained at the border crossing. U.S. torts

claim law and commercial insurances are not valid in Mexico. They

are valid in Canada. See also B.3. 3.

2. Field Access Requiring Air Transportation:

Experiments requiring air transport can also be organized.

NOAA has a number of large shipping boxes (Photos 3 & 4) which will

contain the gravity meter. Dimensions and weights are in Table 3.

Experience has shown that transporting the equipment boxes inside

shipping boxes markedly reduces damage. The large size of the

boxes (usually) means they will be moved by forklift, reducing the

man handling and chances of pieces falling on their sides. There

are two sets of shipping boxes. Some of these boxes will not fit

through the cargo door of mid-sized passenger aircraft, such as a

Boeing 737, Airbus A320, or McDonald-Douglas DC-8 or DC-9. As of

January 1995, only Continental, Delta and United Airlines have

suitable passenger airplanes flying in or out of Denver. Wide-body

and cargo aircraft are sufficient. There are no suitable airplanes

into Fairbanks, AK, or Churchill, MB. The insulated shelter's box

will fit in an airplane's cargo door, but the box may be too long

for some cargo bays. The generator and heat pump each have their

own shipping boxes, all of which should fit on most airplanes.

Shipping of the gasoline generator may require special arrangements

(i.e. hazardous cargo; UN 3166). If a small devoted airplane or

helicopter is used and field party members handle the cargo, then

the shipping boxes need not be used.

In situations where air transport is required and the large

shipping boxes are not usable (e.g. island-hopping work where only

small commercial planes are available), the basic equipment boxes

(Table 2) may be shipped alone (with very small pieces in medium-

sized shipping boxes). As a reminder, mishandling is more likely

and User is responsible for repairs.

For air trips, ground transportation will need to be arranged.

From TMGO the NOAA van and trailer can deliver the equipment and

shipping boxes to U.S. Customs and/or a freight forwarder near the

Denver International Airport. Pickup can also be arranged commer-

cially. At the destination, a truck with a 5+ meter bed length

should be arranged for (so that boxes arrive unstacked) (Photo 5,

although a covered truck-bed is preferred). The truck should have

a powered lift gate, if site does not have a loading dock. While

at the field destination, the shipping boxes can be stored and a

smaller vehicle (outfitted as the van described above in B.1.) may

be used to commute to various sites. If operators can not travel

with the vehicle, a rental car or other ground transportation

should be arranged. It is possible to combine road and air trips

in one campaign, wherein the shipping boxes are carried in the NOAA

trailer. This is not preferred, since gasoline costs go up as does

the probability of road accidents.

There are various people which can arrange the air transpor-

tation of the equipment and of the operators. The NOAA operators

have extensive experience in this. The NOAA/MASC shipping agent

can be involved if NOAA is administering funds, which then issues

Government Bills of Lading (GBLs) and gets government rates on

shipping. User's shipping agent can make arrangements. User's

agent should have arrangements with freight forwarders or airlines

which serve the Denver and destination airports.

There are various avenues to arranging air transport. One can

deal with airlines and cargo companies directly. Or one can go

through a freight forwarder, which is preferred. Freight forward-

ers work through various companies and can get the best schedules

and rates. Their shipments get priority over walk-in customers at

airlines and cargo companies. They have offices at most major air-

ports and can act as customs brokers. They can arrange for local

ground transport of cargo. Also if a trip has multiple legs, one

lump sum payment can be made (although not with GBLs).

Certain air transport arrangements will be insisted upon, even

if they raise costs marginally. Equipment should be flown the en-

tire way from/to Denver. For example, for a campaign in Bermuda,

the equipment should not be trucked (by freight forwarder) from

Denver to Baltimore and then flown to Bermuda. It should be flown

along each leg: Denver to Baltimore, Baltimore to Bermuda. If at

all possible, cargo should be handled by only one airline company.

NOAA's experience has demonstrated that the more the equipment is

handled (or sits) on the ground, the higher the chance for damage

and the slower the delivery. Trans-shipment via different carriers

will be needed if cargo or wide body aircraft of any one airline do

not service both origin and destination airports. See list above

for Denver airport. If possible have the boxes travelling together

rather than splitting onto different planes. If possible, air

transport dates should have some flexibility. Any delays due to

transport or instrument repairs will necessitate later rescheduling

or reduced field time. If a field break-down of equipment requires

certain components to be returned to TMGO and then re-returned to

field, NOAA will make arrangements and payments for shipping.

To date NOAA has not transported cargo by ship (besides fer-

ries). This is due to long times involved when gravimeter would

not be available for other field work.

Cargo rates vary with the urgency of delivery. The first and

last legs of air trips can use a moderate delivery speed. Inter-

mediate legs should use an expedited delivery. Return trips may

need to be expedited if another User is awaiting the gravity meter

or if NOAA awaits NOAA-owned components. Common

shipping times are 3 days to Hawaii or Alaska and one week to

Asia or Europe with advanced reservations.

3. Other Travel Considerations:

When travelling abroad, customs must be dealt with, which is

tiring and trying. Travelling with $400 thousand worth of fragile

equipment does not make the process any easier. The equipment

should be registered (or have Carnet signed off on) by U.S. Customs

before leaving the U.S. (Customs Form 4455). A list of equipment

items, serial numbers, value etc. is available from TMGO. This

permits re-importation upon return. Most "civilized" countries

honor the ATA Carnet which allows temporary import of equipment.

With a Carnet, a bond does not need to be posted, the paperwork

sails through customs and, often, the equipment is not inspected.

NOAA will arrange for purchase of the Carnet.

Entry into non-Carnet-honoring countries (e.g. Mexico, Nepal,

Taiwan etc.) is harder. A bond (~$40,000) may need to be posted by

User. Visual inspection of the equipment is likely. User and NOAA

operator should be present for any inspections, so that equipment

is not mishandled. Getting the cooperation of a foreign federal

agency or a foreign customs broker can sometimes speed the customs

process and reduce bond. Payola may be an unofficial option (for

User). Customs clearance in Carnet-honoring countries may take a

hour to two business days. In other countries it can take up to a

week. It is the User's responsibility to see that the equipment is

legally re-imported into the United States. One expensive and

awkward option to avoid customs is to contract with the U.S. Air

Force to deliver the cargo to an embassy/consulate as part of the

Diplomatic Pouch. Rock and soil (moisture) samples should have

mineral and/or biohazard releases completed. The U.S. Dept. of

Agriculture will otherwise confiscate soil samples.

Reservations for User's and operator's transport can be made

via normal means. Tickets should be fully-changeable (unless User

is willing to accommodate field delays as reductions in data col-

lection time or quality). Refundable tickets should be considered.

Train and ferry tickets and truck rentals usually must be obtained

from the provider. Government rates are often available. VIA

(Canada) tickets are available from AMTRAK ticket sales.

Section D of this manual gives estimated times for various

operational items including set up and data collection. The fol-

lowing paragraph covers the general scheduling of a field campaign.

Table 1 is a "Time Line" of arrangements which must be done before

a field campaign, by User, his agent and/or NOAA operator. A sep-

arate pamphlet, "Time Line for Users of Absolute Gravity Equip-

ment," describes these Time Line activities in detail. Contact

NOAA Coordinator for this pamphlet.

A site occupation will take about 30 to 56 hours of absolute

observations at quiet sites and 70 to 100 hours at noisy sites.

Operators will need to be on site about 15 to 30% of the time.

This includes set up [2 to 4 hours], run time, break down [2 hours)

and relative meter work [4 to 6 hours]. For outdoor sites add 4 to

6 hours and plan for one operator to be on site full time. Site

reconnaissance can take 2 to 8 hours. If sites are close together,

time can be saved by doing reconnaissance and relative meter work

at one site while absolute meter is operational at another. There

is driving time between sites. There is driving or flying time

between TMGO and sites [including customs; 1 hour to 7 days]. Bad

weather can delay logistics and may require more data acquisition

time if data gets noisier. User will need to decide on accuracy

requirements of his survey and together with NOAA Coordinator and/

or Steering Committee Chair determine minimum observation time.

Operator work time needs to be flexible. Field work is often

done on a seven-day-a-week basis with off time normally occurring

while absolute meter is running or while equipment is awaiting air

transport. Outdoor sites require more on-site time. Some sites

may not be accessible on weekends or local holidays. Hazardous-

duty pay may be called for in certain cases. Operator, together

with Steering Committee, may refuse to do a site occupation, if

site or access to site is deemed too dangerous for operator and/or

gravity meter.

It is suggested that a 10% time over-run be budgeted for in

the scheduling. Delays can come from shipping/customs, van down

time, gravity meter down time, extended reconnaissances, increased

observation sessions and real-world problems (bad weather, natural

disasters, political insurrections).

Table 1. Time Line of task completion dates prior to an absolute

gravity campaign. These may be done by User, NOAA personnel and/or

hosting agency. ED=Equipment Departure date from Longmont/Denver;

PD=Personnel Departure date; SOD=Start-of-Operations Date

Air trip Ground Trip

Item Time before Time before

General:

Request time/meter for expt. 2+ months SOD 1+ month SOD

Consult with NOAA Coordinator on 6+ weeks SOD 3+ weeks

operations scenario (5+ weeks for

Canada or Mexico)

Letter of commitment to NOAA 5 weeks PD 2´ weeks

(including proof of insurance) (4´ weeks for C/M)

NOAA travel orders requested 6 weeks PD ~3 weeks

(includes visas request time) (inc HI & AK) (4 weeks for C/M)

Inoculations & pills 6 weeks (for 2 days PD Mexico

shot series)

to 2 days PD

Obtain special items for trip 2 weeks ED 2 weeks ED

Make final operations schedule 1 week PD 1 week PD

Pre-trip observations at TMGO 1 week ED 1 week ED

Transport & Travel:

Equipment transport bids 5+ weeks ED 3 weeks ED

Prepare CARNET 4 weeks ED N/R

Personnel transport reservations 3-4 weeks PD 3-4 weeks PD

Equipment transport reservations 3-4 weeks ED 3 weeks ED

Ticketing 3 days PD 3-4 weeks PD

Lodging reservations 2 weeks SOD 1-2 weeks SOD

Site transportation reservations 2 weeks SOD N/R

GBL request 1 week ED 1 week ED

Service van (& trailer) N/R 1-2 weeks ED

Prep eqpt. for field 1´ weeks ED 1´ weeks ED

Arrange to meet site contacts 1+ week SOD 1+ week SOD

Operator personal arrangements 1 week PD 1 week PD

Pack eqpt. 1-2 days ED 1-2 days ED

Visit Customs & Freight Forwarder 1-2 days ED enroute at border

Table 2. Exterior Dimensions and masses of equipment boxes

FG5-111:

Dropping Chamber 71(l) x 43(w) x 84(h) cm3 62 kg

Superspring 71 x 43 x 69 51

Interferometer 79 x 69 x 61 70

Electronics Rack 74 x 58 x 66 75

Computer 64 x 48 x 28 17

Turbo Pump 61 x 48 x 74 51

UPS(Battery Backup) 43 x 17 x 24 26

Power Converter 47 x 27 x 30 20

Relative Meter 40 x 24 x 38 18

Rel Meter Tripod 80 x 20 x 30 11

~5 Tool & Parts Boxes (small) " 60

Oscilloscope 61 x 44 x 30 9

469

Table 3. Exterior dimensions & masses of available shipping boxes

(Not all are used for any one campaign. Generally, composite box

and 3 or 4 large aluminum boxes OR 3 wood boxes and small aluminum box

are used. Other boxes are also available.)

Basic gravity meter equipment: filled empty

1 Composite box 111(l) x 85(w) x 106(h) cm3 188 kg 80 kg

44 x 33 x 42 in3 414 lb 176 lb

5 Aluminum boxes 114 x 81 x 83 cm3 100-180kg ~40 kg

45 x 32 x 33 in3 220-400lb ~90 lb

1 Aluminum box 79 x 58 x 41 cm3 ~66 kg ~5 kg

31 x 23 x 16 in3 ~145 lb 11 lb

1 Wood box 100 x 88 x 117 cm3 164 kg ? kg

40 x 35 x 46 in3 361 lb ? lb

1 Wood box 109 x 91 x 114 cm3 202 kg ? kg

43 x 36 x 45 in3 444 lb ? lb

1 Wood box 147 x 76 x 107 cm3 194 kg ? kg

58 x 30 x 42 in3 427 lb ? lb

Outdoor mode equipment (optional):

1 Composite box for portable air conditioner (heat pump)

90 x 57 x 86 cm3 91 kg NA

36 x 23 x 34 in3 200 lb NA

1 Wooden box for 5 kW generator (no gasoline)

81 x 79 x 89 cm3 160 kg ? kg

32 x 31 x 35 in3 352 lb ? lb

plus power cables (2) 55 kg

1 Wooden box for insulated shelter (85 kg in 3 bags, alone)

209 x 81 x 74 cm3 ~164 kg ? kg

82 x 32 x 29 in3 ~360 lb ? lb

C. Site Reconnaissance:

Probably the most important aspect contributing to a successful

field campaign is site reconnaissance. The User needs to choose a (pro-

posed) site which best suits his project goals. The User should try to

do as much advance reconnaissance as possible (both by telephone and in

person) before the arrival of the absolute meter. If a proposed station

is near another station to be occupied, field reconnaissance of one can

be done during occupation of the other. The FG5 meter instrumental

accuracy can only be approached at the best sites. Poor site selection

can degrade a measurement's standard deviation by more than 10 microGal.

NOAA operators have extensive experience in selecting optimal observing

sites and will assist if called upon. See Figure 3.

The best site, for data quality, is in a remote building with

isolated concrete piers set into bedrock, temperature control and

electricity. Good examples of this are most seismometer vaults or

observatories. User should look for a place with low micro-seismic

noise, site permanence, minimal soil cover, environmental control and

vehicle access; with noise and permanence being most important.

1. Vehicle Access:

The site should be less than 50 meters from vehicle parking (be it

a truck, van, boat or helicopter -- no pack animals please). The

normally-used equipment has a mass of about 400 kg in about 13 parcels

for in-building mode and about 680 kg in about 18 parcels for tent mode.

See Tables 2 and 3 for dimensions. NOAA vans are two-wheel drive.

Four-wheel drive vehicles may be used. However, keep in mind, that for

every bump encountered the likelihood of disturbing laser or interferom-

eter optics or other damage increases. A disturbed interferometer or

broken superspring flexure requires hours to fix. Damage to laser or

electronics can require days to have it replaced or could end a field

campaign. Delicate equipment is shock-mounted, but extreme care should

still be exercised.

2. Environmental Considerations:

If site is to be in a building (preferred), it should have

electricity and good temperature control. Any AC voltage and frequency

can be accommodated. NOAA can provide a generator, portable heat pump

(air conditioner) and/or space heaters (however, the first two are heavy

and will increase air freight costs), if needed. Temperature at site

should be in range of 18 to 25C (64 to 80F). More important, however,

is that the temperature should remain constant (Ò2C) during the occu-

pation. Spatial temperature gradients should be avoided. For example,

heating or air conditioning vents should not blow directly onto gravity

meter. Meter should not be exposed to direct sunlight. If occupation

is to be done in a previously cold, unheated room, allow about one day

to fully heat floor and room (perhaps doing relative gravity gradients

in the interim). Outdoor sites will use an insulated shelter (Photo #)

with an air conditioner or space heater. Humidity should be kept below

80%, to avoid condensation on the optics. Care should be taken to

choose a site without strong magnetic gradients. These could be caused

by nearby electric-motors, iron pilings/girders or certain ferrous

rocks. Strong, near field, radio transmissions may effect instrument

electronics.

3. Site Geology:

Local geologic conditions and hydrology (soil moisture, snow cover

and ground water) are the sources of the largest uncertainties in making

repeat absolute gravity determinations; either by increasing an observa-

tion's standard error or by introducing a local effect on gravity not

present in later reoccupations. Variations in soil moisture content and

water-table level between experiments can ffect the mass distribution.

Frost heave and swelling of clays can change the elevation. Building

foundations built on soil are also more susceptible to tilting. Non-

indurated and liquifacted soils can transmit stronger and/or longer

wavelength ground motions which the superspring cannot effectively

remove.

Thus sites located on extensive bedrock are preferred. Ground

water in rocks usually remains locked in pores or constrained to frac-

ture zones and therefore hydrologic variability is likely to be minimal.

This is not true of corals, volcanics or other high porosity media.

When a site on soil cannot be avoided, soil samples can be obtained and

analyzed for porosity and moisture content. Typically, soil samples are

collected from a 0.5 m depth. Where relevant, precipitation observa-

tions, water-table observations, lake levels and/or tide levels should

be obtained for each occupation. Copies of soil test borings (made

prior to building construction) and lithologic well logs may be useful

in later analyses. For sites near tide gauges, refer to Section 5, 2.

4. Site Permanence:

For temporal gravity change studies, the site should have an

anticipated permanence (and access) of at least 15 years, given re-

alistic field instrument accuracy and vertical motion rates. There

should be no planned renovations which will drastically change sur-

rounding masses or disturb the floor around the site. Public and univ-

ersity buildings are preferred. Private buildings can be sold or have

access denied. Examples of historically good sites include observato-

ries (seismic, magnetic, radio telescope, astronomic etc.), national or

state park facilities, state metrology labs, buildings on National

Register of Historic Places, university geology or physics buildings, or

museums. It is nice to work in sites where hosts appreciate scientific

research. NOAA personnel have already done some level of reconnaissance

at over 170 cities/villages in the U.S. (Figure 3).

5. Vibrational Quietness of Site:

The site should be micro-seismicly quiet and stable. It ideally

would be distant from major roads, major airports, mining, railroads,

heavy machinery, in-building foot traffic, and strong winds. If the

nature of the research permits, site should be 50 km away from active

volcanic and seismic areas. Again, a building built on bedrock is

highly preferable.

Many regions of research interest will often be located near the

ocean shore. Recent NOAA reconnaissances of tide gauge sites has con-

centrated on establishing sites in buildings as close to sea level as

possible. Given that the entire beach area is likely to be on soft sed-

iments containing water, a station on the beach will have a minimal

fresh water aquifer and water table variations may be tide gauge

determined.

Ground (microseismic) noise affects the gravity meter, with periods

of 0.05 to 30 seconds being most troublesome. Periods near 10 and 30

seconds are particularly bothersome, since these are often the duty

cycles of the meter. Longer periods usually manifest themselves as

tilting of floor with parts of gravity meter tilting differentially and

will appear in the data as a change in the initial velocity. Competent

floors tend to restrict transmission to shorter wavelengths for which

the superspring can compensate.

It is preferable to operate the meter on an isolated pier (a

concrete block which is vibrationally separated from the surrounding

floor). Even better is to have a small pier where just the interferome-

ter sits on the pier and the dropper tripod sits on the surrounding

floor (of same elevation). Pier should not have a thin capstone which

could vibrate independently. If pier is not attached to bedrock, then

it should extend to a depth of more than one meter into soil (or below

frost line) and be wider at the bottom. A concrete pier should be

poured all at once and allowed to cure for at least one week before

occupation. Non-ferrous rebar and 5000 psi concrete are preferred.

Normally, the gravity meter is set up on the floor. An exposed

concrete or stone floor is best. Ceramic or asphalt tile covered floors

are acceptable, if well glued to substrate. Carpeted, dirt or wooden

floors are not acceptable. When set up, the meter and electronics each

occupy a one-square meter area of floor space, plus require some addi-

tional room for walking around the gravity meter. The electronics racks

must be within 3 meters of the gravity meter. See Figure 3 for a foot-

print of gravity meter. A full-sized, transparency footprint is avail-

able and can be sent to User ahead of time for reconnaissance. Tripods

feet should be on uncracked, horizontal surfaces. The equipment should

be operated in a room that can be restricted from foot traffic for the

duration of the occupation. With host's approval, a 2 cm brass plug (or

other survey mark) (Photo #) will be epoxyed flush into a drill hole in

floor to mark the site. Nothing should be permanently built over the

site which would interfere with a re-occupation.

Site must be at lowest level of any section of the building. There

must be no voids under floor, such as crawl spaces, conduits, or washed

out areas. Sometimes floors on rock and piers will have a thin capping

of concrete which becomes separated. Tapping on the floor with a hammer

is an excellent test. If there are any hollow space sounds, reject the

site. Another test is to set up a relative gravity meter. Either look

at beam or galvanometer output on a volt meter. Have second person

slowly sway to and fro with feet about 0.7 meters apart. If relative

meter shows a pliant floor with too much tilting, reject the site.

Outdoor sites should be on a very level exposure of bedrock or

concrete. Jackhammering or grinding may be required to smooth surface

and to remove weathered rock. NOAA has a diamond grinder (Photo #).

Metal disks may be set cemented in rock or "riser blocks" may be used to

provide level areas for instrument feet (Photos # & #). Alternatively,

a concrete pad may be poured on rock surface. It should be at least 10

cm thick and well anchored, perhaps with minimal rebar or aluminum mesh

set in drill holes. There must be no chance of frost heave. There must

be no separation between concrete and rock, which can be detected by

hitting concrete with hammer and hearing a hollow sound. Another

alternative is a thick concrete pad set in soil. The top of the pad

should be near ground level and accommodate an equilateral triangle one

meter on side. See 5 regarding pier installation. There should be a

3´+ meter diameter circle of open ground to accommodate the shelter.

Station will be about 1 meter from the edge of circle (not in center).

It may be necessary to drill holes for shelter anchors (tent pegs).

6. Other factors to be considered:

Doorways must have a minimum of 0.61 meters of horizontal clearance

to permit entry of the absolute gravity meter. It is preferable not to

set a survey mark before site occupation, so as to leave the absolute

field team flexibility in set up.

It is suggested to avoid nearby transmission towers (radio, televi-

sion etc.). The transmisions may affect instrument electronics, al-

though which frequencies are dangerous has not been determined. Also

wind will catch the tower and shake the neraby ground. This is only a

suggestion. NOAA has a very good site in Wausau, WI, at an AT&T (tele-

phone) microwave facility. And FAA (avaiation) VORTAC antennae are

considered good candidate sites. However, these antennae are relatively

short. GPS reception can also be affected.

Excenters, where desired, should be within walking distance (< 100

meters). Usually they are at existing survey marks or they can have

established marks. An excenter is an (outdoor) relative gravity station

adjacent to the absolute site (Photo #). It is generally set on a foun-

dation independent of the foundation under the absolute station. Thus

differential settling may be detected. Relative gravity surveys or

other types of surveys can be done to this station without necessitating

access to absolute site building. GPS sky clearance should be consi-

dered. An excenter might be established at a GPS site, a benchmark or

an existing gravity base station. Or a new disk can be set.

D. FG5 Absolute Gravity Meter Principals and Operation:

This a brief overview. For more details refer to Niebauer, et al,

in Metrologia (1995), the FG5 instrument manual and training sessions at

TMGO.

Figure 4 shows the FG5 system and the optical path used. After

columnation the laser light is divided at the first beam-splitter. The

reference beam passes through to the photodetector. The test beam goes

up into the dropping chamber and reflects off the free falling retro-

reflector (a.k.a. corner cube or test mass). The retroreflector has the

property that if it is slightly rotated (about a horizontal axis), the

incident and returned beams will remain parallel. The beam then goes

through the interferometer body and into the superspring and off its

retroreflector. The superspring removes short period (below 60 seconds)

ground motion from the optical path. The beam is then merged with the

reference beam. As the dropping chamber retroreflector drops, an inter-

ference fringes will be generated in the merged beam and measured in the

photodetector (APD). The control electronics will count the fringes and

assign a time from the Rubidium frequency reference. These give a

series of time and distance pairs which can be solved numerically for

acceleration. Enough drops are collected to give statistical validity

and to fit environmental correction models.

Photo 1 shows the FG5 in operational setup. Table 4 is a list of

operational procedures done after arrival on site with estimates of time

involved. Enter appropriate times and values on log sheets. The NOAA

operator will adjust any optics or electronics, should it be necessary.

User should determine the data collection scheme to be used. This

includes the length of station occupation desired, how much extension is

allowed if data are noisy, number and frequency of sets, and number and

frequency of drops within each set. In the past, occupations have

lasted 24 to 72 hours, with 48 hours being typical. Typical set

scenarios have been 100 to 150 drops hourly, 25 drops every 15 minutes

and 250 drops bihourly; all with drops spaced 10 seconds apart. Another

option is for drops every 30 seconds in 110 drop sets hourly for near

continuous data over entire occupation. Data is split into sets for

computational convenience. The usage of sets derived from computer

limitations on the predecessor JILAg absolute gravity meter and the

desire to switch modes on the HeNe lasers. It is more common to do

statistics on set averages, rather than on all drops together.

Table 4. Instrument Set up and Operation Summary.

Obtain access to site (e.g. permission, keys etc.)

Prepare site area (e.g. grind feet areas, clean, remove obstructions).

[10 minutes to 2 hours]. Set mark, if outdoors.

If site power is not available or reliable, set up generator.

If outdoors, set up shelter and heat pump/space heaters. [2 hours]

(See Reconnaissance section paragraph on outdoor sites.)

Bring site, including pier/floor, to operational temperature. Site

should be out of direct sunlight. [10 minutes to 1 day]

Unload boxes from vehicle. Some equipment needs to go into shelter

before shelter is constructed. [30 minutes]

If 110 VAC not available, setup power convertor (Elgar), with ground.

***[here to first systems check w/o pump down: 45 to 90 minutes

depending on experience and site conditions]

Set up surge protector and battery backup unit (UPS).

If ion pump is off, do turbo pump down. [20 to 60 minutes]

Set up racks, connect AC line cord and turn on MASTER AC and MASTER

DC power. Be sure SS loop is OPEN.

Wire ion pump to rack power supply and turn on.

Put legs on interferometer and set over site with laser cables oriented

to north or south. Level with bubble level. Remove 2 plugs &

telescope cap. Put on telescope. Position top pegs outward.

Affix laser cables. Turn on laser: Power switch, key switch.

Air dust interferometer and dropper bottom.

Put legs on tripod triangle and set on interferometer. (no feet)

Put dropper on tripod and lock in place.

Level interferometer with alcohol on floor. Mark laser spot on floor if

at a new unmonumented site.

Put feet under tripod and raise until dropper is level.

Lower interferometer or raise dropper until pegs are clear. Check

levels of interferometer and dropper. Sites on uneven rock may

require may special care or grinding or riser blocks.

Unclamp dropper motor.

Make rings and dip stick measurements.

Run cables to racks: Green from APD on interferometer, Orange from

dropper motor, Blue from motor shaft encoder, White from dropper,

BNC from TTL to computer TTL, BNC from ANALOG to oscilloscope.

Hang superspring, remove plug, level and un-travel-lock. Check return

beam path for clipping. If bad, adjust SS mount. Connect Yellow

cable from SS to rack. Monitor ring down on oscilloscope.

Raise or lower sphere to zero. Close loop when quiet (<15 mV).

Check interferometer verticality and SS and dropper levels. Check that

Rb oscillator has locked (light on power supply).

Connect computer cables: ribbon, HPIB, power, White (barometer) cable

from COM1 to rack, Trigger BNC from dropper cont. and CLOCK BNC.

Connect/check environmental cables: 1H to temp probe, 2H to SS SPHERE,

3H to ion pump METER MONITOR and 4H to Laser OUTPUT (set to 1F).

Check cables: Laser LOCKED(back) to Scaler Counter STAT 1(back). S/C

THROW INIT to Dropper EXT IN (to computer TRIGGER).

Lock laser, probably on peak E. Maximize fringes in oscilloscope.

Do systems check. [10 minutes] (see paragraph below)

Wait for laser for fully warm up and SS to calm. [1-3 hours]

Table 4. Instrument Set up and Operation Summary (Continued).

Set up C:\NEWTON\FG5COMND.DAT file (refer to Micro-g FG5 Software

Manual). Have coordinates ready (preferably WGS 84 and NAVD 88).

If available, reset polar motion coordinates. If available, COPY

site's ocean loading file to C:\NEWTON\OCLOAD.DAT. Set computer

clock. [10 minutes]

Do another systems check. When laser temp OK light is on, measure 1F

voltages (can be done during data collection); possibly resetting

values in FG5PARAM.DAT. [10 minutes]

Collect data: Run current version of OLIVIA program. Systems checks

when scheduled. Note/correct computer clock drift.

[Run-time is dependant on site noise and project requirements.]

When done, transfer data to 2 back-up media. [30 minutes, which will

overlap the following items]. Remeasure 1F voltages.

Open SS loop. RESET dropper. Unlock laser and turn off. Knob lock SS

and travel lock dropper. Disconnect cables without disturbing

interferometer. [here to Remove legs... takes 30 to 60 minutes]

DC MASTER power switch off.

Remove SS. Remeasure rings and dip stick.

Remove dropper and hook to portable, ion pump, power supply.

Remove and disassemble tripod. Remove telescope. Cover it and plug 3

holes and tape pegs. Remove legs from interferometer.

At this point second operator can start relative work. [2 to 6 hours

depending on project requirements, noise & weather]

Unplug power to rack, computer (after data transfer), UPS, heat pump and

Elgar. Pack up components into boxes and vehicle. [30 to 45 min]

If outdoors, pack up heat pump/space heaters, generator (drain gasoline

and oil before flight), power cables and shelter. [2 hours]

Anytime during site occupation: collect rock and soil samples (if any),

get precipitation records, do mark setting, tape measurements for

description, make site maps and orientation diagram, take photo-

graphs and do general cleanup.

Restore site to pre-arrival condition or as appropriate.

Transmit data to TMGO for processing and archiving.

A Systems Check is an inspection of the gravity meter and environ-

mental sensors immediately before, during (2 or 3 times daily) and after

the data collection process. Checks (and adjustments) are made on laser

beam verticality, dropper chamber and superspring chamber levels; super-

spring test-mass position; interference fringe intensity; dropper hold

voltages; computer time; general data quality; laser power & body tem-

perature; ion pump current; air temperature and air pressure. Comments

are written on any events affecting the data collection. A Systems

Check takes about 5 minutes. Adjust or stabilize air temperature and

humidity where needed. It is a physical disturbance to the gravity

meter. The Superspring (and thus data) will be noisier for about 10

minutes after a Systems Check. The Check will not affect the instru-

ment's height.

E. Operation of Peripheral Equipment:

In addition to the absolute gravity meter, various peripheral

devices may be needed for operations. Most of the below items have

operations/repairs manuals which should be read.

1. Relative Meters and Tripod:

NOAA currently uses two LaCoste & Romberg Gravity Meters , Inc.,

Model D land gravity meters modified with electronic levels and electro-

static feedback units (EFU) in association with absolute measurements.

Surveys are routinely done for determining vertical gravity gradients

and transfers to excenters. Surveys could also be done to intra-city

and inter-city stations. Generally only one meter is available for User

surveys. L&R Model G or Scintrex gravity meters with the same features

are also usable for gradient surveys.

Gradient surveys (Photo 2) are run to transfer the absolute gravity

value to the floor or to a height to compare other absolute meters, to

establish a link between height and gravity changes and for use in post-

processing floor-rebound effects to absolute meter observations.

Current NOAA convention is to run gradient surveys (12 loops) between

the floor and a height of 131 cm (the top of an FG5 drop) and between

the floor and a height of 91 cm (the top of a JILA-type absolute meter).

If this is a reobservation and gradients were measured at other heights,

generally the other heights will be repeated. While an occupation by a

JILA-type instrument is not likely, the 91 cm survey provides a check on

the 131 cm survey and a check for non-linear gradients. See Section

C.6. 2 for a description of excenters. Eight loop surveys are made to

excenters.

Relative gravity meter should be on power for at least 12 hours

before operation. EFU power should be on (plunger switch at top NW

corner). Battery should be fully charged. If meter has travelled more

than 10 of latitude, 700 mGals or received a major hit since its last

levels checks, then these checks should be redone. Instructions are

included in meter case.

Extend tripod legs and adjust table to desired height above the

floor. If survey mark and floor are at different heights, note the dif-

ference but use the floor since relative meter will sit on floor, not on

the mark. Trivets are not generally used, except at some excenters.

Set tripod height with a tape measure to within 1 mm. Watch for back-

lash in screw. NOAA measures relative meters oriented to north (i.e.

the side with one leveling screw is to the east). The northwest screw

is fixed to a height of 20 mm from floor to the bottom of the white

case. By convention, the height-of-instrument is 20 mm (unless all

heights are modified by difference to mark height). The actual obser-

ving mechanism is about 100 mm above the bottom of the case. All dial

motions should end with a clockwise (combined) turn of one or more full

rotations.

Place meter at first site. Level it. Dial should be near 100.000.

Unlock beam. Adjust crude screw to bring beam into range. Adjust fine

screw (dial) to crude center. Enable EFU, recenter if necessary and

wait 5 minutes. Toggle switch from (null) to R (reading) and make

dummy reading. Toggle to and lock beam. Make real reading (unlock,

toggle to R, wait few minutes (3 to 6), observe, record datum, toggle to

, and lock). Move tripod into place (never lift by table, always by

legs). Move meter GENTLY. Level. Make reading. This comprises one

loop (of two stations). Do 12 loops for gradient surveys and 8 loops

for excenter surveys (6 and 4 loops for 2 meter surveys). End survey

with one final observation at first station. The desired standard

deviation for these surveys is 5 MICROGals or better. If an obvious tare

has been made, add another loop.

The EFU has a dynamic range of about Ò1300 MICROGals. For excenter

intervals >1200 and <2500 MICROGals adjust dial during dummy reading. For

intervals >2500 MICROGals, use EFU in conjunction with dial (at 2 repeated

settings) or dial alone. Use trivet at excenters only if necessary and

note additional height of instrument over mark.

2. Elgar Power Convertor:

The Elgar power convertor will change any stable input AC voltage

(>80 V AC) of any frequency to an output of 117 V AC at 60 Hz (1 kVA

maximum). Do not plug anything in output receptacles until READY light

is on. Check wall receptacle for correct phases. Watch for floating

grounds, which may necessitate connecting Elgar case to one of ground

lines. A variety of foreign plug-type cables is available at TMGO. If

input voltage varies after Elgar is activated, the output voltage will

vary proportionately. Unit will not work if air fan filter is dirty.

Wash in water and restart unit.

Almost all components of the FG5-111 and peripherals can be

reconfigured to operate at 230 V AC, 50 Hz. A major exception is the

heat pump.

3. Topaz Uninterruptable Power Supply:

The Topaz UPS model SQD 81100VR is a 1 kVA battery backup with

some voltage regulation. Input should be 110-120 V AC, 60 HZ and output

is the same. Use a surge protector in line before UPS unit. Batteries

should run the gravimeter (without space heaters or heat pump) for one

half to one hour. Relative meter and ion pump will run off their own

batteries after UPS batteries have been exhausted. When battery backup

is activated, a loud, repeated beep will be heard. The laser may unlock

if UPS is activated or if output voltage varies too much. Turn off unit

before unplugging it from power source. This may be replaced by most

any commercial UPS with sufficient power.

4. Honda Generator:

The 5 kVA Honda generator will run the gravimeter plus heat pump

or space heaters. It is acoustically and vibrationally noisy and thus

should be located as far from gravimeter as cables (and line drop) will

allow. Placing generator on foam and placing boards around it will re-

duce noise transmitted. In extreme-weather conditions, the generator's

gasoline tank will allow for about 6 hours of full-load operation before

refilling. In lower-load operation, 9 hours may be possible. See

manual for operations and maintenance. Gasoline and oil should be

drained before transport. Gravimeter itself requires under 1 kVA. So

a smaller generator may be used if this is the only load. Starting and

running the heat pump or space heaters necessitates the extra 4 kVA.

5. Ariagel Heat Pump:

The Ariagel PortaTemp heat pump is used as a portable air condi-

tioner. For heating, use space heater(s). Place small unit outdoors.

Allow some place for condensation to drain from outdoor unit. Large

unit may be used in or out of case. Controls are self-evident. The

unit will cool 10,800 BTU per hour. Unit requires 40 Amps to start and

12.5 Amps to run at 115 V AC, 60 Hz (or 4600 and 1450 VA). Since Elgar

power convertor only has a through-put of 1000 VA, an independent source

of 115 V AC must be located.

6. Insulated Shelter:

The insulated shelter or tent can hold the operational absolute

gravimeter, heat pump and some peripherals (Photos # & #). It is

octagonal, 3 m in diameter and about 3 m tall. Its aluminum frame can

be adjusted to differing ground heights. It has an insulation layer and

a canvas layer which stretch around and over the frame and are held in

place by Velcro. There is an access portal for the heat pump small

unit, but since it zips closed the wrong way, it is easiest to run small

unit out under the frame. The canvas skirting should be held down with

soil or rocks to keep wind out of shelter. Winds can cause shelter to

"breath" (i.e. inflate and deflate) causing undesirable air pressure

changes. Shelter is not stable in high wind areas, even though top may

be guyed down and pegs may be set through frame feet. Shelter is

waterproof, except for some ground seepage. Care should be given not to

rip canvas or insulation. Installation is fairly straight forward,

although a manual is available. At least two persons are needed as well

as a step ladder and a pole (e.g. broom handle). After site has been

prepared, assemble frame ("the Spider"). The Velcroed cross braces of

the frame go on top facing outward. Position door so predominant winds

and sunlight will not enter and upset gravimeter (located along opposite

side). Insulation and then canvas are stretched from the door around

the frame and then the roof is applied. Do not erect when windy. A

lead light is useful for subsequent operations.

7. Auxiliary Survey Equipment:

TMGO has a second L&R relative gravimeter which may be available

depending on other commitments. TMGO has some optical levelling eqpt.,

a total station surveyor (EDM) and a few tripods available. CIRES has

a GARMIN portable GPS unit (not geodetic) and bar code levelling eqpt.

which may be borrowable. These items have their own manuals. User may,

of course, supply his own equipment. At TMGO there are a lathe, mill,

band saw, drill press, grinder, welder and hand tools available. TMGO

has a multipycnometer, oven and balance available to analyze soil mois-

ture and density. GPS, a rainfall gauge, a soil moisture gauge, a cryo-

genic gravimeter and seismometers operate continuously at TMGO. TMGO

regularly hosts other absolute gravity meters.

8. Mark Setting (etc.) Tools:

There is nothing too high tech here. A hammer-action electric

drill with a -inch, carbide-tipped, masonry bit is used to drill mark

holes. Using this in tandem with a star drill (four-faceted hand

chisel) is very effective. A ´-in bit is used for tent pegs. A non-

hammer-action drill can be used in concrete. For remote sites a gaso-

line powered drill (Roybi ) may be borrowable. For absolute station

plugs, do not drill too deep. For survey disks countersink a hole

around central hole so that lip of disk is at or below concrete/rock

surface. Use a combination of drill and chisel. Clean out drilled

holes with water or compressed air before setting mark. Do obtain

permission before drilling.

Before drilling hole, stamp letters into disk. For absolute sta-

tion plugs, about 12 or less characters can be stamped around the edge

plus a 4 digit year at bottom plus a 2 character code in the center

(using -inch dies). For survey disks, designation can be stamped above

the central cross and the year below. Note: the edge of the die is not

the edge of the stamped letter. All marks should be aligned so that

they are readable facing north. A chiseled or inked arrow pointing

north may be added nearby.

Survey disks should be set with anchoring cement. Plugs may be set

with anchoring cement or 5-minute epoxy. Do not cover plug with clear

epoxy. A disk without a stem being set on concrete may be set with

liquid steel solder or epoxy. This last option is not preferred. Do

not allow setting agent to freeze before hardening.

The grinder has a diamond encrusted pad. Note that diamonds are on

outer half of pad's radius and along edge. Pad may be used wet or dry.

Dry is dustier and less messy. Wet will lengthen life of pad.

9. X-Y Detector:

The X-Y Detector tracks motion of the laser beam coming out of the

dropping chamber onto the floor (no superspring in place). If there is

no motion of the beam, then the interferometer and the dropper are

aligned with respect to each other (i.e. bubble levels are aligned) and

the dropper rods are not bent. The detector will only be used by NOAA

operator and only when something major has happened to the system. The

X and Y outputs go to Channels 1 and 2 of the oscilloscope and the power

line comes from the back of the electronics rack.