Geophysical Exploration for Archaeology: 

An Introduction to 

Geophysical Exploration 

Midwest Archeological Center 

Special Report No. 1 

1998 
By

Bruce W. Bevan

Geosight

Midwest Archeological Center

Special Report No. 1

United States Department of the Interior

National Park Service

Midwest Archeological Center

Lincoln, Nebraska

1998

 Sources of equipment 

 Survey preliminaries -------------------------------

 Setting up a grid 

 General survey procedures --------------------------

Resistivity Surveys 

 Instruments ----------------------------------------

 Mapping 

 Sounding -------------------------------------------

Magnetic Surveys 

 Instruments ----------------------------------------

 Before the survey 

 Simple procedures ----------------------------------

 Magnetic interpretation 

Conductivity Surveys ------------------------------------

 Instruments 

 Interpretation procedures --------------------------

 Mapping 

 Detailed procedures --------------------------------

Radar Surveys 

 Equipment ------------------------------------------

 Survey procedures 

 Interpretation -------------------------------------

Self-Potential Surveys 

 Equipment ------------------------------------------

 Procedures 

 Survey details -------------------------------------

Special Topics 

 Soil chemistry -------------------------------------

 Multiple instruments 

 Maintenance, reliability, safety -------------------

 Special survey conditions 

Notes on the Figures ------------------------------------

References 
ii

a microfiche (sheets of microfilm that can be read with a ma

lens or with a special microfiche viewer). You may obtain a 

this microfiche at no charge by writing the author at:

 Bruce Bevan

 Geosight

 356 Waddy Drive

 Weems, VA 22576

 USA

 Volume A of the report has a general description 

Petersburg Battlefield where this survey was done, and it su

the history of the site; this volume also has guidance abou

geophysical survey might be specified and how an excavatio

follow a geophysical survey. Volume A includes the pub

references and notes about the figures in all three volumes

volume A is not technical, volume C is very technical. V

describes how geophysical measurements change with time and 

it also shows how detailed methods of geophysical interpreta

add information to a survey. Volume C also discusses the e

procedures of resistivity pseudosections and seismic surveys.

 This is the third edition of the report. The first and 

edition was photocopied for a National Park Service course w

held at the Colonial National Historical Park; that edition 

24 April 1995. The second edition, dated 26 January 1996, c

typographical errors in the first edition; the second edi

available only as the microfiche mentioned above. Further 

have been corrected in this third edition.

 Note that the National Park Service does not endorse 

models of the geophysical instruments which were applied 

geophysical surveys. There are many different instruments 

many different manufacturers which are also excellent for geo

surveys of archaeological sites.

 Additional Note. This is the third edition of a 

originally published as Geosight Technical Report Number 4, V

which was last revised March 6, 1998. The original title pa

the notice “This report may be freely copied and distributed

main text pages of the present volume match the pagination 

original, except that the designation “B- ” has been omitted 

page numbering scheme in this most recent printing. For exam

text on p. B-23 in the original document corresponds exactly 

text as it appears on p. 23 in this version. All figures an

notes are numbered as in the original, but the pagination i

to this reprinting for the References and Figure Notes s

which originally appeared in Volume A.
1

You will be able to do it just as you wish and when you 

you will probably be excavating the site after your surv

easily be able to compare the excavation finds to your 

results. You may even wish to start an additional surve

excavations have begun in order to understand the cha

feature which extends outside your excavation.

 You may also do geophysical surveys in the bottom 

sides of your excavations. These may reveal invisible f

behind the excavated surface. A geophysical survey on 

excavation surface can provide an objective map of the 

the soil through a shallow depth into the unexcavated so

the different types of survey can be done within exca

major difference is that the measurements are made 

together.

 I believe that any archaeologist could do good 

surveys; the mental and physical requirements of arch

geophysics are very similar. There are some differences 

geophysics is easier on the knees, it also requires pre

doing repetitive work. You definitely do not need 

technical background to do geophysical surveys, but yo

time if you can make simple electrical repairs; cables an

frequently break. Geophysical surveys can be done (an

done) in weather which would halt most excavations. If 

mind doing a walking reconnaissance as a search for ar

sites, you do not mind collecting spider webs on your 

pass between the trees; geophysics is the same. If you 

good work, you will have to do at least one or two 

surveys a year; if you do fewer, you will forget the proce

 In this volume, I will be emphasizing topics which h

described very completely in other publications. 

duplicate the clear introductions which others hav

resistivity and magnetic surveys. However, I will descri

resistivity soundings, and I will also describe simple pr

magnetic mapping and interpretation. My discussions of 

and radar surveys will be more complete than usual, be

little has been written about practical procedures for t

before.

Sources of equipment

 Geophysical equipment is expensive. You should 

buying equipment for the first few surveys that you do

rent or borrow the instruments. Most of the comp

manufacture geophysical instruments will rent, and som

their instruments so that prospective buyers can become f

them before they consider a purchase. There are also ma

which rent a wide selection of geophysical instruments

which I have heard of are listed below:
2

 phone 512-346-4042 phone 303-863-8881

and

 Mitcham Industries, Inc. Terraplus USA Inc.

 Post Office Box 1175 625 Valley Road

 Huntsville, Texas 77342 USA Littleton, Colorado 801

 phone 409-291-2277 phone 303-799-4140

There are also two directories which list the manufactu

geophysical instruments; both of these are published once a y

of these is called The Geophysical Directory, and the addre

Volume A. The other is the yearbook of the Society of Exp

Geophysicists; this is published in every May issue of their 

called The Leading Edge.

 If you are familiar with electronics, you may constru

instruments yourself, such as a proton magnetometer (Harkne

Steponaitis and Brain 1976) or a resistivity meter (Williams 1

 In the following sections, I will give additional inf

about specific types of instruments.

Survey preliminaries

 If you are doing a survey at a site which is new to 

should learn enough about it so that you can decide which ins

might be suitable. Ground-based or aerial photographs will 

good idea of topography and trees, and perhaps also the brus

site. If the site is very remote, you may find that only s

photographs are available. Be sure to check if any other geo

surveys have been done in the general area of the site. C

surveys at nearby archaeological sites; if you can, also c

geophysical surveys which have been done for other applica

the area. These publications may reveal difficulties with t

type of soil and rock which will affect your survey. Try 

copies of historical maps and photographs of the site. Con

of these maps to the scale of your site map so that they 

compared.

 If you cannot find a detailed geological map of the 

world map may have enough information to help (Choubert an

Muret 1976). The voltage and frequency of the local powe

should be checked (U.S. Department of Commerce 1975)

anticipated temperature and rainfall during the survey perio

also be estimated (Rudloff 1981).

 You should check for all of the sources of difficu

interference which are listed in Fig. A94 and Fig. A95. If t

buried pipes or wires at the site, try to get maps of them. 

is a likelihood of many unknown utility lines, you may need t

locate them before you start the survey. You could use 

tracing instrument to map conductive pipes and wires. There 

separate components to these instruments; one component ind

alternating current with an audio frequency onto the condu

the other component allows you to follow the signal and delin

conductor.

 It may also be valuable to make a preliminary test of 
3

Setting up a grid

 The normal procedures of archaeological mapping 

always suitable for setting up the grid for a geophysical 

analysis of magnetic maps is somewhat simpler if thei

aligned close to magnetic north. When errors are 

geophysical survey, they can create false anomalies 

extended along the direction in which the measurements ar

this reason, it may help if the grid is not align

suspected direction of buried structures.

 Sometimes, a geophysical survey is done to map a f

approximate location is known. Even if the area is kn

the area of survey should go well outside the feature. 

allow the geophysical character of the feature to be com

surrounding earth where there are no features. The to

survey is likely to be 3 - 5 times the area of the feature

 If a geophysical survey is to be done on the 

moderately steep slope, it may be necessary for coordi

determined by distances on the sloping surface, 

horizontal distances. If it is not possible to 

measurement locations to a horizontal surface, this 

mentioned in your report.

 Your usual methods of coordinate nomenclature will 

for the geophysical survey. If the grid is aligned 

north, these coordinates can be called East and North, re

point being described as E141.5 N52.0, for example

archaeological sites, a coordinate label specifies an are

10 m square area for excavation; since these types of co

not specify points, they are difficult to use for a 

survey. It is easier if the coordinates are small numb

state plane coordinates of the USA or the coordina

Universal Transverse Mercator system are used, these num

truncated for simplicity of recording. In most geophysic

systems, positive coordinates are North and East, whil

West are given negative values; a designation of South 23 

as a North value of -23. In this report, the borders o

contour maps have negative values for south and west 

grid north is always toward the top of each figure

coordinate systems, the labels X and Y will sub

approximate East and North directions.

 A geophysical grid is defined by a reference p

direction, or by two reference points. If there is a

building at the site, it may be practical to align the 

grid with the direction of that building, and make a co

building the reference point for the grid. This is wh

Petersburg; see Fig. A77.

 If the area of survey is larger than 200 m long

necessary to use a good transit, theodolite, or EDM 

Distance Measurement) instrument to locate the stakes. Si

of my staking at Petersburg covered a distance of about 7
4

still readily visible (as it was in the winter), the stakes w

to find. If the paint was lost, I had to relocate the stake

tape measure. Then, I placed flags on tall plastic rods to m

stake location for the week of my survey; I removed these at 

of the week. I left the flags in the wooded area between 

but these were gradually lost during the two years of surv

average, it took me 1.8 hours to relocate and mark these stak

time I returned to Petersburg.

 Most surveys are done in smaller areas, and simple equi

adequate for setting up the grid. One of the most valuable 

an optical right angle sight. These small instruments us

pentaprisms to allow quite accurate 90 degree angles 

determined. They cost about US $100 and can be bought from c

which sell equipment to surveyors. A tape measure can be 

make a 90 degree angle with the aid of a triangle whose sid

multiple of 3, 4, and 5 m (Gardner 1964), but the optical 

much easier and more accurate.

 It is much better to have geographic north (also cal

north) on the site map rather than magnetic north; this is 

the direction of magnetic north changes from year to year 

1995). In the USA, magnetic north has changed direction by 

degrees during the last century. At London, where records h

kept longer, magnetic north has changed by over 30 degrees 

last 400 years. For all practical purposes, true north is a 

direction. While magnetic north can be estimated to an error 

than about 3 degrees with a compass, different procedures ar

to find geographic north. While these procedures are somew

difficult, they are also more accurate than a compass meas

Five different methods are outlined below.

 Get an accurate map of the area (such as a U.S. Ge

Survey topographic map if the area is in the USA) which has 

north arrow or a latitude / longitude grid. See if there i

linear feature near your area of interest; this feature mig

straight section of road or railroad track. Measure the tr

of this feature on the map with a good protractor to an acc

about a degree. With a transit, theodolite, or EDM, trans

angle from the feature to your site. As an alternative, mea

azimuth of the linear feature with a magnetic compass (if 

iron at the feature, put a pair of stakes offset from the fe

at least 10 m and measure their angle); subtract the measu

angle and the result is the magnetic azimuth of true north.

 At Petersburg, the straight section of the park road, 

Fig. A68, may be adequate, but it is rather short. I measu

angle, but it gave me an error of about 2 degrees.

 If you know the magnetic declination at the location, t

allow you to convert magnetic north to true north. First, c

your compass (without this, you should expect an error of as 

about 5 degrees); a precise needle compass, such as a su

compass or a trough compass on a theodolite is needed. The 
5

for any location can be determined by contacting t

Geophysical Data Center (NOAA, Code E/GC4, 325 Broadw

Colorado 80303). Fig. C17 is a plot of the change 

declination at Petersburg which I have determined from 

program USHD which is available from the NGDC above. 

USA, the current magnetic declination can be estimated f

approximation of the magnetic field of the Earth; this 

described in Volume C.

 At Petersburg, my measurements with a magnetic comp

that magnetic north is about 7 degrees west of the grid n

used; I estimate that the error of this magnetic nor

could be about 2 degrees. As seen in Fig. C17, true nort

time of this survey was about 9 degrees east of magnet

true north should be about 2 degrees to the east of grid n

 The procedures above are adequate if you wish to d

direction of true north to an accuracy of about 1 degree; 

an angular accuracy of one minute or better you could 

direction of the Sun, or a planet or star. For this 

method, you will need to know the precise time, and the 

the measurement to a moderate accuracy. The direction to 

be calculated for any time and location, just as was d

photographic analysis of Fig. A85. True north is de

measuring the actual direction to the Sun and comparin

calculated direction (Davis and others 1986 p. 430).

 Accurate time is best gotten from short wave rad

such as WWV from the USA (5, 10, 15 MHz), CHU from C

14670 kHz), or the BBC (the start of the 6th pip marks th

accuracy to within about a half second is needed. The 

your test can be determined with adequate accuracy using a 

 The angle or the azimuth of the Sun can be meas

transit, theodolite, or compass, depending on the accurac

Electronic Distance Measuring instrument probably is not 

observation of the Sun, for the Sun's brightness can 

sensors of the EDM.

 The true angle of the Sun can be calculated by on

approaches. If you will do only infrequent measureme

north, it may be easiest to get an almanac (Elgin, Knowle

1994). The angle of the Sun may also be determined by t

calculations with a simple hand calculator (Duffett-Smith 

 My solar measurements at Petersburg indicate that t

an angle of 2 degrees 2 minutes east of the grid north w

I measured this direction twice (in 1991 and 1993) 

difference between the two measurements of less than 2 min

 The geographic coordinates of two staked locati

determined with a GPS receiver; differential measur

usually be necessary for sufficient accuracy. These coo

be converted to a geographic direction relative to tru

gyroscopic compass may be suitable if the site is under

clouds prevent a solar observation (Davis and others 1981 
6

measurements are often made in a checkerboard pattern. The 

points is a square pattern and the spacing between the meas

is the same in the two perpendicular directions. Widely-spac

of measurement may be suitable for locating large features 

defining the boundary of a site. These may be single, 

lines; however, sometimes a group of two or three closel

parallel lines can give more reliable data. These groups 

may be widely separated. The lines of measurement may also 

from a point; this may be suitable only for the survey of a mo

 During the geophysical survey of a rectangular grid, po

most readily located with the aid of a guide rope which cro

grid. Tape measures may be placed along the two opposite 

the grid and the guide rope may be stretched betw

corresponding points on the two tape measures. Instead 

measures, which will blow aside, it may be better to place t

markers at the points where the guide rope will be set; small 

stones, or spray paint marks can be used for this.

 While fiberglass or cloth tape measures are accurate, 

difficult to use if the site is windy. Calibrated ropes, cal

chains or surveyor's ropes, are much easier to work with. T

essentially non-elastic ropes which have permanent marker

their length at 0.5 m intervals. These can be pulled quite t

there is no problem with them turning the wrong side upwards

rope chains can be bought from several manufacturers of 

equipment.

 A normal rope which has been calibrated is also excel

the guide rope which defines each line of measurement. If it 

elastic, it will be more accurate. However, if there are 

the way, or if the surface is steep, a non-elastic rope wil

short at its ends and points will be in error. For these sit

an elastic rope will be better. A normal cotton or nylon rop

stretched by about 1 m in a 100 m length. This rope can be 

with markers along it which indicate each measurement point

markers may be painted bands; distinctive marks should indic

multiples.

 After each line of measurement is finished, the guide 

be moved parallel and a new line begun. For conductiv

magnetic surveys, you will be able to estimate short d

visually from the rope to the measurement points, and it may 

necessary to move the rope for every line of measurement. D

on the accuracy needed, it may be possible to estimate the 

distance from the rope for one or two lines of measurement o

side of the rope. This will save time. For most types of su

is good to try for a maximum error in the location of meas

which is about a quarter of the distance between measurements

 Unless there is a good reason for another orientation, t

rope should always be extended the long direction of the 

rectangle. This will reduce the number of times that the ro
7

one measurement interval and make measurements moving tow

and then across it to the other side.

 During my survey of the Taylor House grid at Pe

preferred direction for the guide rope was east-wes

traverses were 170 ft (52 m) long. For the Fort Morto

magnetic and conductivity measurements were made along 

north-south lines which were 600 ft (183 m) long. Dur

Morton surveys, I moved the rope after every 3 or 5 

traverses. I placed a line of flagged poles to mark the 

the rope; because of the hill there and the woods, 

possible to see from one end of the survey line to the ot

wooded area, the rope could not be moved sideways, bu

pulled in and reset on the next line; each shift took 

minutes.

 Many geophysical instruments are computerized now, 

no need to write down any measurements. However, 

important to write down many notes about the survey. 

will indicate the spacing between measurements, the dire

traverses, the direction in which parallel lines progre

settings of the geophysical instrument. The notes will a

the date of the survey and the start and stop times. The 

be noted, for it may affect the measurements.

 If the measurements are recorded with pencil and 

should be written as a matrix of numbers on a page; a n

lines ruled in perpendicular directions will simplify 

have found it best to write the numbers at the inte

lines. The numbers should not be written as isolat

numbers, for this makes it difficult to compare one 

traverse to the next. As you do the survey, you will wan

your current measurements to those you found on the a

which you just finished. If you find that they are 

higher or lower than those numbers, it will alert you t

error. Also, you may be able to see the pattern of measu

this may suggest that you should extend your area of sur

to explore the geophysical anomaly further.

 If your measurements are stored in the internal me

geophysical instruments, you may have little idea of what 

has found until you make a map of your measurements lat

wait until the end of your survey to do this, unless you 

to gamble that you made no error which will cause you to 

all of your measurements.

Resistivity Surveys

 If you have never done a geophysical survey before, a 

survey should be the first one you do. Resistivity 

simpler than all of the other surveys, and there are few

go wrong. While the measurements can be rather slow, 

surveys give good results at more sites than will any o

survey. Good introductions to resistivity surveying hav
8

stone, and very dry soil have a high electrical resistivity

which has a large fraction of clay, or which is wet or salin

low electrical resistivity. Metals have the lowest resistiv

short list of approximate resistivities is given below:

 Resistivity, Conductivity

 Soil ohm-m mS/m

 -----------------------------------------------------------

 Sand, gravel 1000 - 10,000 0.1 - 1

 Silty sand 200 - 1000 1 - 5

 Loam 80 - 200 5 - 12.5

 Silt 40 - 80 12.5 - 25

 Clay 10 - 40 25 - 100

 Saline soil 5 - 10 100 - 200

The values for electrical conductivity are also listed; as 

see, these increase as the resistivity decreases. Conduct

measured with a quite different type of instrument; it 

described later.

Instruments

 Resistivity meters are the least expensive of all 

geophysical instruments. This is partly because they are 

simple in their electronic circuitry; it is also due to the f

they are widely used for many applications. While geo

exploration is a common application, resistivity meters are a

to determine the quality of a grounding rod of an electric

station or a transmitting antenna. A few years ago, I 

summary of the resistivity meters which I could locate and wh

available for sale (Bevan 1993); this list may still prov

help to your selection of a meter. If you are famili

electronics, you may be able to build your own resistivit

(Williams 1984).

 The basic parts of a resistivity meter are very simple. 

shows that you can construct one with a very few components

simple meter will be excellent for your initial tests and w

you the general principle of a resistivity meter; it may 

helpful as an emergency meter. This resistivity meter will b

to operate than a more expensive meter, but its accuracy 

almost as good.

 The major expense for this meter will be the purchase o

digital multimeter for measuring voltage and current. The on

parts for this meter are: a 6 - 12 volt battery, metal 

electrodes, and wire.

 You will be able to buy the equipment for this resistivi

at many electrical stores. Digital multimeters are small h

instruments for measuring voltage, current, and resistance. 

instruments are powered by a battery, perhaps a 9 V bat

several small 1.5 V cells of the AA size. The measurements a

on a liquid crystal display (LCD); there will be 3 - 4 d

precision shown on the display.
9

principal disadvantage of a VOM is that difficult to r

with sufficient precision. Digital multimeters also ha

input impedance (generally over 10 megohm) than a VOM; 

that the meter itself does not change the measurements by 

much power from the circuit being measured.

 Your next purchase will be a battery for sending a

current through the earth. A voltage in the range of 6 

probably be fine. You could use a common lantern batter

lightweight and rechargeable motorcycle battery (12 V). 

in Fig. B1, you may also make a 9 V battery, with 6 me

cells (each with a voltage of 1.5 V) wired in series. 

were held in a plastic battery compartment. Since the cu

from this battery is very small, this battery would las

year's worth of measurements.

 The voltage of the battery is not very important. 

to measure the earth to a depth greater than 10 m, or if 

quite dry, you may need a higher battery voltage in o

enough current to flow in the soil; a lower precision DM

also require a higher voltage for the battery. 

resistivity meters can apply voltages of 40 - 1000 v

earth. At these higher voltages, painful or dangerous 

possible if one is not careful; since you know that you c

terminals of a 12 V battery without harm, this lower volt

safe.

 You will also need four metal electrodes; these wi

into the soil to a depth of about 2 - 10 cm. The elect

made of iron, aluminum, brass, or any other metal. 

screwdrivers will work fine. Meat skewers made for b

ideal; these are iron rods which are 25 cm long and whic

to minimize rusting. They are pointed on one end and ha

the other; this allows them to be easily pushed int

Surveyor's pins cannot be used because of the paint whi

them.

 The final major item which you will need is wire

length of about 5 - 10 m. This wire will connect the bat

to the electrodes. This wire should have a plastic insul

must not be cracked or damaged so that bare metal 

Stranded copper wire would be fine; solid copper wire 

readily than stranded wire. The diameter of the copper 

can be in the range of 18 to 22 gauge (a diameter of 0.6 

 You can probably use as many as eight connectors 

contact from the ends of the wires to the battery and the 

Alligator clips are excellent, and the name is a good de

the jaws; these clips can have screw terminals. You will 

and a screwdriver to prepare these wires with the clips 

If the spacing between your electrodes will be 1 m, t

probably be easiest to make four wires, each about 2.5 

each with alligator clips at each end of the wires. There 

advantage to having two red wires and two black wires
10

different names. The Wenner configuration is excellent f

applications.

 If a resistivity profile or map is desired, the spacing 

the electrodes remains constant for all of the measuremen

features are thought to be underground at a particular dep

spacing between the electrodes can be about the same as tha

With this spacing of 1 m, features between a depth of about 

1.0 meter will be accentuated; however, deeper and shallower 

will also be detected.

 The depth of the electrodes is not very important. An i

depth of between 2 cm and 10 cm will probably be suitable. 

best that their depth not be more than about five percent 

spacing between the electrodes, otherwise the depth may 

affect the measurement. It is also best that the electrode

go deep enough to reach moist soil. If the soil is very 

measurement accuracy may be improved by pouring water arou

electrode. If you find it necessary to change the location 

of electrodes during a measurement, you may have to st

measurement over again.

 For Step 2, connect the digital multimeter to the middle 

electrodes. If the leads from the DMM are too short or do 

clips on them, you could use extra wire and alligator clips 

the distance. Remember that no bare conductor or clip shou

the soil or vegetation on it; if this happened, there wou

different path for current flow and an incorrect reading. 

prevent this by placing bare connectors on small plastic sheet

 Set the DMM to read a DC voltage of about 200 mV (if t

is adjustable) and turn on the meter to measure the voltage 

the inner pair of electrodes. Since the battery is not yet c

to the earth, you might expect that the voltage would be zero

the voltage you measure will be small, it will not be zero. 

because the chemical reaction of corrosion is generating a vo

the electrodes; because of small differences in soil mois

chemistry at the two electrodes, there is a net voltage mea

the DMM. This example shows a measured voltage of 19 mV. Co

soil resistivity meters use alternating current rather than 

current for their measurements; there is no residual volta

this 19 mV with those meters.

 At first, you should watch the voltage at the inner el

for a minute or two to see if it remains rather constant. 

changes by a significant amount (perhaps 20 percent), you ma

able to use this simple meter at the site. Before giving 

should test different electrodes to see if they show less cha

might even consider stainless steel or lead electrodes. Y

also try measuring the voltage at a constant delay time after 

each group of electrodes.

 The voltage connections of the digital multimeter are la

plus and minus (and the standard leads from the meter are col
11

 A different voltage will now be measured. This w

have a magnitude which is much larger than the voltage 

step 1. For the example of Fig. B1, the measurement i

voltage of V1 = 93 mV. Note that this is much smaller 

of the battery; most of the battery's voltage is "lost" v

the two outer electrodes.

 If the measured voltage was -10 mV rather than +93 

mean that the battery did not change the voltage very 

middle pair of electrodes. It would also mean than it 

possible to get a very accurate measurement of resistivit

help to try watering each electrode or driving them dee

soil; you might also use a higher battery voltage.

 The final measurement is Step 4. The current going 

earth from the battery is measured. The DMM is switche

current; on some meters, it is also necessary to change 

of one of the wires going into the meter. The connect

inner pair of electrodes are removed and the DMM is put 

of the battery. Fig. B1 shows the polarity require

positive terminal from the DMM can be connected to the p

from the battery.

 The meter would not be damaged by having its 

reversed; the only change would be that the current w

negative value. The example in Fig. B1 has a current of 

coming from the battery.

 If this current was less than 0.5 mA, it could ha

trouble with the measurement. A low current is an ind

the resistivity measurement is going to be less acc

procedures mentioned in step 3 could be used to try to i

current.

 Since the battery voltage was 9 V, Ohm's la

calculation of the electrical resistance between the ou

electrodes. This resistance is the ratio of 9 V divided 

or a value of 4100 ohms. This is a low resistance and 

there is no difficulty with the measurement. A resistan

ohms would indicate that the soil was quite dry and 

resistivity measurement would then be less accurate.

 With Step 5, the earth resistance can be calcul

simple equation of Fig. B1. Be careful with the signs 

The current, I, should always be a positive value. Vol

usually be positive also; however, V0 may be either 

negative. The difference of the voltages must also be pos

 The calculation of this example gives a resistance 

If you are doing a resistance profile or map, you can plo

of resistance; by convention, it is plotted at a point 

the midpoint of the four electrodes.

 You may ask why four electrodes are used for this me

resistance, for you may have noticed that the DMM has th

for measuring resistance directly. Also, in step 4, th

between the outer pair of electrodes was calculated; th
12

 The resistance value of this calculation would be very d

if we used a different electrode spacing or if we otherwise 

the arrangement of the four electrodes. It can be convenien

convert the resistance values to the more universal va

resistivity. This is Step 6.

 The conversion of resistance to resistivity is done w

simple equation shown at the bottom of Fig. B1. The resistan

is multiplied by the electrode spacing and then by a consta

of 6.28; for this test, the calculation yields a resistivity 

320 ohm-meters.

 Unlike a value of resistance, a resistivity value can b

up in a handbook and it can be readily compared to other s

surveys. This value of 320 ohm-m indicates that the soil 

location must be quite sandy or rocky. It says that there de

must not be a large fraction of silt or clay in the soil.

 Stores which sell equipment for gardeners have a soil 

meter which will indicate if the soil needs watering. Thes

have a single metal rod with an insulated electrode at its t

meter measures the electrical resistance between the two met

of the rod; some meters will have the rod made of two di

metals so that it acts like a battery also (Logan 1945 p. 1

principle, these gardening meters can be used to meas

resistivity of the soil. Their principal disadvantage is t

are very sensitive to changes just at the junction between 

parts of the electrode; if the insulating gap was larger

meters would measure a larger volume of the soil. The rods 

be inserted to the depth at which measurements are wished.

Mapping

 If you have many measurements to make, your survey will 

faster if you have a good resistivity meter. A good res

meter sends an alternating current into the soil, and this el

the constant voltages which would otherwise be measured 

electrodes. While the home-built meter described above may 

more than a minute to make each measurement, a standard res

meter may only require about 10 s to make each measurement.

 Resistivity surveys are commonly done by making meas

along a straight line. If the four earth-contacting electr

equally spaced, the survey is said to be done with the 

configuration of electrodes. A sketch of this is shown at th

of Fig. A89.

 When the Wenner configuration has the four electrode

together, the measurement emphasizes soil contrasts which a

to the surface. If the spacing between the electrodes is lar

measurement is affected by strata which are deeper in the s

you are searching for a moderately compact object (ideally, a 

in the soil, the distance between the adjacent resistivity el

should be roughly the estimated depth to the middle of tha
13

people to help, four people can each move one of the elec

this approach, you can move each electrode any distance y

it is generally adequate to move them a distance which 

the spacing between the electrodes. Instead of having 

move the electrodes, you could construct a rigid bar to h

electrodes at the proper spacing (Ehrenhard, Athens, 

1984); then, one person can move all four electrodes at a 

 As another approach, you could leapfrog the electrod

time. Just have someone move an end electrode and its 

opposite end of the array. The person carrying the resis

will move the wires and connections on the meter at th

There are ingenious switching systems which will elimin

for the meter operator having to move the connectors (J

1982 personal communication).

 Each measurement is plotted in your notebook at 

between the four electrodes. You should also record the 

the line of electrodes which you used; the measuremen

somewhat different if the line of the four electrodes wa

90 degrees.

 Resistivity surveys are most easily done at a sit

moist soil; if the soil is very dry, it may be difficult 

electrodes into the soil and poor contact with the soi

errors in the measurements. You can check for these erro

each of the four electrodes by a small amount; an error 

if there is a large change in the resistivity then. If 

dry, a better electrical contact to the electrodes is p

small amount of water is poured around each electrode. 

will affect the reading, the error will be smaller 

electrodes were not watered. Resistivity measurements ca

hard or concrete pavements if absolutely necessary. Mo

sponge with saltwater and put a metal plate on top of i

of these, an adequate contact to the pavement may b

Electrodes may be driven into the mortar of a wall, but 

between the electrodes may not be uniform, and you may ha

your measurements for this (Tagg 1964 p. 17). If a rock 

prevents you from inserting an electrode at the correct 

the electrode a short distance perpendicular to the line 

electrodes.

 For normal soil, any type of metal rod can be a goo

screwdrivers or meat skewers are suitable, but they are 

may add extensions to these so that you do not have to 

place an electrode. Since normal iron electrodes will r

use stainless steel to minimize any cleaning that you have 

 For your first surveys, the Wenner configuration is 

and easiest to begin with. Since I do geophysical survey

the Wenner array is not practical for me. I apply a met

pole-pole configuration (Sumner 1976 p. 23); for this co

two electrodes, rather than four, are moved during the 
14

sites, for the long wires going to the reference electrodes m

difficulty if there are people walking in the area; you ca

shorter distances to the reference electrodes if a slightl

error in the measurements is acceptable (this error is 

gradient in the measurements, and it is generally correctable

 During my surveys at Petersburg, the reference electro

located in different areas for the surveys which I did on 

sides of the park road which separates the Taylor House grid 

Fort Morton grid. The electrode locations are plotted in Fig

 It takes additional time to place the distant r

electrodes for the pole-pole array. However, the resistiv

generated with the pole-pole array can be simpler than th

with the Wenner array. This is because Wenner resistivity 

have auxiliary or spurious patterns adjacent to small f

Instead of finding just high resistivity measurements over 

resistivity feature, there might also be abnormally low va

resistivity before and after the feature along the 

measurement (Weymouth and Huggins 1985 p. 224). This pro

reduced with the pole-pole array.

 The pole-pole array is not the same as what has been ca

twin-electrode, two-electrode, or two-probe array (Clar

Aspinall and Lynam 1970; Kelly, Dale, and Haigh 1984). F

other arrays, the two reference electrodes are placed close 

other; this makes it easier to set the two reference ele

However, the resistance values measured with the twin-electro

cannot be converted to resistivity. This means that the type 

cannot be determined, and it also means that a resurvey of 

cannot be compared to an earlier survey. While the pole-po

takes more time to set up, the benefits which it offers can 

the time.

 For the Petersburg surveys, the spacing between the tw

electrodes of the pole-pole array was fixed at 5 ft (1.5 m

array detects features to the same depth as does the Wenn

(Roy and Apparao 1971), so my survey accentuated features 

depth range of 0.7 to 1.5 m. For these resistivity surveys, 

two calibrated ropes along parallel lines, spaced by 5 ft (1.

made measurements with the electrodes along one line, and a

the electrodes perpendicular between the two lines. The mea

sequence is illustrated in Fig. C80. The measurements were 

at the midpoint between the two electrodes. The conversi

resistance to resistivity is the same for the pole-pole arra

is for the Wenner array; see Fig. C87.

 The locations of my resistivity measurements in the Tayl

grid are shown in Fig. B2. The resistivity map of the 

plotted as Fig. B3. It took me 5.5 hours to make t

measurements of this map; this does not count the time req

place or retrieve the reference electrodes.

 The negative numbers at the bottom border of the con

indicate that those coordinates are to the west. Numbers on 
15

these anomalies may be due to a poor contact at an elect

is very little correlation between the small-area anomal

this survey and the anomalies detected by the radar, m

conductivity surveys.

 This map has a small-area irregularity, no

graininess; this makes it difficult to see some of its pa

larger patterns can be clarified by smoothing the measure

map. This has been done for Fig. B4. The smoothing 

operation. Each original measurement has been replaced by 

of the sum of that measurement and the eight 

measurements. This simplifies the map of Fig. B4 an

general pattern of the resistivity. However, this sm

removes small-area anomalies, and it also increases the 

anomaly. These changes must be kept in mind 

interpretation.

 The resistivity is highest on the east side of the 

the soil there must be sandier than the soil to the wes

values throughout the map indicate that the upper layers 

be quite sandy. There is a U-shaped pattern near E20 N

centered on the eastern side of the Taylor House cellar 

map to Fig. A3). The low resistivity may be caused by th

of the cellar, but the anomaly does not correlate with 

where the cellar wall is closest to the surface. Perhaps 

could be partly caused by a trench which was dug to rem

the brick from the wall during the Civil War.

 There is not much to be done for the interpretat

resistivity map. The areas of high and low resistivity 

distinguished; this has been done in Fig. A21.

 It can be valuable to check on the reliabil

resistivity measurements, and a good way of doing this 

repeat the measurements on one line more than once. Th

did at the end of my measurements for the Taylor Ho

repeated lines are plotted in Fig. B5; these show an acce

difference between them. This was not a complete test ho

did not move the guide rope between the repeated surveys.

 A resistivity measurement will change if the array o

is rotated to a different direction. Features within 

cause these differences, and the soil itself may have a 

which is dependent on the direction in which the electr

flows. It is possible that plowing the soil will 

directionality, for it shears the soil. The ar

excavations in the Taylor House area found that the so

disturbed by plowing to a depth of about 0.3 m (Brooke 

personal communication).

 Natural changes in the formation of the soil or duri

sition of the deeper sands could also influence the direc

resistivity measurements. I tested for this directional e

Taylor House with a measurement of azimuthal resistivity 

Fleming 1988). The location of the test is shown in F
16

trenches. The original measurements are plotted in Fig. B8 

smoothed version is given in Fig. B9.

 By comparison with Fig. A36, it is seen that the trench 

detected most distinctly by the resistivity survey. All 

trenches are detected as higher than normal resistivit

suggests that high resistivity sandy soil from near the sur

have replaced the silty soil at the bottoms of the trenches w

were filled in at the end of the war.

 The resistivity map also suggests why the other trenc

not as readily detected. There is an area of low resistivit

middle of the Bombproof detail. This covers part of the b

trenches; it also degraded the radar profiles in this area. 

possible that this low resistivity soil was from the mounds 

over the two powder magazines which were just to the wes

excavations for these two powder magazines were probably dee

any of the other fortifications here; at that greater dep

would have encountered a lower resistivity soil which would h

brought to the surface and mounded over the top of the ma

When these mounds were leveled, the soil might have been sh

the east and this lower resistivity soil might now co

bombproof trenches and thereby mask them.

 Fig. B10 is a surface or "fishnet" plot of the data of 

The low values in the foreground are near the northern magaz

southern magazine does not have this anomaly. These types 

allow the relative amplitudes of the patterns on geophysical 

be easily seen.

 The resistivity measurements over the Trench detail are 

in Fig. B11. The line of the main fortification trench is m

high resistivity along line E480. Therefore, this is t

polarity as the anomaly of the trench found in the Bombproof 

The smoothed map of Fig. B12 does not add much, for the origi

had a simple pattern already. The resistivity is rather hig

northern part of the area; there must be a greater thickness 

soil there.

 Metal objects are only rarely detected by resistivity 

this is generally good. A large metal object should be detec

a very low resistivity, and this is just what was found w

resistivity survey over the Iron detail; see Fig. B13. This 

the lowest resistivity of any place tested in the Fort Mort

however, the west side of the Taylor House survey area had ev

readings of resistivity.

 The other geophysical surveys delineated a buried iron 

the northwest corner of the Fort Morton grid. Resistivity 

were made across this pipe; the line of the profiles is show

top of Fig. B7 and two sets of measurements are plotted in F

Both measurements indicate a drop of about 30 ohm-m, indepe

the orientation of the two electrodes of the pole-pole array. 

do not know if this low resistivity is caused by the pipe its
17

resistivity of soil layers in an area. This information 

valuable for a prediction of the depth and type of soil w

encountered by an excavation. The depth of bedrock mi

determined by a resistivity sounding.

 With a resistivity profile, the separation 

resistivity electrodes remains at a fixed value, and th

are shifted about the area to be surveyed. With a 

sounding, the center of the array of electrodes remains 

location, and the spacing between the electrodes is cha

the electrodes are more widely separated, the mea

resistivity averages through a greater depth. An anal

measurements of a resistivity sounding will then estimat

of soil layers.

 The Wenner array is excellent for resistivity so

basic electrical schematic of a resistivity meter is sh

B15, and the separation between adjacent electrodes is 

S. This figure shows the first resistivity sounding whi

the Taylor House. For that sounding, the center point 

four electrodes was at E66 on line N85; this is east o

House cellar. The 12 measurements which I made are 

circles in the graph; I changed the spacing between the e

steps from 0.1 to 3 m. Note that both of the axes of t

logarithmic; this is the typical way of plotting soundin

the best way.

 The graph of Fig. B15 reveals some interesting 

values of resistivity are higher than would be found a

site; the soil must have a large fraction of sand or g

it. Had the soil underground been quite uniform and unc

depth, the sounding would have plotted as a horizontal 

not, and so the soil is stratified. The resistivity i

the electrode spacing is larger; therefore, the resistivi

with increasing depth underground. Finally, the soundi

rather flat for small and large electrode spacings; 

gradually decrease from high to low and do not show more 

changes. This suggests that the soil stratification is ra

there are only two distinct layers which were detected.

 A simple analysis can furnish an estimate of the 

the upper soil layer, and it can also give good estim

actual resistivity of the soil in the two layers. The 

done by the procedure called curve matching (Keller and 

1966 p. 135). The measurements are plotted in the s

shown in Fig. B16, and then these measurements are comp

calculated curves which are in Fig. B17.

 The curve of Fig. B16 is just slid over the surface 

(without rotation) until the measurements most closely m

the calculated curves in Fig. B17; this match can be see

the two sheets in front of a light. A reference point in 

then transferred to Fig. B16; this point is where the hor

marking a resistivity = 100 intersects the vertical lin

that the spacing = 1; these are both solid lines in Fig
18

resistivity measured with S = 0.8 m could be an error. The l

with S = 0.1 m could also be an error, but it is also possi

there is a thin layer with lower resistivity right at the su

the soil. This analysis has assumed that there are only two 

if there are actually three or more different layers, the res

sounding curve might show smooth oscillations between three 

high and low values of resistivity. While a qualitative anal

be enough for you, if you wish to analyze the depths of the d

layers, a more complicated curve matching is needed (Orel

Mooney 1972; van Dam and Meulenkamp 1975).

 The analysis of this resistivity sounding assumed that 

had a very simple stratification, and that the resistivity 

soil did not change in a lateral direction; this means 

resistivity map of the area would show no features. 

assumption is not correct, the resistivity sounding curve 

affected by these lateral changes. Fig. B19 illust

resistivity sounding which I did over the cellar of the Taylo

The measurements, indicated with circles, show the same 

resistivity decrease which was found to the east; that res

sounding is shown in Fig. B15. However, there is an additiona

the measurements when the electrodes were separated by a dis

about 4 and 5 m. It is impossible for horizontally stratif

to cause this abrupt a change in the sounding curve; theref

cause of the dip must be a lateral change in resistivity, rat

a vertical change. The outline of the buried cellar is esti

the figure; notice that the outer electrodes cross from in

outside the cellar at a spacing of about 5 m. This lateral 

is the cause of the unusual values of the sounding.

 Volume C describes methods for the measurement and ana

resistivity soundings which are more complicated and precise.

Magnetic Surveys

 After resistivity surveys, magnetic surveys should be 

procedure you should consider doing as you begin to lear

geophysical exploration. With a magnetic survey, there a

detailed procedures to learn than are needed for a res

survey.

 After you have made a magnetic map, you will be able to 

the depth and mass of the buried iron-containing objects. T

give you valuable information, but it will require additio

for learning the method. The basic procedures for doing 

surveys, and interpreting the measurements, have been 

described by several writers (Breiner 1973; Weymouth and 

1985; Weymouth 1986; Clark 1990; Logachev and Zajarov 1978).

 A magnetic survey is typically done with two people, 

may be fewer than the number needed for a resistivity sur

magnetic survey is much faster than a resistivity survey, an

readily be done in wooded areas. The cost of a magnetometer 

more than the cost of a resistivity meter.
19

nT). The average strength of the Earth's magnetic fie

50,000 nT. The amplitude of the magnetic anomaly f

archaeological artifacts may be in the range of 1 - 10

means that the changes in the measurements during a sur

small. With a resistivity survey, the ratio of the hi

lowest measurement may be about 10; with a magnetic 

ratio may be much smaller, perhaps about 1.01. 

magnetometers are very accurate, and most will measure to 

of 0.1 nT.

 The names for the different types of magnetometers d

fundamental aspect of their physical principle of oper

most common instruments are called proton (or proton 

cesium (or alkali vapor), Overhauser, or fluxgate m

(Dobrin and Savit 1988 p. 660). All of these differe

magnetometers are suitable for archaeological survey

differences in their capabilities are not always important

 Any of these different types of magnetometers can b

in two different modes; they can have a single sensor and 

total magnetic field of the Earth, or they can have two 

measure the difference in the field at the two sensors. 

type of instrument is called a magnetic gradiometer. Wh

magnetometers are almost always configurated only 

gradiometers, the other instruments can be changed from 

the other. Fig. A87 has a sketch of a magnetometer in t

modes of its operation.

 There are advantages and disadvantages to the t

operation. A magnetic gradiometer allows an automatic co

changes in the strength of the Earth's magnetic fie

survey; a total field instrument requires a mor

correction. A magnetic gradiometer is not affected b

distant iron objects; these may be cars passing on a 

Also, a magnetic gradiometer can distinguish feature

closer together than can a total field magnetometer. Ho

necessary to make measurements at a closer spacing with a 

than with a total field instrument (Reid 1980). 

gradiometer will be more difficult to carry, since it has 

rather than one. A gradiometer will indicate the wrong 

if it is tilted; a total field magnetometer does no

problem. Since a magnetic gradiometer measures the 

between two nearby sensors, the anomalies which it meas

smaller than those measured with a total field magnetomete

 Some modern magnetometers allow a great flexibili

operation; this will allow them to be applied to a wid

sites and circumstances. In addition to being able to be 

both total field instruments and as gradiometers, the spa

the sensors can be changed in the gradiometer mode and 

height can be set at any value. These options allow the r

the measurements to be changed. These modern magnetometer
20

these factors when you decide on which instrument you will 

buy for your survey. An earlier review of many instrum

getting moderately old now (Bevan 1991), but it might give 

information to help your decision.

 Magnetometers measure the total field or the strength 

Earth's magnetic field; they do not measure the direction 

Earth's field. A magnetic compass measures only the directio

field, and not the strength (although the oscillation period 

compass needle is related to the strength of the field). El

magnetometers were first developed in the middle of this 

before that, magnetometers were entirely mechanical and w

dissimilar to magnetic compasses or dip needles (Swanson 1936

1931; Howell 1959 p. 361; Haanel 1904). This type of me

magnetometer was used for an early archaeological survey 

1970); however, the speed and accuracy of these instruments 

If you are seeking a massive iron object, a compass may be 

need to locate it. This is was done in 1956 for a successfu

for a boat which was sunk on the Mississippi River during t

War (National Park Service 1971). The massive iron object n

Morton at Petersburg is quite detectable with a magnetic comp

circles and squares in Fig. B20 show that the apparent dire

magnetic north changed by about 2 degrees near the iron-fill

The deflection of the compass needle is indicated in Fi

although it is exaggerated by a factor of 20 there for clarity

Before the survey

 For your first surveys, you should try to select sit

will have magnetic anomalies which have a rather large amplit

these will be easier to survey; historic sites usually ha

larger anomalies than prehistoric sites.

 Before you begin a magnetic survey, it is good to check 

is very much modern iron trash in the area of the survey. 

be on the surface of the ground and quite visible. If the 

sensor for your survey will be at an elevation of about 1 

objects the size of a metal can lid or larger are best remo

the survey area. If the magnetic sensor will be lower than 

might also remove every iron object larger than a small nail. 

do not have enough time to clear a site of this magnetic tr

will need to analyze the magnetic measurements and indica

trash is likely to remain. There may also be additional ir

shallow depth in the soil. You can search for this with 

metal detector or with an audio-indicating fluxgate magnetom

it is necessary to do a careful search for buried iron tra

site cleaning may take longer than the magnetic mapping of the 

 It might be valuable to do a quick and unrecorded explor

the site before you begin your mapping. This will tell you t

of measurements to be expected. It might also indicate tha

small part of the site should be mapped; the other areas may 
21

the magnetometer which you will be using. Just tie 

sensor to a tree at the same height that the sensor will 

Make a few magnetic measurements with no one near the s

have the sensor carrier stand closer to the sensor than 

make a few measurements as that person rotates at 90 d

around a circle. Finally, make a few more measuremen

carrier away from the sensor, in order to make sure 

measurements are about the same as they were at firs

maximum change with the carrier close to the sensor is on

2 nT or less, then that person will probably have n

affect on the measurements of the magnetic map.

 You should decide what the height of the magnetic s

be. For almost all surveys, this will be in the range 

m. If you seek to locate small and shallow objects, you 

low sensor; for larger and deeper features, a higher sens

can be suitable. Perhaps a good guide would be to have t

an elevation which is roughly the depth of the features 

If there is modern iron trash in the soil, or if the s

soil are magnetic, you will need a sensor height which 

than normal. Otherwise, you will generally wish to ha

height which is as low as possible (Smekalova, Myts, 

1993). This will increase the amplitudes of the magneti

but it will also require that the measurements be m

spaced. If the features are quite broad, you may use a h

and more widely-spaced measurements.

 It is generally best that the spacing between your 

be about the same as or smaller than the height of the 

the ground or above the features. Most archaeological 

done with a measurement spacing which is between 0.5 an

magnetic maps which I measured for the survey at the 

will show you the effect of changes in the measureme

Remember that measurements with a magnetic gradiometer wi

smaller measurement spacing than measurements with a 

magnetometer.

Simple procedures

 Expensive and complex magnetometers are neither n

sufficient for high-quality magnetic surveys. You will b

excellent work with a simple magnetometer and you will 

computer. In this section, I will describe the simpl

which were used to make a magnetic map at the archaeolog

Fort Laramie, Wyoming, during a National Park Service cour

 The part of the survey illustrated here was a 5 x 7 

The survey was done with a single proton magnetometer

magnetometer could have stored its measurements in i

memory, they were written on paper instead; see Fig. B22

of the magnetic sensor was fixed at an elevation of 0.7 

anywhere in the range of 0.4 - 1 m would have been good. 

between the measurements was set at 0.5 m, slightly le
22

location.

 For the magnetic survey at Fort Laramie, these tempora

were made at a point near E0 N0 in Fig. B22. A few pre

magnetic measurements near that point showed that there was 

magnetic anomaly there; had there been one, the temporal chec

have been in error if the exact point was not relocated 

measurement. Before the start of each line of survey, the 

field at the base station location was measured; these val

written at the bottom of the page of measurements, just b

broken line in Fig. B22.

 The magnetic survey was done with three people. One 

the measurements, one carried the magnetic readout, and one 

the sensor. The equipment was similar to that at the bottom 

A87, except that there was only one sensor on the vertica

rather than two. The sensor carrier stopped near each mea

point and set the sensor so that it was directly over the poi

person with the readout was at a distance of about 2 m so 

somewhat magnetic readout console had little effect 

measurements. Measurement traverses were also made in o

direction to further reduce the effect of iron in the readou

readout operator called out the measurement to the per

recorded the measurements as a matrix in a notebook.

 The numbers recorded in Fig. B22 are abbreviations of t

values. The actual field at this site was around 55,700 nT 

changes from this value are recorded. For example, a value 

the map means a field of 55,729 nT and a value of 697 in 

means a field of 55,697 nT. This simplifies the recording 

measurements. Additional information is recorded at the sid

map in Fig. B22; this should be on each page of field dat

direction of north and a scale are essential, along with the 

the survey. The sensor height and the traverse direction sho

be noted.

 The base station measurements, written below the broken 

Fig. B22, indicate that the change in the Earth's magnetic f

larger than normal. This survey was done on two days, and t

between days was between lines E2 and E2.5. The Earth's 

field was definitely stormy on the second day; this was the 

the abrupt change in the base station value for the last line

an adequate correction for this change was possible, if yo

large changes during your survey, you might just wait for 

day to do the survey, just as you might wait for a thunderstor

 The correction of the shift in the magnetic field can b

to do. Just adjust each column of measurements so that the 

value of the Earth's field at the base stations remains at a 

value; this is done by adding numbers to each column. F

greatest reading at the base station; this is 53. Then determ

number should be added to every other base station reading 

the sum is 53; this will cause the Earth's field to be eff
23

preliminary step before starting to draw. Examine each a

of numbers, and check to see if one is greater than 50 a

is less than 50. If so, place a dot on the page at abo

contour line will go between the numbers; for a good map

an approximate linear interpolation between the numbers. 

shows how I have placed these points for the 50 contour li

 This first contour line is drawn in Fig. B24. The 

simply connects the dots. At the right hand side of the 

the contours indicated with broken lines. The four num

square there form what is called a "saddle surface", an

dots on all four sides of the square. There are two alte

of contouring this square; either one could be used, an

line contour would be just as accurate as the solid 

which I drew. Choose one or the other, whichever ma

simpler or easier to visualize.

 After the first line is finished, select other 

values in multiples of perhaps 10 or 100 and draw these l

continue to draw contours with lines spaced between these 

until the map is generally filled with contour lines. 

each contour line, you will see that high numbers will be 

of your advancing line, while low numbers will be on the 

It is best to draw smooth contour lines, for this clarifie

 The resulting contour map is shown in Fig. B25. Th

the pattern of the numbers easy to see; the patt

measurements is very difficult to visualize in Fig. B23

draw a contour map, lines should not obscure the mea

being drawn through them, unless you are working with a 

original measurements.

 The numbers show those areas of the map which have 

and which have low; however, it can be helpful to finish 

distinguishing these polarities. Just get a red and blu

shade in the anomalies; it is conventional to use a re

high values and a blue color for low values. You may al

perpendicular tick marks (hachures) on the contour line

low values; most of the other maps in this report 

convention. You can draw the contour lines of anomalie

values using solid lines, while the contours of low 

given broken or dashed lines. Another possibility is tha

random dots in the areas with either high or low values.

 An adequate interpretation of a magnetic map is si

and it adds much information which the map itself does 

This procedure is summarized in Fig. B26. The first st

locating the magnetic anomalies. These are just the area

where the measurements are abnormally high or low. For 

map, the highs are generally the most important; ther

distinctive areas with high measurements on this map. F

value at each high anomaly. Then, estimate the magnetic 

but away from the anomaly; this value is called the backg
24

is approximately equal to this diameter. To get th

underground, just subtract the sensor height. Next, the 

buried iron is estimated. The equation at the bottom of 

shows the calculation; the equation gives an estimate of the 

iron mass, in kg.

 For the final step, the iron is located on the map. 

often a small amplitude magnetic low to the north of each 

high. This low is also caused by the magnetic object. It is 

possible to tell that a magnetic low is associated with a 

high by noting the close spacing of the contour lines betwe

Even if the associated low is not in a northerly directi

object is probably along a line from the peak high a short 

toward that low. Sometimes there is no clear low associated 

magnetic high. The magnetic high at E4 N6 is an example; t

lows to the northeast and southwest, but there is no concentr

contour lines from the high to either low. For this anom

source can be estimated to be at the anomaly peak.

 It is more likely that these estimates of depth and mass 

large than too small. This is because the analysis has assu

the objects are compact or spherical. If the object is 

spread out along a line or in a lens shape, the estimates 

depth and mass will be too large. The mass estimate has also 

that the object is iron; in fact, it may be fired earth or a 

stone. A magnetic survey cannot usually distinguish these d

magnetic features. If the feature is actually fired earth, s

kiln, fireplace, or furnace, the mass of fired earth could be 

100 times the weight of iron which was estimated from this a

If the feature is a concentration of bricks in a wall or fou

the mass of brick could be about 1000 times the iron e

Igneous rock can be similar to brick in its magnetic propertie

 One must always be cautious about geophysical anomali

extend in the direction of the measurement traverses; these 

caused by errors. The left side of the figure shows an e

anomaly which is probably caused by a short, abrupt change 

Earth's magnetic field during the measurement of that line. 

station readings were not frequent enough to correct for this 

Magnetic interpretation

 This section will explain some of the basics of 

interpretation in greater detail than was done in the prior 

Even more detailed procedures are described in Volume C.

 There is a tendency for beginners who do magnetic sur

filter, "enhance", and otherwise display or modify their 

ments, but not to interpret them. With interpretation, one 

describe the buried feature itself (its material, shape, dept

and orientation) rather than just describing the geophysical 

ments. It is indeed more difficult to interpret a magneti

than to only present the magnetic map of a survey. However, t
25

simple survey above.

 A justification for this approximation is given in 

mass of 1 kg of iron is assumed to be buried undergroun

of 0.5 m, while the magnetic measurements are made at an 

0.85 m above the ground. This figure shows the calcula

map which would result if this iron object was at Pete

circular contours reveal the high magnetic values. The m

which is north of the high, is indicated with a hach

line.

 The peak of the anomaly is 21.3 nT, and the solid 

is half this value. The average diameter of this half-w

is 1.38 m; this is almost the same as the depth of the ir

sensor, which is 1.35 m. This is the half-width rule: 

the object below the sensor can be estimated from the dia

anomaly at half its peak amplitude.

 The calculations of Fig. B28 have been made assumi

survey was done at Petersburg, and also that the measu

made with a total field magnetometer. However, the hal

is a good approximation for any location and with 

magnetometer. This is revealed by the nine calculated 

B29. The three rows of maps show the calculations at 

poles (the dip of the Earth's field is 90 degrees), in 

the Earth's field dips by 45 degrees (in countries such a

Tanzania, and Jordan), and in areas where the Earth's ma

is horizontal (countries such as Brazil, Nigeria

Philippines). The three columns have the calculated a

three different types of magnetometers. The column on 

for a total field magnetometer, such as a proton magnet

middle column assumes that the total field magnetome

sensors, one above the other, and the gradient is calcula

difference in the field at the two sensors. The left col

magnetic gradiometer which measures only the vertical 

the magnetic field; fluxgate magnetometers are this type.

 For these calculations, a steel mass of 1 kg is ass

m below the elevation of the magnetic sensor; this i

sensor of the gradiometers. The broken line conto

intervals of either 10 nT or 10 nT/m. The solid line 

drawn at a level which is half the peak anomaly. For 

maps, the average diameter of this contour line is about 1 

 The maps reveal two other interesting things. 

pattern of the anomaly is very similar for the three magn

each value of magnetic dip, except for the different ma

with the vertical component gradiometer at the magnet

Second, the iron object is assumed to have only 

magnetization; the magnetic anomaly at the equator is hal

is at the poles, and it is also a negative value at th

magnetic low.

 If the magnetic object is not small, the magnetic 

indicate something of the shape of the object. Buried ir

usually very apparent because of the linear patter
26

pipes in the northwest corner of the Fort Morton grid; see Fig

 As features become larger or longer, a magnetic map wi

to reveal their shape. However, the shape will still be blu

it will only approximate the actual shape of the feature. 

square magnetic feature will result in a magnetic map li

illustrated at the top of Fig. B31. The interpretation of a 

map would be simplified if a uniform pattern of high 

readings was found over a magnetic object; this does not 

Instead, this figure shows that there are low readings at on

the object and high values at the other end. The map also 

interesting fact. If there is a hole in a magnetic object

detected predominantly as a magnetic low; the small square 

middle of the figure locates this hole.

 The map at the bottom of Fig. B31 shows another type of 

which can be encountered by an archaeological survey. Norma

magnetic anomaly of objects measured in the northern hemisph

have its magnetic low on the north side of the high. However

object is above the sensor, the magnetic high will be north 

low. Elevated magnetic objects are commonly found as el

transformers on utility poles and as roofs which have iron s

ceramic tiles.

 Several different magnetic surveys were done at the 

House. These were done to illustrate the effect of different 

between the magnetic measurements. Fig. B32 shows a low re

survey; Fig. B33 shows a medium resolution survey; and Fig. B

a high resolution survey.

 The magnetic map of Fig. B32 was made with measurement

by 5 ft (1.5 m). While this survey was fast, it detected mos

features in the area of survey. Solid line contours are at i

of 10 nT; a few broken line contours interpolate to a level 

Note that the magnetic field is not indicated on the contour

is because the actual value at each contour line is n

important; it is the relative amplitude and the lateral grad

the anomalies which are important. This magnetic map was int

using the half-width rule, and the magnetic objects were as

be located at the peak of the magnetic highs. This interpret

given in Fig. A15. A broken line in that figure marks th

magnetic pattern which is caused by the southern brick wall 

cellar. The width of the anomaly along a north-south line 

assumed to be suitable for the half-width rule, even though 

an elongated magnetic anomaly; there is a fairly good a

between the depth determined by this approximation and the 

determined by more detailed procedures for the interpretation

 If the spacing between the measurements is halved, four 

many measurements are needed for a survey. The magnetic map 

B33 has a measurement spacing of 2.5 ft (0.8 m), half that of 

in Fig. B32. Note that the area of the anomalies is typically 

in this map; this is the result of the higher resolution. As 
27

of comparison, there are only 714 measurements in the mag

Fig. B32, and that map required only about 1.5 hours of fi

 There are three different contour levels in this map

a few broken line contours; these are at intervals of 

solid line contours indicate 10 nT intervals, while the 

contours are at 100 nT; the switch between these tw

generally apparent by a gap in the contour line spacing. 

the small details in Fig. B34, the magnetic map is enlar

sections as Fig. B35, Fig. B36, and Fig. B37.

 A very detailed magnetic interpretation was made of 

Fig. B34, and the results are shown in Fig. A13. For th

mathematical models were made of each of the most import

anomalies. This procedure is described in Volume C.

 The area of the Taylor House magnetic survey is far 

the park road that passing cars caused little difficu

survey, however, it was necessary to stop the survey whe

were moving on the track to the west. The magnetic anom

distant iron objects can be greatly reduced with 

gradiometer. The improvement is shown in Fig. B38. 

show how the magnetic anomaly from an object like a c

with increasing distance. At a distance of 10 m from 

magnetic anomaly detected by a gradiometer will be about 

anomaly which would be detected by a total field magneto

m, the anomaly with the gradiometer is reduced to only 

that of a total field magnetometer.

 The Taylor House grid was mapped with a magnetic 

also. The map of Fig. B39 was surveyed with a measuremen

2.5 ft (0.8 m), the same as with the total field map of F

two maps show similar detail. The magnetic sens

gradiometer measurements was somewhat higher above the 

this reduced the amplitudes of its anomalies.

 The magnetic map of Fort Morton shows hundreds o

small objects. The map, in Fig. B40, is drawn with thr

contour intervals: 2.5, 10, and 100 nT. While the same 

is drawn for each interval, it is generally possible to s

contour interval changes, for these shifts are near 

"terraces" where the spacing between lines abruptly chang

maps often have a very wide range in the amplitud

anomalies, and it can be necessary to draw them with sev

line intervals. The magnetic map is enlarged in the two 

Fig. B41 and Fig. B43.

 The relative amplitudes of the high readings on the 

are indicated with the surface plot of Fig. B45. An iro

the former park road is on the right side, while the pi

former farm house cause the group of highs on the left 

small cluster of highs in the near corner of the map is 

Civil War battery. The highest peak in the map is ca

iron-filled well.

 This survey required five days of field work and 
28

place for each day's survey. If you change the location of y

station, you can just remeasure the last line you surveyed t

day and this will allow you to determine the difference bet

two base stations.

 The interpretation of the magnetic survey of the For

grid was done using the approximation of the half-width rul

estimated values of the mass and depth of iron are marked in 

and Fig. B44. As with the interpretation map of Fig. A30, d

symbols indicate the approximate mass at each anomaly. Th

number of each pair shows the mass, while the lower number i

the depth. The distribution of these depths is shown in F

Almost no objects were detected below a depth of 2 m. As 

Fig. B47, small objects were generally shallow, while massive 

were deep.

 Fig. B40 shows one anomaly which covers an unusually lar

this anomaly is centered at about E550 S150. The interpret

this magnetic anomaly is that it is caused by iron objects w

in a well. The pattern of the magnetic anomaly appears to be 

to that of the calculated map shown in Fig. B28. There is 

high, and north of it, there is a wide area with low magneti

the contour lines with tick marks show this low. However, t

depth extent of the iron in this well causes its anomal

different from that of a small object. Because of its 

character, the anomaly is given a detailed analysis in Volume 

 A detailed magnetic map was made of the 100 ft (30 m) s

the bombproof trenches of Fort Morton. This map is plotted 

B48. While many magnetic objects were detected, there 

indication of the trenches which were detected by the ra

resistivity surveys. The interpretation of the magnetic map 

in Fig. A39.

 Note that there is a tendency for some of the contour 

be extended in a north-south direction. These reveal an erro

measurements and these effects are called striations; they 

along north-south lines because that was the direction 

measurement traverses. The map shows that these striations 

amplitude of about 0.5 nT. I made my measurement traverses 

one direction in an attempt to minimize these striations; 

iron which I was carrying contributed to the striations. The 

magnetic field changed rather fast on the day of this survey

is possible that the base station magnetic measurements w

sufficiently frequent for the temporal correction; the int

these measurements was 30 s. While some of the measurements 

to be affected by lightning noise also, this noise is app

errors in scattered measurements, and it is unlikely t

striations were caused by the lightning.

 I am disappointed that I did not notice that the hat whic

for some of my magnetic surveys at Petersburg was magnetic. T

an iron wire around the brim of the hat. When I finally di
29

blurred. Their peaks have reduced and their areas are i

interpretation of this smoothed map would indicate depth

too large.

 The magnetic anomalies of the Trench detail are fou

in only part of the survey area. In Fig. B50, the main f

trench is along line E480 and the magnetic survey detecte

the trench itself. However, strong magnetic anomalies ar

to the east of the trench, and it appears that some 

objects may be within shelter pits dug by the soldiers 

siege. The interpretation of this map is given as Fig. A4

 Several magnetic maps were made of the intense ano

Iron detail. These are plotted in Fig. B51. The map 

right was measured with a magnetic gradiometer. This 

reveals a magnetic anomaly at E513 S137 which is lost 

magnitude of the three total field maps which are also on 

 The spacing of the contour lines shows how the ampl

anomaly decreases as the height of the sensor increases 

0.8 to 1.16 m.

 Striations are evident on each of the maps; these a

positional errors of about 10 - 20 cm during the surve

smoothing filter could improve the appearance of these ma

have not been modified. Striations are always most e

magnetic measurements are made with a spacing which are 

they need to be; since this anomaly has a smooth circu

the locational errors are particularly evident.

Conductivity Surveys

 Conductivity surveys are much faster than resistiv

although conductivity surveys are more complicated to do. 

their speed, I do many more conductivity surveys than 

surveys. This section will be somewhat more detailed th

sections. This is because relatively little has been w

doing conductivity surveys; I will describe the practica

which I have found to be helpful in doing the field work 

interpreting the results of a conductivity surve

conductivity instruments can also be used to measure 

susceptibility of the soil; since this is the m

application of the instruments, the discussion of this 

Volume C.

 The unit of electrical conductivity is the siemens 

abbreviated as S/m. Conductivity is simply the re

resistivity. For example a resistivity of 100 ohm-m is t

conductivity of 0.01 S/m. Because most conductivity va

much less than one, the most common unit for conductivity 

(millisiemens per meter), and the resistivity of 100 o

same as 10 mS/m. In the earlier section on resistiv

volume, there is a short table which lists the 

conductivity of different types of soil.
30

coils are hidden within the ends of the other two instrument

figure. All of these instruments are manufactured by 

Limited, and the model numbers are listed in the figure. 

manufacturers have different types of electromagnetic i

meters for sale. These instruments from Geonics are the on

commonly applied to archaeology. This is partly because two 

instruments have a depth range which is excellent for archae

features. These instruments have also been designed to dis

actual conductivity of the ground, rather than a more t

parameter. The interest of the company in archaeology is ill

by the fact that it has prepared a collection of reprints 

"Selected papers on the application of geophysical instrum

archaeology".

 The EM34 is designed for detecting features at depths 

from 10 to 50 m. This is suitable for only a small fra

archaeological sites. However, if large mounds are to be e

or the course of an ancient river is to be traced, this in

may be excellent. The depth of exploration is determined 

spacing between the two circular coils of the instrument; thi

set at 10, 20 or 40 m. Because of this large separation, it 

that two people operate the instrument, one carrying th

transmitter coil and its electronic box, and the other carr

larger receiver coil and the display console. The co

typically set on the ground, either flat as shown in Fig. 

vertical.

 The principle of the measurement is as follows: An alt

current is sent through the transmitter coil; the frequency 

current is in the audio range. This current generates a 

field about the coil and this magnetic field readily goes 

earth. When this magnetic field passes though conductors 

soil, an electrical current is generated in them; these co

may be just strata of the weakly conductive soil; metals 

necessary. These electrical currents in the soil generate 

magnetic field, which is detected by the receiver coil of 

instrument. This indirect coupling from the transmitter coil, 

the soil, and to the receiver coil, allows the measurement 

electrical conductivity of the earth. While these instrum

detect metal, they are not made for this application; t

designed to detect the much weaker conductivity contrasts 

different soils.

 The other two conductivity meters, the EM31 and the EM

have a pair of coils; their coils are hidden within the end

instruments. These two instruments detect shallow featur

strongly; the maximum effective depth at which the two cond

meters will detect features is dependent on the spacing bet

coils. With the EM31, this maximum depth is about 6 m; for t

the maximum depth is about 1.5 m. If features are within t

range of the EM38, it can be best to use this instrument, for 

allow a higher resolution with its geophysical maps; the E

detect smaller features than those which can be detected w
31

EM31 is more difficult to carry through the woods, an

almost impossible to rotate the 4 m long boom of the ins

horizontal direction if trees are close together.

Interpretation procedures

 These conductivity meters are excellent for locatin

of soil contrasts. If there is a broad pit which has b

sandy soil and then it has been refilled with organic 

might be detectable as a high conductivity anomaly. A l

rubble from a building, now buried in silty soil, may be 

a low conductivity area. Had the pit been dug in silty 

the building rubble was in gravelly soil, both featur

invisible to a conductivity survey, for the archaeologi

would then be too similar to the surrounding soil.

 Conductivity surveys will define the location and 

shape of some features, but quite different features 

similar conductivity anomalies. Fig. B52 shows the cross

three features which are detectable as high conductivi

figure, the dotted soil strata have low conductivity

hachured areas are higher in conductivity. Each of these 

of features can cause similar conductivity highs.

 While these conductivity meters are not designed 

metal, they do it, and sometimes too well. If there are 

and wires in the area, the conductivity anomalies which 

can hide other features in their vicinity.

 Fig. B53 illustrates how readily the EM31 will det

pipe; the middle profile line is mapped at the top of F

three parallel lines reveal the same pattern. This pip

by the magnetic, radar, and conductivity surveys of Fort 

radar indicates that this pipe is about 0.6 m undergroun

below the EM31. The distance between the anomaly maxi

(5.8 m); the interpretation manual from Geonics suggest

distance should be about the depth of the pipe below th

this case, this interpretation must be wrong, and 

determination from the radar is more reasonable.

 For the profiles shown in Fig. B53, the long bar of 

oriented perpendicular to the pipe. If the bar is alig

to the pipe, the conductivity profiles are different; s

The anomaly is entirely positive in this case, and its h

double that of Fig. B53. There is a sharp drop right ov

but this does not go to a negative value of conductivity

and Fig. B53 show that both conductivity anomalies are 

distances from the pipe greater than 10 ft (3 m). 

detectable for a lateral distance of more than 30 ft (9 m

are several pipes at a site, the conductivity map of t

show little more than just the pipes.

 The EM38 is affected by pipes over a narrower spa

illustrated with a comparison of Fig. B55 to Fig. B53. T
32

metal objects; each was square and 0.55 m on a side. The me

gives the strongest anomaly. The EM31 was hung from a tre

elevation of 1.5 m. These objects were slid on the ground b

EM31 and along the length of the instrument. The vertica

line marks where the objects were directly under one of the 

the instrument; this is called a magnetic dipole of the ins

With two of the objects, the maximum anomaly was detected 

point.

 With these same three metal objects, the EM38 detects 

different patterns shown in Fig. B57. The anomalies have o

conductivity and the peaks of the highs are at the middle 

instrument, rather than at the dipoles or coils. The metal 

screen still gives the strongest anomaly, and the metal lo

gives the smallest anomaly.

 A closed loop of wire causes almost the same anomaly as 

plate of the same size. If, however, the wire is made into 

but the two ends are not connected together, then the wire ha

effect on the conductivity meters.

 Both the EM31 and EM38 will detect large metal objects

the EM38 will detect small metal objects, the EM31 will not

B58 illustrates the conductivity lows which were measured ov

objects which were 14 cm on a side. Note that these prof

rather similar to what the EM31 measured over metal objects 

a side; there are conductivity lows close to where one of t

of the EM38 is centered over the objects. I do not know why 

gave a larger anomaly than the thick plate.

 It is likely that many of the metal objects which are 

area of my survey at Petersburg are projectiles from the 

during the Civil War; while many projectiles were salvaged du

after the battle, there must still be many in the soil. I 

four shells or shell fragments from the park's collection an

them; the projectiles are sketched in Fig. C41. The cond

anomalies of these iron artifacts are plotted in Fig. B59. W

whole mortar shell gave a strong negative reading, the oth

fragments just gave weak lows. Lead bullets must be very c

the soil; the EM38 will only detect them if they are very 

one of the coils of the instrument. For my conductivity ma

Petersburg, I deliberately raised the EM38 above the ground 

these hundreds or thousands of small objects would not be dete

 For the illustrations above, I moved a metal object 

ground along a line below the stationary conductivity ins

This same procedure will furnish a map, and an example is ill

in Fig. B60. The square at the middle of the map shows the 

the metal foil; the long rectangle shows the size of the EM

sides of the map indicate dimensions, in meters. I subtra

approximate conductivity of the soil from the measurements 

map.

 There is a conductivity high in the middle of this ano

has a peak value of about 3 mS/m. A contour line at half of t

value, or 1.5 mS/m, has a diameter of about 0.5 m. This is th
33

object; the spacing between the two lows is just the 

between the coils.

 When the instrument is raised in the air, the anomal

a positive value. The amplitude of this anomaly decre

EM38 is lifted higher in the air. The width of the anoma

their peak value appears to be equal to the depth of the 

the EM38. Both of these positive anomalies indicate th

of the object could be about 0.45 m. This object was a

by the magnetic and radar surveys; an analysis of the da

of those surveys confirms that the depth of the object is 

 With this confirmation of the suitability of thi

rule for the EM instruments, I applied it to the inter

the conductivity highs which were detected by the sur

Petersburg battlefield.

 For large features, the conductivity instruments 

true conductivity contrast of the feature. If there i

conductive or clayey soil which is 5 m wide within sandy 

has a low conductivity), the EM31 and EM38 will indicate 

with measurements which actually show high conductivi

conductive feature. Unfortunately, if the feature is r

the conductivity contrast indicated by the EM31 or EM38 

opposite polarity of the actual feature. If the conduc

above was a clay-filled pit in sandy soil, and the diam

pit was only about 1 m, then the EM31 would indicate thi

a low conductivity anomaly, and not its true high conduc

the EM38, it is possible that this switch in anomaly 

occur for features smaller than about 0.5 m in size.

Mapping

 If you are doing a conductivity survey with the EM

have to check for metal in your clothing which may 

measurements. If the EM38 is carried close to the ground

be particularly careful with your shoes. Just set the 

ground and watch the measurement as you move your foot 

of the end coils of the instrument. If you see any ch

reading, you will have to keep your shoes well awa

conductivity meter. If the EM38 is carried low on the g

at waist height or higher will have rather little effe

should remove metal objects larger than about 15 cm in s

are carrying the EM38 above the ground, you will have to 

careful with metal which you are carrying. Most of the 

objects which you could carry have no effect on the EM31.

 The measurements of these conductivity meters are di

meter dial; the meter's needle points to the value alo

scale. These readings can be read from the dial and writt

notebook. For sites with rather silty or clayey soil, the 

conductivity, and the conductivity anomalies, are typi

high and this method works well. If the soil is ra
34

electronic instrument converts the analog or voltage signal 

conductivity meter to a digital value and records it in the m

the logger. These readings can later be sent to a compute

data logging allows more precision in the measurements, and 

allows a much greater speed of measurement. However, it a

another complexity to the survey, and something more to g

Still, it is usually the best way to operate the conductivity 

 An example of the speed of a conductivity survey using 

logger is illustrated in Fig. B62. The 735 measurements 

Taylor House map were made in only 1.5 hours. It was adeq

locating the high conductivity area on the west side of th

House cellar, at W10 N100.

 I resurveyed the area a few months later; this time 

slightly longer (1.8 hours this time) and was more careful 

measurements. The EM31 has a short delay time before its mea

settles down to the correct value; for this resurvey, I w

each point about 2 s before recording the measurements so th

accuracy would be better. However, there is little di

between the maps of Fig. B62 and Fig. B63, even with the t

delay in measuring the second map.

 There is not much which can be done for the interpret

these conductivity maps. As shown in Fig. A17, I just ma

areas of high and low conductivity. The small-area low cond

anomalies could be caused by shallow metal objects, and I 

these differently. The long conductivity anomaly at the so

of the grid is almost surely caused by a pipe going east-w

outside the area of the grid; this pipe was detected by the 

and radar surveys also.

 The measurements of the conductivity meters will be diff

the instruments are rotated about a vertical axis. This di

is sometimes valuable for indicating buried metal and othe

changes in conductivity. However, this procedure will incr

time needed for the survey, although it may not double

measurements of a conductivity meter should not change if t

and back of the instrument are interchanged; this is becaus

principle of reciprocity (the measurement should be the same 

location of the transmitter coil is switched with the r

coil).

 I resurveyed the Taylor House grid, making measurements 

spacing of 2 ft (0.6 m), rather than 5 ft (1.5 m). The map 

B64 shows the conductivity patterns when the long bar of the 

oriented east-west. It would have been possible to make a 

measurements at each point, swinging the bar of the EM3

degrees between them. If the measurements are close toge

these are, it is easier to do a separate survey with the ba

EM31 going north-south. This map is plotted in Fig. B65.

 The two conductivity maps are rather similar. The acc

the conductivity data can be improved by averaging the two ma

has been done in Fig. B66. This map is just the average of 
35

(1.5 m, shown in Fig. A17) is small; however, this highe

survey would be more trustworthy in its location of bo

large features.

 Fig. B67 is a plot of the difference between the ma

the two orientations of the EM31 (Fig. B64 - Fig. B

differences will be largest where there are metal obj

ground, and the several large anomalies are indeed l

caused by metal. There is an interesting high in the m

cellar, at W10 N85; this is not apparent on either cond

by itself.

 This difference map accentuates errors in my measure

two original surveys; for this reason, the map of Fig. 

smoothed. With earlier maps in this volume, I have shown 

transformation for smoothing replaces each measuremen

average of the nine measurements centered on that point. 

small variation on this which is more suitable when the 

show a striated pattern. The same nine measurements a

but the weighting of the three rows is different. 

direction of the striations, the six measurements of th

rows are added together. The three measurements in the 

are added together and the sum is multiplied by two. 

are then added. Since the sum of these weights is now 1

+ 1x3) rather than 9, the overall sum is divided by 12; 

replaces the measurement at the central point. This tr

can be called striation smoothing, and this has been ap

original data of Fig. B67.

 Just as with the EM31 surveys, I also did a fast EM

the Taylor House grid. The measurement spacing was 5 ft 

the data plotted in Fig. B69 required 2.5 hours to surve

emphasizes features which are at a shallower depth than 

predominate on the EM31 maps. The Taylor House cellar is 

as an elongated high conductivity anomaly. Notice also 

area peaks, probably caused by metal, are now detecte

rather than as lows with the EM31. This has allowed t

some of the objects to be estimated, and the interpretat

in Fig. A19.

 I resurveyed the Taylor House grid with the E

measurements spaced by 2 ft (0.6 m). Just as with the EM

separately mapped the conductivity with the bar of the E

east-west and also north-south. These two maps are plo

B70 and Fig. B71. The average is given in Fig. 

irregularity of the contour lines is primarily cause

features which were detected, but measurement noise also 

to it. The smoothed map of Fig. B73 clarifies the 

anomalies. However, the interpretation of this survey w

the unsmoothed map of Fig. B72. This interpretation i

Fig. A18. Notice than many more objects were located b

resolution survey than were detected by the lower resolu

interpreted in Fig. A19.

 Both surveys mapped an oblique line of anomalous 

which starts at W70 N0; this is a refilled trench that wa
36

conductivity surveys did not; in the Taylor House ar

conductivity survey detected a trench, but the resistivity 

(Fig. B3) did not.

 The surface map of EM38 conductivity, in Fig. B74, sh

high values at the cellar in the foreground; the conductivit

pipe in the background is so high that the values have been t

in the figure. Note that this EM38 conductivity survey sh

metal objects as conductivity highs, while the EM31 survey 

them to be apparent lows. This difference is caused by the f

the metal is "close" to the EM31 but "far" from the EM38. 

conductivity instruments have a nonlinearity for very co

features. As the conductivity detected by the instruments go

extremely high value, the reading on the meter can actually d

when the instruments are very close to metal (relative to 

spacing of the instruments), the reading will be negative.

 Before I began the survey at Petersburg, I thought that 

conductivity meter would readily detect the refilled trenche

Fort Morton grid. I made a few long conductivity profiles ac

grid; these lines were spaced by 50 ft (15 m). They did not 

nice fort-like pattern, so I filled in the conductivity meas

with lines at a 25 ft (7.6 m) spacing, and I thought that th

help. The resulting map is plotted in Fig. B75. While a bur

and a pipe are very apparent in the map, there is still no 

the fort.

 Since the quick survey did not work, I did a slow and 

survey of the entire grid, making measurements at intervals 

(1.5 m). The 11,811 measurements mapped in Fig. B76 took me 

of field time, but they still do not reveal the fort.

 Buried pipes and a wire are very apparent. The linear 

at the top of the page are caused by remnants of pipes 

farmhouse which was there early in this century. The long li

to the south end of the map is a buried electrical wir

provides power to a tape recorder that has an announcemen

describes the history of the area for visitors. A metal pipe 

near the southeast corner; this is an iron culvert under th

park road. The current park road is at the northwest corner, 

area was not surveyed.

 While the contour interval in the map of Fig. B76 is 0.

the lines are not labeled, for they are too irregular and 

The complexity of these contour lines is caused by electric

and by irregularity of the soil. While it is not apparent 

map, the highest soil conductivity is found in the northwest 

near the former farm house.

 The great irregularity of the contour lines in the origin

mapped in Fig. B76, makes the patterns very difficult to 

smoothed data plotted in Fig. B77 reduces most of that irreg

However, it also broadens and blurs the anomalies. This 

apparent for anomalies which are small in area and high in am
37

 Many metallic objects were detected by the EM31, a

shows these as conductivity lows. The locations where th

can be accentuated by subtracting the smoothed map of F

Fig. B76. This has been done in Fig. B78. Most o

patterns there are caused by buried metal objects. The p

wire are very clear in this map; notice that the pipe whi

line like a backwards L at the top of the map ends with 

on the west.

 The map of Fig. B78 provides the start of the inter

the conductivity measurements of Fort Morton. High 

anomalies which are isolated and small in area could b

metal objects which are rather deep underground; they are 

circles in the interpretation map of Fig. A32. If t

conductivity in the middle of the anomaly, the metal 

shallow, and an X in Fig. A32 locates these points. The 

extent of broad-area conductivity anomalies was also d

the interpretation. The linear patterns on the east side 

are along a side of the former road; perhaps there wa

ditch there at one time, and it has been filled with s

different from what was originally there.

 Because of the low conductivity of the soil in the 

grid, I had difficulties with the EM38 conductivity surv

ways of reducing my difficulties, but only after I f

survey. While the map shown in Fig. B79 does not have 

data, it is adequate to reveal many things. The many sm

black patterns there are mostly caused by shallow me

these objects are larger than nails, and are probably a

20 cm in size. Pipes and a wire cause the linear patt

that these patterns are not as broad as those found with t

 The large patterns in the EM38 conductivity map 

apparent by smoothing the measurements; see Fig. B80. 

striation smoothing was done, it was first necessary to 

of the effect of metal objects from the map. This 

reveals the large-area conductivity features which are pl

interpretation map of Fig. A33. The small features were 

the aid of the difference map of Fig. B81. The diameter 

patterns in this map increases where the anomalies hav

area or amplitude; the diameter is therefore an indic

clarity of detection of the objects. The EM38 conduct

did not locate the linear patterns along the edge of the 

road; these were quite clear with the EM31. I would 

these features should be shallow, and therefore likely t

detected with the EM38. The failure of their detection m

the different depth weighting of the EM38 and EM31; perh

low conductivity strata cancel each other out with the EM

 I resurveyed three small parts of the Fort Morton gr

the EM31 and EM38. Since these measurements were spaced 

m) rather than 5 ft (1.5 m), it was possible to detect sm

much better. The EM31 conductivity map of the Bombproo

shown in Fig. B82. The two powder magazines at the fort w
38

the ovals is in the direction of the EM38, north-south. S

full range of the measurements has not been contoured, the m

these anomalies can be white; this helps to show the hachure

contour lines and therefore it reveals that most of th

anomalies are lows. When this map is smoothed, the cond

pattern is simplified (see Fig. B84). The difference of the 

and the smoothed maps, in Fig. B85, reveals only low cond

anomalies. These are probably shallow metal objects and 

marked with + signs in the interpretation of Fig. A41.

 There are so many metal objects in the Bombproof deta

they interfered with the visibility of the larger patterns 

data. For Fig. B86, each of the small-area, high amplitude a

were removed from the data. This finally reveals more 

anomalies. Two high conductivity anomalies near the bottom 

survey area could be caused by deeper metal objects, and th

been indicated in Fig. A41. A few other anomalies are appar

central low value which is surrounded by high conductivity

could indicate metallic objects at an intermediate dep

different symbol (a plus in a circle) locates these in Fig. A4

 None of the EM38 and EM31 conductivity maps of th

revealed anything of the bombproof trenches. While the res

map of Fig. B9 shows one of these trenches clearly, neither 

nor the EM38 conductivity map give any hint of this trench

lack of detection by the EM instruments does not indic

fundamental failure of the instruments. The difficulty is 

just caused by the stratification of the soil here. 

resistivity and conductivity are just reciprocals of each 

there is no way that the maps measured with a resistivity su

be simply converted to maps which are identical to those 

with the EM38 or EM31 instruments. This is because th

instruments measure depth-weighted averages of the soil's res

or conductivity. Since this weighting is different for the d

instruments, the readings cannot be directly compared. 

different weighting can cause one instrument to detect a 

clearly and another to miss it completely.

 The conductivity surveys of the Trench detail are plo

Fig. B87, Fig. B88, and Fig. B89. The total range 

conductivity measured with the EM31 in this entire area w

about 1 mS/m. If the analog meter of the instrument was r

anomalies here would be impossible to record, for the defle

the needle would be too small. The digital recording of 

logger has allowed the very faint changes to be mapped.

 The EM38 conductivity is mapped as Fig. B88. Low read

marked by hachured contours; these may be caused by metal 

Note that two objects (along line E502 at S28 and S45) are lo

are surrounded by a ring of high values. With the smoothed 

Fig. B89, two local regions with only high conductivity are a

at E465 S52 and E485 S18. These two anomalies are probably c
39

 The EM31 conductivity map of the well in the Iron de

very simple pattern; it is at the upper left corner of Fi

predominantly low values just mean that metal is rather 

comparison to the 3.66 m coil spacing of the EM31. The 

circular low, which is surrounded by a very weak ring of 

This map is the average of the readings which were made 

of the EM31 going north-south and east-west. The differe

two readings is mapped in the upper right part of the 

both maps, the contour interval is 0.25 mS/m.

 While the EM31 shows the anomaly of the well to be 

EM38 shows it as a high; this map is at the lower left 

Because of the high conductivity which the EM38 detected 

the depth can be estimated. This is indicated as 2.4 m 

The EM38 also located several other metal objects in the 

these are also mapped in the interpretation figure.

Detailed procedures

 The procedures described below can make it more dif

conductivity surveys, but these procedures may resu

accurate and reliable conductivity anomalies, particular

where the conductivity of the soil is not high.

 All of the conductivity meters are rather s

electrical interference. At Petersburg, the greatest dif

from the sferic noise of lightning; this is the same noi

be heard on radios as static crashes. The effect of lig

is obvious in Fig. B91. The same line was measured when 

thunderstorm nearby, and when none was near. While the 

sensitive than the EM38 to the static of lightning, the 

can still be severe. All of the measurements which we

this thunderstorm was near had to be discarded, and t

later re-surveyed. Because of the low amplitude of the 

anomalies at Petersburg, the conductivity instru

particularly sensitive to noise. If the amplitude of t

had been about 5 mS/m or more, as it can be at some 

lightning would have caused little difficulty.

 At other sites, I have detected the most severe 

nearby electric lines. The source of this interference 

not the 50 or 60 Hz frequency of the electrical current. 

likely caused by abrupt changes in the electrical curren

the line, which is caused by heavy machinery starting 

even light dimmers in houses can create electrical nois

wires (Skomal 1978). If you plan to do conductivity 

cities or near high voltage power lines, it is importa

test the noise at the site.

 At Petersburg, there was little noise from industr

voltage wires. However, I monitored the noise during my 

surveys, often making one or more tests each day. I did 

by just keeping the conductivity meter stationary and mak
40

power line noise which I have measured with the EM38; th

detected by the EM31 is half these values.

 Noise standard deviation, mS/m

 --------------------------

 City 0.2 - 0.6

 Village 0.1 - 0.3

 Rural 0.05 - 0.1

 The EM38 is more sensitive to interference than the EM31

the measurements of Fig. B93 were made on a rather quiet d

summer at Petersburg is the most common time for thunderstor

magnetic coils in the EM38 are located at the ends of the ins

In the normal mode of operation of the EM38, measurements 

with the thin edge of the instrument vertical; the control k

the meter are then on top of the instrument. In this mo

magnetic field from the coils is directed in a vertical di

and this the called the vertical dipole mode. If the instr

set with its large surface on the ground, this is cal

horizontal dipole mode. While there is not much difference 

the north-south and the east-west orientations in horizonta

mode, the lightning noise in horizontal dipole mode is mu

than the noise measured with a normal, or vertical, orient

the dipoles.

 Measurements with the EM38 resting on its side examine 

to a shallower depth, and this can be valuable for some site

measurements with horizontal dipoles, combined with measureme

vertical dipoles, allow one to estimate how the conductivity 

soil changes with increasing depth. There is some tendency 

horizontal dipole mode to be most sensitive to lightning nois

vertical dipole mode is most sensitive to power line noise.

 Another example of severe interference is illustrated 

B94. This graph shows that it can be impossible to g

measurements when lightning is nearby. Three different co

are tested in this figure. The short broken lines indic

noisiness which I measured with simple measurements. Note 

major effect of the noise is usually a decrease in the v

apparent conductivity. If maps are made when this noise is s

series of low values are seen along measurement traverses in t

 The noise from lightning and power lines can be red

averaging a series of readings with the conductivity meter s

at one point. In Fig. B94, the solid line shows the great r

which is possible with this averaging. I programmed the dat

to sample the measurements from the EM38 at intervals of 0

also set the logger to average these samples for about 5 s, 

about 9 samples. On ten different occasions, I compared the 

the EM38 with and without this averaging; I calculated the 

the standard deviation of the unaveraged readings to those 

ments where about 9 samples were averaged. The average of th

was 3.2, which is approximately equal to the square root of 9

noise had a normal statistical distribution and each sam
41

instrument is moved along a line of measurement.

 Seasonal changes in the noise measurements with t

plotted in Fig. B95. Lightning is most frequent in summ

in late afternoon; so is the noise. The EM31 tests i

same peaks; however, Fig. B95 shows that the EM31 noi

Lightning noise can be a great problem in tropical areas

is most severe, it can be best to survey at first l

morning.

 The electronic circuitry within the EM31 has 

averaging which helps to reduce the noise of its measur

effect of this averaging is seen in Fig. B97. The horizo

the graph shows the time of survey, and the asteris

measurements which were made at about 1 s intervals a

point along line N80. Note that the EM31 tends to o

measurements. That is, if the measurements at the prio

low, the first readings of the EM31 at the new point w

low. There appears to be an inertia to the measurements. 

the 1 s integration time of the EM31, it can be necessary 

a second or two at a point before reading the measuremen

does not have this built in averaging; while this ma

susceptible to noise interference, it also makes it respo

changing conductivity along a measurement traverse.

 Conductivity surveys with the EM38 are typically d

instrument setting on the surface of the ground. This m

conductivity anomalies of buried features, since the i

then closest to the features. However, when the instru

directly on the ground, it is very sensitive to featur

the surface of the soil; these may be small bits of meta

your site is rather trashy, it may be necessary to elev

above the surface.

 The effect of elevation is illustrated in Fig. B98

the apparent conductivity decreases as the instrument is 

is because the reading with the elevated instrument 

conductivity of a layer of air, which is zero. Note al

the EM38 is directly on the ground's surface, the readi

irregular. This is almost surely due to the density of 

objects at this site. When one of the two end coils of 

very close to one of these small objects, it causes 

reading, or sometimes a high. Because of the irregula

readings which are measured when the EM38 on the ground, 

procedure is needed; if there is metal trash at the site

necessary to raise the instrument, even though this 

anomalies of interesting and deeper features.

 Since there were so many shallow metal objects in 

Petersburg, all of my EM38 measurements there were ma

instrument elevated above the ground surface. This height 

0.3 m in the Fort Morton grid to almost 0.7 m in the 

grid. The upper illustration in Fig. A88 shows the EM38 b

in a sling; this can be used to lift it above the ground. 
42

in the carrying frames, and the metal of the data logger 

carried well away from the EM38. The EM31 is much less sens

moderate sized metal objects which are nearby.

 The measurements of apparent conductivity are decreased 

EM38 is lifted in the air. It is not necessary to correct 

decrease, but I did it so that the surveys in the differe

could be more readily compared. I determined the correctio

from tests like that shown in Fig. B98. I multiplied the 

with the elevated instrument by factors which ranged from 

1.54; these factors would be different for a site with differ

stratification.

 The measurements of the conductivity meters will chan

changes in temperature. This effect is most noticeable w

EM38, and at sites with low conductivity, such as Petersburg. 

part of my EM38 conductivity survey of the Fort Morton grid, 

of traverse moved between wooded and open areas. The di

between the sunny and shaded areas caused the instrument to 

cool. As an example of this, I once set the EM38 on the gro

sunny area while I moved the 600 ft (182 m) long rope which g

lines of measurement. During the 22 minutes that the in

rested in the sun, the apparent conductivity indicated by t

rose by about 6 mS/m; it took about 23 minutes with the instr

the shade for the instrument to cool down and return 

conductivity of the former value. This drift of the EM38 w

large relative to the low conductivity of the soil here, 

later correction for this drift complicated the analysis 

survey.

 While other tests have not shown as severe a thermal dr

found during the Fort Morton survey, temperature changes are 

cause of difficulty for EM38 surveys. This difficulty can be 

reduced by encasing the EM38 in insulation. After this For

survey, I made a shell of expanded plastic (Styrofoam) wh

outside the EM38; this insulation is about 0.9 cm thick. I 

thin layer of laminated wood over this plastic to protect 

abrasion. My measurements suggest that this insulation red

thermal drift rate of the EM38 by a factor of about 10.

 The manual for the EM38 gives a good description 

procedures for calibrating the instrument. Fig. B99 may hel

understand the operation of the two controls which cont

calibration. Notice that if the QP (quadrature phase) zero 

incorrectly set, all readings shift up or down by a constant. 

IP (in phase) setting knob is incorrect, the amplitude rang

measurements is accentuated or muted. That is, the QP zero 

determines a constant which is added to the measurements, w

IP zero setting determines a multiplying factor for the measur

 I analyzed this effect by measuring a line of conduct

the Taylor House three times. For the first survey, I set 

correctly. For the other two surveys, I incorrectly set one 

the other of the two calibration controls. Fig. B99 just s
43

Radar Surveys

 Petersburg is an unusually difficult site for 

surveys; it is also unusually good for ground-penetr

surveys. You may wish to think that the success of t

Petersburg will be duplicated if you try radar at yo

probably will not. At many sites where I do geophysica

never even consider a radar survey; this is because it 

that the radar is unsuitable. If I were to rank those s

have done radar surveys in order of the success of the 

site at Petersburg would be in the upper 5 per cent.

 If you are beginning to do your own geophysical 

should not begin with radar. When the factors of equ

ease of survey and interpretation, and suitability for g

are considered, ground-penetrating radar is ranked 

resistivity, magnetic, and conductivity surveys. That 

section is placed in this part of the report.

 At their best, however, radar surveys reveal a grea

of detail of underground features than that furnished b

type of geophysical survey. Radar surveys also det

variety of features than other surveys do. Because the r

so many features, it may be less able to furnish an ident

them; different features may look rather similar. H

capability of radar for identification is improved by 

depth information which it provides. When radar profil

along parallel lines, a rough approximation of the three

form of a buried feature can be determined from the pro

form or shape information is usually the best gu

identification of the features which are detected by the r

 Radar is an interface detector. This is quite di

resistivity, magnetic, and conductivity instruments. Th

detect the volume, area, or mass of features; thei

increase as these parameters increase. Radar is best f

rather abrupt changes in the soil; if the soil contrasts 

or graded, the radar may not be able to detect them. For 

sandy or gravelly soil, the water table is a rather shar

and the radar can map it clearly. If the soil is silty

very broad region where the moisture of the soil changes 

water-saturated; the radar will probably be unsuccessful 

this interface. The burrows of animals and insects in 

also blur the boundaries between soil strata.

 Radar will generally detect features of a given 

greater depth than will other geophysical instruments. 

radar is not suitable for detecting thin soil strata

pavement below a prehistoric house will probably not be 

the radar, unless there is a good contrast in the soi

below the pavement.

 Radar is generally excellent for mapping soil strati

there is a modern fill soil layer over an important cultu

radar survey is suitable for searching for this de
44

never be able to detect the sides of the pit; it may readil

the debris in the pit, but the radar image will not reveal th

is a pit there. The radar image of this pit would probably 

same as that of a boulder which was underground there.

 The soil at a site has a major effect on the suitabili

radar survey. If the soil is stratified, this stratificatio

contrast with the features you seek. Most archaeological 

are rather small; if there are small features naturally in t

the difference between the cultural and natural features 

impossible to determine. Stones in soil are a particular s

confusion; if bedrock is shallow, the soil will probably b

Soils in or near glaciated areas can also show complex pattern

 If you are trying to locate flat surfaces, such as the f

structures, with the radar, your success will be better if t

no natural and rather flat interfaces in the soil. However, 

is planar stratification visible on the radar profiles, feat

into and through these strata may be more apparent. In gene

radar gives the greatest success where there is little strati

visible on the radar profiles; Petersburg is such a site.

 The type of soil and its conductivity have a major infl

the depth to which the radar can profile. In clayey, sa

silty soil, the profiling depth may be so shallow that the 

worthless. If you know the type of soil at a site, you can 

good prediction of the suitability of a radar survey (Dooli

Collins 1995). Measurements of the electrical conducti

resistivity of the soil also provide a good prediction.

 If it is not possible for you to get a resistivity mete

site for tests of the soil resistivity, you might have a s

soil sent to you for tests; a volume of about a half liter 

adequate. You can put this soil in a small plastic bowl and 

the soil with distilled water. Then, you can measure the res

of the soil using small pins rather than large electrodes. 

pins can be stuck into a small wooden or plastic stick so t

are spaced by about 1 cm. These pins can then be touched 

surface of the soil and a measurement of resistivity made 

Wenner configuration using your normal resistivity meter. 

spacing between the pins is much smaller than the distance 

pins to the container, you will get an adequately good readin

the soil will be wet, you may underestimate the actual resist

the soil at the site, but this is better that an overestimate

Equipment

 For many years, there were only one or two manufactu

ground-penetrating radars. Now, there are at least five d

suppliers of these instruments. There is a good range in t

and in the capabilities of the radars. There are light-we

portable radar systems that are readily carried to a distant 

which operate from simple batteries. There are also comple
45

frequency modulated (FM) radar. Both types of radar 

suitable for surveys at archaeological sites.

 Fig. A86 has a sketch of the ground-penetrating ra

used at Petersburg. This is a pulsed radar. It is bulki

modern radars; it is my opinion that it gives higher qual

than many other radars provide. A gasoline-power

generator provides the 200 W of electrical power requ

equipment. Most of this power is used by the recording equ

 The radar images are displayed on the graphic recor

and white shades on a paper scroll. The recorder shown 

has electro-sensitive paper. A small spark burns off a w

on the paper and exposes a black layer below. This 

feeds as a profile is being generated, allowing a

indication of the findings of a radar traverse. The 

sets the maximum depth range of the radar, and also oth

parameters. The complete data of the radar profiles is s

tape recorder. This provides a backup copy of the paper 

data can also be used to make additional, and possibly l

copies of the radar profiles. Further processing of th

is also possible from this tape copy.

 The central part of the radar is within the boxy an

With my radar, a long electrical cable connects th

equipment to the antennas. Other radars use a fiber opt

this connection. With some systems, the recording e

combined with the antenna, and no cable is needed. Th

cable for my radar is long enough that I can readily make 

long as 100 m; this is a convenient length for profiles.

 The radar antennas shown in Fig. A86 also 

electronics for the transmitter and receiver. The partic

sleds which are sketched there have separate antenn

transmitter and the receiver, but this is not necessar

same antenna can be used for both the transmitter and re

antennas are mounted on the bottom surface of the boxes. 

radar pulse goes into the soil below the antenna, howev

radiated into the air. The power of this radiated sign

low, less than the power radiated from hand-held trans

for communications. Should you wish to limit your expos

radar signals, you should investigate the frequency and 

particular radar in more detail.

 The two antennas which I found most suitable for t

Petersburg are sketched in Fig. A86. As this figure sug

frequency antennas have a larger size. I have desc

antennas by the predominant frequency which is apparent 

profile made with the antenna resting on rather typical s

facturer may describe an antenna by the frequency of the 

the antenna is elevated in the air; these frequencies wi

that the frequencies of my description. For example, 

antenna has a frequency in air of about 300 MHz, while 
46

nearby transmitter antennas near a site before considering 

survey.

 You will select a radar antenna on the basis of what 

about the soil and also the size and depth of the archae

features at a site. If the features are larger than 2 m 

most of the common radar antennas will be suitable. The 

antenna of this survey can detect some features as small as a

m in size, while the 315 MHz may be able to detect features 

as 0.1 m in size. These small features may have to have 

which is less than 0.5 m to be detected. Remember that t

many natural features in the soil which are small in size. 

are trying to locate post holes, there are also many stones

tunnels, and tree roots which are about the same size and wh

be detected at least as readily. If you need to search the u

m of the soil, you will need a fairly high resolution 

probably the 315 MHz antenna or one with even higher frequency

 There is a good correlation between the depth at which 

antenna will profile and the electrical resistivity of th

Fig. B100 shows some of my measurements. Depths and resis

are plotted there for 79 different sites where I have measur

If the resistivity is greater than 1000 ohm-m, the 315 MHz 

has generally been the best; if the resistivity is less than 

m, the 180 MHz antenna is generally best. While the maximum 

which features are detected is dependent on the contrast 

underground features, the depths here have been estimated 

maximum depth at which distinct echoes were detected at each s

 A specific radar antenna will be optimum for exploratio

a particular range of depths. You will be able to select an 

which is most suitable for your site. The antennas are disti

by the frequency of the radio wave which they transmit

frequency antennas allow the detection of small features at a 

depth; lower frequency antennas will detect features wh

deeper, but the features must be larger and they must not 

shallow. Radar antennas are primarily limited by the maximum 

which they can profile, but they are also limited by the 

depth at which features can be detected. As the frequency 

antenna is reduced, both the maximum and minimum depth of exp

increase.

 I tested four different radar antennas during my su

Petersburg. Radar profiles made over the cellar of the Tayl

and also the bombproof trenches of Fort Morton illustr

tradeoff between resolution and profiling depth. These ante

described by either their resolution or their charac

frequency; the adjectives "low", "medium", "high", and "ve

distinguish both the resolution and the frequency of t

antennas and their profiles.

 A low resolution radar profile across the cellar is ill

in Fig. B101. This antenna is not sketched in Fig. A86; it i

than the two antennas there and it is about 1 m square
47

different for this profile and that of Fig. B101.

 There is a very pronounced difference with the 

frequency antenna, whose profile is illustrated in Fig. 

very thin echo bands indicate its higher frequency and it

for detecting thinner strata. The flat echo bands whi

apparent below a depth of 1.5 m are just caused by a d

radar.

 The highest frequency antenna gives the rather fain

detailed echoes plotted in Fig. B104. This ante

illustrated in Fig. A86; it is only about 0.25 m long. 

echo on the profile at N70; this is from an object wh

0.25 m underground. It is detected weakly with the 315 

but not detected at all with the two lower frequenc

However, the detection of the cellar is so weak with 

that this antenna is much less suitable than the other 

features at a depth of greater than 1 m. However, this 

detect features as shallow as about 10 cm.

 Four radar profiles from Fort Morton also illustrate 

of resolution and depth. With a low frequency antenna

from four trenches of the bombproof are difficult to is

profile of Fig. B105. During my 1979 search for Fort 

this antenna, I could not identify the fort on the prof

made with this antenna; see Fig. C150. The trenches of 

more distinct in the higher resolution profiles of Fig. B

B107. With the highest resolution profile, shown in Fig

one of the trenches is detected with a strong echo.

 If a radar survey is to be done in a city, it 

important to select an antenna which will detect less 

from the many other radio transmissions. I have alway

the 315 MHz antenna detects much less noise in cities 

MHz antenna. Even though the 180 MHz antenna should 

profile to a greater depth, I have found that noise ca

profiling depth to be less than that of the 315 MHz durin

cities.

Survey procedures

 Radar profiles are easiest to interpret if they 

long spans. If the profiles do not extend to a good dist

a feature, the feature might not be recognized as anyth

As with my other surveys, I find it best to make the ra

go the long length of a site rather than the short width.

 A site will usually be surveyed with the radar by 

of parallel profiles across it. The spacing between th

is determined by the time allowable, and also the size of 

be mapped. For some applications, widely spaced profiles 

suitable. For most of my surveys at many sites, I have 

a line spacing of 1 - 2 m. If interesting features are 

go back and make profiles midway between the original 

order to resolve the shape of the features better.
48

perpendicular direction.

 The lines of profile can be marked with a rope wh

calibrated marks along its length at fixed intervals. I find 

to set this rope slightly off the line of the profile, so 

antenna goes just next to it during the traverse. The 

traverses with the antenna should be made in only one direct

not in a zig-zag fashion. If alternate profiles are in 

directions, they are difficult to interpret; however, it 

possible to make an electronic correction of the profile di

If there are trees in the area of survey, it will usu

necessary to profile in only one direction in order to keep t

from the antenna to the recording equipment from getting 

around trees.

 I can operate my radar by myself, since I can turn the r

on and off from where I am at the radar antenna. Some radar 

will require two people for their operation. The recording e

for my radar is usually in the back of my car. After I have 

a line of traverse, I sometimes walk back to the recorders 

the location of the traverse on the profile and to take a 

what the profile revealed. Otherwise, I have no immediate in

of the findings of the survey.

 It is possible that the long electrical cable connec

radar antenna to the recording equipment will become 

(actually, rotated) during a survey; this will make it mor

that the cable will become caught in weeds or brush. This 

can be prevented by pulling the antenna so that it effective

S-patterns on the ground; you can make these S-patterns if y

pull the antenna over its cable (while you can actually c

cable, you should quickly cross over it to the original si

cable twist is generated each time the antenna is pulled aro

circular loop. The cable will be easiest to pick up and sto

is not twisted. I find that a light wooden spool is best for 

the antenna cable; an axle allows the cable to be quickly unsp

 Whenever I can, I follow a convention that the right s

profile should be on the north or east side of the line of 

this convention is also common for drawing archaeological s

However, some site conditions will make it easier to traver

different direction. Sometimes it is easier to pull an 

uphill or downhill, depending on the slope. I usually walk b

when pulling a radar antenna; in severe weather, it is more 

to have a strong wind on one's back. During the radar survey 

Morton, I made radar profiles to the south, rather than 

conventional north direction. This was because of the woo

were on the south side of the grid. If the antenna is pul

the wooded area, the cable will not get caught on a branch o

If the bend of the antenna cable, between its stationary an

sections, is in a wooded area, it is likely to get caught on 

projection such as a short stump of a bush or root; this w

the traverse until the cable snag is cleared.
49

vicinity of 1 meter per 5 seconds. If markers on the gu

at 1 m intervals, I just spend my day counting up to five

 Each radar can be adjusted to set how rapidly ech

built up on the radar profiles. With my radar, these ec

vertical lines on the profiles; they are usually so cl

than the separate lines cannot be distinguished. The 

these echo scans, the traverse speed of the antenna, an

setting of the radar determine the aspect ratio of 

profiles. Fig. B109 illustrates two ratios.

 The aspect ratio is the compression of the hori

relative to the vertical scale. For typical radar pr

compression is usually quite large. At the left side 

profiles in Fig. B109, the rectangular shapes indicate a 

m on their horizontal and vertical sides. A large 

compression allows slightly different depths to 

determined. While you could make the horizontal and ver

of a radar profile to be the same, this would require a v

narrow paper profile. Remember that a high compression 

apparent slopes of dipping interfaces to be exaggerated

ratio of about 15 is generally good and this is about wha

use.

 Radar antennas should not normally be set direc

surface of the ground. If they are, small and shallow 

the soil have a marked effect on the antenna; this 

undesirable. If the antenna is lifted slightly, these 

greatly reduced. This difference is shown in Fig. B110. 

side of the figure, the sharp vertical lines in the prof

features more difficult to locate. The antenna can 

slightly with runners or wheels. I find that flat-botto

can be lifted by putting foam plastic on their bottom s

duty abrasion resistant plastic sheet can cover this 

density polyethylene is excellent). The foam plast

antenna must not be electrically-conductive; thick, cond

are relatively rare, and a resistance meter quickly checks 

 The slightly elevated antenna has two disadvan

somewhat reduces the strength of the echoes from 

features, and it increases the echoes from overhead featu

electric wires or tree limbs. In some circumstances

better to have the antenna directly on the ground. Howev

directly on the ground surface, a small pile of grass 

significant and confusing radar echo. These are most a

the higher frequency radar antennas.

 A rainstorm can cause a noticeable change in the a

radar profiles. It is not always clear whether radar 

better if the soil is dry or wet, but they are distinctly 

after rain has started to seep into the soil. I tested t

rain several times during my survey at Petersburg. Fig

one comparison. There was a total rainfall of 7.4 cm i

between these two profiles. Note that the profile is si
50

surveys, like resistivity surveys, appear to give the best 

when the soil is neither too wet nor too dry. I have not don

radar surveys on frozen ground to have seen any noticeable 

however the interface between frozen and unfrozen soil shou

an echo on a radar profile. Frozen soil has a higher el

resistivity, and this may improve the profiling depth at some 

Interpretation

 The first step in the interpretation of a radar surv

determination of the speed of the radar pulse in the soil. W

information, you will be able to estimate the depth of the 

which are detected.

 I find that the best method for this speed determinati

simple calculation based on the shapes of radar echoes wh

encountered on the profiles of a survey. The two profiles 

B113 show arc-shaped radar echoes; the width or roundness 

arcs is proportional to the velocity of the radar wave in the 

 Select echoes which show clear and smooth arcs. Pipes a

are usually excellent for this measurement. Since a tren

have been dug for the pipe or wire, the soil directly over 

may not be the same as the surrounding undisturbed soil. 

for most of the echo arc, the radar signal will have traveled 

the unexcavated soil. If you suspect that the echo might b

broad feature, such as an underground tank, it should not be u

 These radar echo arcs are mathematical hyperbolas. T

simply the result of the changing distance from the radar an

the reflector. The radar antennas send a broad beam of r

into the soil; therefore, the antenna detects a feature befor

reached it along the line of the traverse. As the antenn

over the top of the feature, the distance to the object de

and then it increases again as the antenna goes away from the 

 The geometry of this is drawn at the top of Fig. B11

letter "r" indicates the changing distance or range to the 

while "d" is the depth or minimum distance to it. The vel

the radar pulse is "v" and the time required for the pulse to 

the antenna to the object, or back again, is "t1", meaning 

way echo delay time.

 The simple equations at the top of the figure are solved 

velocity; this result is at the bottom of the figure. The 

calculation is based on three measurements which you will 

each echo arc. These distances are marked with broken line

bottom of Fig. B114.

 For the first step in the analysis of an echo, draw a ho

line across the arc near the lowest point where the arc i

Measure the distance across this arc on the paper copy 

profile. Then convert this paper distance to the actual grou

of the arc. This conversion can be done from the distance 

tick marks at the top of the profile.
51

just three half-cyles of a very short radio wave). 

distance to the white line between the first and second ec

 With these two times and the one distance, the vel

determined with a simple calculator. Measure a few of t

see if they give a consistent value for the velocity. 

more detailed discussion of my measurements of radar pu

in Volume C. I found that a good estimate of the velo

Taylor House grid was 0.090 m/ns; the velocity for the 

grid was 0.094 m/ns. These velocities have calibrate

scales of the radar profiles and maps in this report.

 Fig. B115 is a plot of the velocities of the radar s

I have measured at 79 sites; these are the same site

included in Fig. B100. The average pulse velocity w

measured is about 0.1 meter per nanosecond (m/ns). Th

about a third of the speed of light (or the radar pulse) 

speed of the pulse is somewhat higher in high resistivity 

as sand, but the resistivity does not have a strong ef

velocity. In principle, there should be a differe

velocities determined from radar antennas with different 

However, I have never seen a noticeable difference.

 After you have estimated the velocity of the radar s

soil, you can prepare a paper scale which shows the 

This scale can be set next to a radar profile and the d

easily read from it.

 You can then begin to map the plan position of t

types of radar echoes. Cluster the echoes by their ap

plot the location of the echoes with symbols on a map. I

use a symbol which gives a hint as to the character o

Small and compact symbols can mark small and compact ech

symbols indicate the location of a long echo pattern fo

traverse. Use different symbols to distinguish the cla

echoes also. Fig. B116 has a key to the symbols which I 

for my radar maps. The profiles which are illustrated 

definition of these different types of echoes.

 There is typically no clear boundary between the

classes. Also, I find that my definition of different ty

echoes can change somewhat as I do an interpretation; t

cause much difficulty. My major goal is to make su

important lateral contrasts in the echoes have been mark

why I sometimes put a short vertical line on the interp

to mark where there was a change that I could not readily 

 Here are approximate definitions of the different sym

B116. Irregular strata appear to be the result of cont

earth, such as lenses of differing types of soil. Chaoti

fine-scale complexity appear to be due to many small ob

soil; chaotic echoes appear most likely to be due to meta

to air pockets, typical of rubble or trash undergroun

pattern for irregular strata is smoother than the patte
52

echo bands caused by each interface; with reverberations, t

many more vertically-aligned echoes from each interface. Ty

these reverberations are found where the radar signal 

several times between metallic objects and the surface of t

In some cases, they will be caused by echoes between two 

objects which are underground, or between the upper an

surfaces of a sharp contrast in the soil.

 All radar echoes cannot definitely be categorized into 

types; examples can always be found which are in between 

classes of echoes which have been defined. Radar interpretat

not have to be perfect, but a good interpretation should 

extract as much information from the radar records as 

justified by time and cost. You may wish to have a procedu

will be completely automatic, so that you do not have to m

judgments about the radar information and so that an interp

can be done by any unskilled person. This cannot be do

Indeed there are computerized processes which will alter 

image (some people might call this an enhancement), but most 

processes give little benefit. However, some procedu

extracting and mapping radar echoes from selected depths can 

an objective display of their spatial patterns; these proced

be done with a computer (Battelle 1981; Goodman and other

Grumman and Daniels 1995) or with paper copies of p

Computers are very valuable for eliminating much of the d

preparing radar echo maps and illustrations. The figures w

report illustrate drawings which have been drafted 

automatically, even though the interpretation itself was n

with a computer.

 It is easiest to see changes in the character of an echo 

profile; it is more difficult to spot the changes from one 

profile to another. A few profiles, made perpendicular to t

direction, can help this.

 On my first interpretation of a set of radar profiles, 

evaluate each profile separately, without reference to th

profiles. This gives me my most unbiased map of the locatio

features are distinct. After I have mapped these echoes

interpret the profiles, taking a closer look near those areas 

have already mapped an echo. I try to see if there are fain

of the same type of echo which I did not see on th

examination.

 The radar surveys at the Taylor House will illustr

interpretation of radar profiles. The first survey which I 

the area was in 1979; my interpretation of those radar pro

given in Fig. B117. That survey was done with a low frequ

low resolution antenna; while it was adequate for locat

cellar, the higher resolution antennas of my 1992 survey h

much more suitable for this site.

 The echo maps from these higher resolution radar surv

plotted in Fig. B119 and Fig. B122. In order to generat

figures, a total length of 27,740 ft (8.5 km) was profiled 
53

multiply these numbers by 0.3 to convert them to dept

These map pairs, which separate the echoes by the d

traverse, help the interpretation, for they can show lin

more clearly than a map which combines echoes from both d

traverse.

 When radar profiles have been made along perpendicul

is possible to locate features more accurately, although 

always be needed. Fig. B125 shows the procedure. Points 

on the radar echo maps (like those of Fig. B120) to in

the object was closest to the radar antenna along 

traverse. The object may not be directly under the 

traverse; instead, it may be off to the side of the 

radar. This is because the signal beamed from the r

spreads out to the left and right of the antenna. 

interpretation maps in this report, such as those in Fi

this refined location of features, which is derived from 

echo maps.

 A selection of the most interesting radar profiles 

with this volume; additional radar profiles are in Volu

report. An index of the radar profiles in this report, 

Taylor House grid, is plotted in Fig. B126.

 The cellar floor of the Taylor House is clearly in

depth of about 1.5 in Fig. B127; a perpendicular 

illustrated in Fig. B128. Tick marks at the tops of th

indicate 5 ft (1.5 m) intervals along the profile. 

markers on the radar profiles when I did the field trav

is a button on the handle of the radar antenna; I 

whenever the middle of the antenna passed 5 ft points alo

rope. At 50 ft (15 m) intervals, I put two markers on 

this allows points to be quickly and reliably located.

 Black bands on the profile indicate radar echoes. 

indicate stronger echoes. The radar signal alternates f

to negative to positive in its polarity. All of these 

are identically printed in this profile as black bands. 

can be valuable to distinguish the positive and negative 

but I have not done that for the profiles here. The wh

the radar profile locate regions where the amplitude o

echo was low. The amplitude is low as the polarity 

changes, so there are white lines between the dark echo ba

 The upper black bands are rather horizontal and 

These bands are primarily a result of the transmitted 

directly to the receiver; echoes from the surface of th

also appear there. These bands can sometimes hide the 

features which are at a shallow depth in the soil. Th

which are below these horizontal bands reveal inform

underground features.

 Echo bands are typically triplicated, although som

will be two, four, or five echo bands caused by one 

interface. This triplication is just caused by the three 

of the transmitted signal, a sinusoidal wave going from 
54

show my estimation of the depth of the objects; these depth

meters.

 The radar profiles of the Taylor House cellar show a re

flat lower surface and also a moderately complex group of 

above that. These shallower echoes might be caused by th

which fills part of the cellar. The profiles of Fig. B127 

B128 also show echo peaks or arcs at a depth of roughly 0.5 

ends of the cellar. It is possible that these are caused 

intact brick foundation. I mapped these apparent wall depths 

A9; they are in fair agreement with the findings of the most 

magnetic survey.

 After the radar echo maps are prepared, clustering o

will be apparent and the boundaries of these clusters 

estimated. For the Taylor House, these interpreted boundar

drawn in Fig. A11 and Fig. A12. Broken lines in those 

indicate boundaries which are not as distinct. For som

surveys, I find that it can be valuable to distinguish the d

types of clusters of echoes on the interpreted map; for this 

those distinctions have not been strong and so I have only i

the clarity of the clusters.

 The conductivity and resistivity maps of this grid (F

Fig. B66, and Fig. B73) all show high resistivity on the east

of the area. One possible interpretation of this is tha

sandy soil has been spread on the surface in this area; perh

was done when the fortifications were leveled. The radar pro

not reveal this soil interface, but the depths of the fea

suggest the possibility of fill soil. The statistics of th

from the high resolution antenna are plotted in Fig. B12

suggest that the radar echoes are slightly deeper on the east 

the survey area. Perhaps a layer of soil has been spread 

surface there.

 Many features are immediately apparent on the radar 

even as the field work is being done; the cellar of the Tayl

is obvious by a quick look at a profile just after the tra

finished. Some features only become apparent after a 

examination and mapping of the radar echoes. The radar ech

Fig. B122 shows a line of echoes which goes from the southwes

of the Taylor House cellar to near W80 N20; this is almos

caused by a trench which was at this location during the C

(see Fig. A6). This feature is so indistinct on the radar 

that I never noticed it until I had completed this combined ec

 Fig. B130 illustrates one difficulty in determining the 

a feature. At the point labeled Gt6 there are echo arc

indicate that there is an object at a depth of 1.4 m. I susp

this object is actually at a depth of about 0.6 m, where th

very faint echo. The arcs at 1.4 m are probably a second 

reverberation, from the object; the radar signal made two rou

between the surface and the object. The arcs at the very b

the profile probably indicate the third echo from the objec

type of echo is relatively rare; it can usually be identifie

fact that there are vertically-aligned echoes which have ro
55

Fort Morton was 55,355 ft (16.9 km). The three areas of 

resurveys are readily apparent in the figure.

 The combined echo map of the main area of survey i

Fig. B133. The backwards-D shape of the fort can be seen

refilled fortification ditches were mapped by the radar

maps are split by the direction of traverse in Fig. B1

B135. The buried power line is apparent near the left 

B134; notice that this wire was not detected very well o

south profiles, as revealed by the map of Fig. B135. Thi

the traverse then crossed the wire at a much smaller a

B136 locates the radar profiles from the Fort Morton ar

illustrated in this report.

 The radar profile of Fig. B137 goes lengthwise down 

bombproof trenches of the fort; the radar indicates tha

of the trench is about 0.9 m underground. Several small 

detected by the radar as small arcs, but these are not m

bottom the profile. The unchanging gray bands at a dept

m near E475 are caused by an electronic ringing withi

system, and not by any soil stratification. Another 

bombproof trench is given on the left side of Fig. 

profile shows a rather flat lower surface there; the 

shows a complex pattern above that surface which could 

debris in the soil which fills the trench, or by the mix

soil itself. The radar echo from the main fortificatio

not very clear, but two crossings of the covered way 

trench are very distinct.

 Fig. B139 is a profile which crosses the front of 

The radar detected no trace of the earth ridge or parape

this was scraped back into the excavated area when t

leveled. The excavation shown in the figure was made by 

as a source of the soil which built up the parapet; th

soil behind the parapet so that they could make their 

while they were protected by the parapet. The cross-se

fort, at the top of Fig. A73, shows this excavation belo

line. The buried wire going to the tape recorder causes 

the left side of the profile, at E259.

 On the right hand side of Fig. B140, there is a very 

of echo arcs which is caused by an underground pipe. The 

three very strong arcs with additional, but faint, echo

below those three. There was formerly a farm in the nort

of the Fort Morton grid, and most of the echoes here a

underground remnants of the demolished structures.

 The two profiles in Fig. B141 appear to locate a wir

radar antenna crossed over this wire at a shallow obliqu

left profile), it revealed a pattern which is similar t

broad soil interface. Perpendicular radar profiles 

narrow echo arcs along this line; the feature cannot be 

soil contrast. Because of the 10 ft spacing between the p

interpretation of the radar in this part of the survey 
56

 The geophysical summary of Fig. A22 identifies an area o

near the northeast corner of the Fort Morton area; the pr

Fig. B143 illustrates this. The undulating symbol marks a sp

the soil strata are irregular. While the area of this irre

is well-defined, it does not correlate with the locations 

Civil War fortifications. This echo pattern may be caused b

area excavations which were made during the siege, but the p

to the park road suggests that this could be an area whic

layer of recent debris. The basin-shaped echoes on the right 

the profile are caused by the fill of the former park road

the surface is now grass-covered, there is probably a layer o

in the ground below.

 A dense mesh of radar profiles resulted in a very 

survey of the Bombproof detail; it is illustrated in Fig. B14

some of the trenches in this grid were easy to detect with th

others were detected only faintly. However, the closely spac

allowed these faint echoes to be delineated with a good reli

Fig. B145 is the combined echo map. The directional radar 

Fig. B146, Fig. B147, and Fig. B148 show the depths of the 

The southern powder magazine, at E340 S250, and the nor

trench, at E360, were not at all clear on the radar profil

could be mapped and identified because of the close spacing 

radar profiles. Even with this close spacing, the radar did 

clear information on the area near E380 S245. The resistivit

(Fig. B9) and EM38 conductivity survey (Fig. B86) both hint 

possibility that rather silty soil may have been spread in th

and this has attenuated the radar signals here. It is possi

this soil was spread during the leveling of Fort Morton a

war.

 In Fig. B149, a drawing of the cross-section of the bomb

compared to a radar profile of the existing trenches. Note 

right-most radar echo band, which appears to be caused by 

bombproof trench, is not shown in the cross-section or the ma

fort. It is possible that this trench was dug after the map 

fort was prepared.

 The lines of radar profile which were made at the Trenc

are mapped in Fig. B150 and the combined echo map is shown 

B151. If the purpose of this survey was the location of 

line of the Civil War fortification, the lines were much 

together than they needed to be. However, some smaller fea

the right side of the area would only be clear with rather 

spaced lines. The findings of the separate surveys are pl

Fig. B152, Fig. B153, and Fig. B154.

 The two radar profiles in Fig. B155 reveal the main forti

trench. The main trench is on the left side of the profile

are extensions of the trench, like bays, going to the righ

appear to be dugouts or similar excavations which are along 

wall of the main trench. In addition, there are isolated pit

the main line of the fortification. The profile of Fig. B15
57

B158 show the echoes which were detected by the two dif

antennas.

Self-Potential Surveys

 Self-potential, or SP, surveys measure voltages at 

of the earth. The equipment needed for this survey is 

and inexpensive; however, the survey speed is quite s

surveys have not been used very much for archaeolo

relatively little is known about the benefits and best 

of SP surveys. This is an excellent technique for experim

 It is difficult to find practical information abou

field work and interpretation for most types of geophys

SP surveys are different; several authors have writte

descriptions of the procedures (Corwin 1990; Wynn and Sh

Corry, DeMoully, and Gerety 1983; Burr 1982; Corwin and Ho

 Self-potential surveys are used extensively for 

applications, measuring the corrosion rate of metallic 

are underground (Lattin 1979). I have seen very interest

about the procedures of these engineering applications, 

never found a good and complete summary. As near as 

there is very little communication between those in this 

discipline and those who apply SP for exploration geoph

as a search for minerals.

 Water which contains a small amount of salt beco

when this water moves through the soil, the moving charg

are an electrical current. These currents create voltag

be measured at the surface; even the very slow moveme

seeping in the soil can generate detectable voltag

archaeological features can cut into natural soil strat

alter the rate at which rainwater will seep through the 

earthen types of features might be detectable with an SP s

 There is an interesting converse to the fact 

groundwater will generate electrical currents. Just as 

of an electrical generator is a motor, if voltages are ap

earth's surface, groundwater can be pumped. This is cal

osmosis (Probstein and Hicks 1993; Gray 1969), and 

applied to the preservation of building foundations an

walls by preventing moisture from rising within them.

 Another interesting aspect of self-potential is th

burrowing animals, like moles, can detect electrical vol

soil (Conniff 1994); I do not know if they may be affe

small voltages which are measured by an SP survey. The

be many mole tunnels in the Fort Morton grid, but I neve

see if there was a change in the density of their exca

the Iron detail, where the SP voltage is rather large.

Equipment

 The very simple items which you need for a self-pote

are illustrated in Fig. A90. Probably any digita
58

SP surveys also; you should use an analog meter if you cann

digital voltmeter. If you have one of these analog meters wh

not have a sufficiently precise voltage scale, you might 

measure with the meter's most precise current scale; t

sometimes allow good measurements.

 It would be nice if the leads from the voltmeter could 

touched to the soil surface to measure the voltage there. T

not usually work, however, because chemical reactions betw

metal of the electrodes and the soil changes the small voltag

you wish to measure. Instead, it is necessary to use 

electrodes to contact the soil; these are called non-po

electrodes.

 The essential aspect of a non-polarizing electrode is 

electrical current flows from a metal (such as copper) to a 

of a salt of that metal (such as copper sulfate in water) 

into the soil. For the simple electrodes shown in Fig. A90, 

from the voltmeter connects to a copper wire which goes 

water solution. The ceramic of the flower pot just all

solution to very slowly seep through it and moisten the botto

pot. This pot is just set on the soil to make the 

measurement.

 This type of non-polarizing electrode is called a coppe

sulfate (Cu-CuSO4) electrode. It is the type which is most 

applied to self-potential surveys. Another type of electrod

different metal, this is the silver-silver chloride (

electrode. These Ag-AgCl electrodes are most commonly f

chemistry laboratories, where they are used to measure the ac

solutions; they are called reference junction half-cells 

field. These Ag-AgCl electrodes are less sensitive to cha

temperature than are Cu-CuSO4 electrodes; however, they are a

expensive. Cu-CuSO4 electrodes can be purchased from geo

supply companies or companies which sell equipment for c

testing; you can also make your own Cu-CuSO4 electrodes if 

to. I tested several types of non-polarizing electrodes d

survey at Petersburg, and I got the best results with the el

which I made myself.

 The basic ingredient is a flower pot. Any ceramic, no

pot will do. If there is a hole at its bottom, this can be 

with a cork or with epoxy putty. It is possible that there a

within the ceramic which might affect the measurement, so 

washed my pots out with fresh water. I just filled the p

water and let them set for a week in the air; water will slo

through the pot and I rinsed this off every day.

 To complete this pot as a copper-copper sulfate electro

bend a short copper wire so that it goes from the edge of 

down to the bottom. Then, add a quantity of distilled wate

pot to a depth of about a centimeter. Finally, put in copper 

crystals, and add more later as these dissolve; make sure th

blue crystals are always visible at the bottom of the pot. 
59

it.

 You will need at least two of these copper-cop

electrodes, but a few more may speed the survey. Coppe

not particularly dangerous. I recall reading somewh

damages leather, but I have not tested this. Copper 

commonly used to kill algae in ponds. If too much is use

be killed also (Environmental Protection Agency 1973 p. 

only small amounts of this chemical seep through the SP 

you may reduce this by making the solution into a jelly 

Perhaps the copper sulfate which seeps into the soil 

there a long time and cause difficulty for a future 

survey which is capable of locating the element copper.

 You will also need some small electrical clips to 

voltmeter to the copper wires at the pot. Finally, you 

insulated wire which is probably as long as the larges

you wish to survey, but a maximum length of about 40 m wo

A tape measure or calibrated rope can be used to define 

each measurement traverse.

Procedures

 I made self-potential measurements in three are

Morton; these are located in Fig. B159. The surveys g

results in the Iron detail, where the other geophysical 

indicated that there was a large mass of iron under

measurements of the earth voltage, or potential, are plo

B160. The voltage is distinctly low at E526, over the iro

 I used two of the flower pot electrodes for this sur

one stationary at E509 S145, the end of the line. T

reference electrode and I measured voltages relative to 

convention for SP surveys that the negative lead of the 

connected to this reference electrode. I scraped awa

below this reference electrode so that it's bottom woul

soil. Measurements of voltage were made at intervals of 

along a line by placing the moving electrode at those 

grass was scraped from these points also.

 Fig. B160 shows that the voltages are very small, b

a clear pattern. There are two possible causes for this 

feature there is almost surely a well which is fille

artifacts. Water from the surrounding soil could be seep

well shaft, creating this voltage anomaly. As a second 

the iron and other metal in the well could be rusting. 

reaction of rusting metal creates currents and voltages 

this action is rather similar to the operation of a batter

 The measurements in Fig. B160 required 15 minutes. 

slightly moist, and the electrodes made good contact wit

soil had been quite dry, I could have dug a shallow ho

slightly moist soil is found. If the soil is very dry, 

not be put on soil to artificially moisten it for this wi
60

 The value of non-polarizing electrodes is illustrated 

B162. The voltages are large, but erratic, when stainles

electrodes are driven into the soil. While the voltage is lo

middle of the line, the anomaly is small relative to the var

of the measurements.

 You may notice that the voltage which you measure betwee

of non-polarizing electrodes slowly changes. If you see the 

drifting, just wait about 15 seconds or so for it to stabil

you find that you need to wait for each electrode to stabiliz

soil, your survey will be faster if you have several el

moving along the line. While measurements are made at one el

the rest can be sitting in the soil, so that little or no w

be necessary when the voltage is to be measured.

 If you use several electrodes, you will have the ad

complexity of correcting for the different inherent voltages 

the electrodes. You should number each electrode, and per

electrode number 1 for the reference. Before you start your 

put all the electrodes close together in a container which co

shallow layer of copper sulfate solution. Now, measure the 

to each pot relative to the reference electrode; these can b

calibration voltages. When you make your measurements al

ground, subtract the appropriate calibration voltage fr

measurement. If your calibration test of pot number 3 ind

voltage of 3.1 mV relative to the reference pot, just subtrac

from every voltage reading with pot number 3.

 For most of my measurements at Petersburg, I used 

electrodes. These electrodes are about 15 cm long and about 

diameter. In order to support the electrodes, and for better 

to moist soil, I cored 2.5 cm diameter holes into the soil 

points of measurement; these holes went to a depth of about 1

set the electrodes in the bottom of these holes. When I m

electrodes across the bottom of the holes, the voltage I 

would change by 3 - 10 mV. While I do not know the reason f

I believe that it might be due to the sensitivity of th

electrodes to slight changes in the type of soil which t

contacting. The electrodes which had a large contact surface 

have this problem, perhaps because they average the effect 

soil particles. In this case, lower resolution helps.

 Fig. B163 illustrates repeated measurements with these 

electrodes. While both surveys found the same low voltage 

the irregularity between the surveys is about 5 - 10 mV a

would make it difficult to detect weaker SP anomalies.

 This is exactly what I found with my continuing self-p

surveys at Petersburg. Three lines of measurement are plotted 

B164, Fig. B165, and Fig. B166. The line segments at the bo

those graphs locate features which were along the mea

traverses; none of these features was detected by the 

measurements. While there were lateral gradients in the v
61

pattern shown at the upper right in the figure.

 These measurements were made with the Ag-AgCl electr

assumed that these electrodes would give me the mo

readings; I tested the simple flower pot electrodes a

"good" electrodes only after my surveys were almost 

Petersburg. Perhaps some time I will be able to use th

electrodes to remeasure all of the lines where the Ag-AgC

found so much irregularity. It is possible that an 

electrode surface area would help; possibly the sur

electrodes could be as large as 0.2 m in diameter for eve

data.

 I have not done enough SP surveys to have determin

trees will affect the measurements. I suspect that ther

significant effect; you should note the location of n

Buried pipes may also affect these surveys.

Survey details

 Some details of SP surveys and their interpretatio

You will probably not have to concern yourself with t

during your first few self-potential surveys.

 If the soil below the SP electrodes is very dry, 

measurements will probably be erratic. It is possible 

if the soil is so dry that a poor contact is made with a

Just measure the electrical resistance between a pair o

while they are setting on the soil. During my surveys at 

I measured the resistance between the two flower pot elec

about 9000 ohm. Because of their smaller surface area, 

electrodes had the much higher resistance of 110,000 ohm.

 While this test will show you if there is a prob

electrode, or its wiring, it does have a disadvan

resistance measurement polarizes the electrodes; this is 

measurement sends a current through the electrodes. 

return to the normal value in an exponential fashion, as 

B168, and it may be 5 - 10 minutes before the electrod

enough to their normal conditions. This polarization is 

the current which was applied during the resistance meas

also on the duration of the resistance measurement.

 There is a large difference in the voltage of one se

electrode as compared to another; this effect must be ad

you have made your measurements. It is a good idea to 

on these changing voltages, for it may alert you to diffi

an electrode. The measurements of my Ag-AgCl electrode

in Fig. B169. The voltages there are in millivolts (

there are large differences from one electrode to a

differences are fairly constant with time.

 My measurements with two different Cu-CuSO4 electrode

in Fig. B170. The drift in the voltages of these electrod

than the drift of the Ag-AgCl electrodes. With my field m
62

 Another effect is seen if you move the wires to the el

while you measure the voltages. When the wires move in the 

field of the Earth, you are making an electrical generator 

will see the voltage change. If there is a long span of movi

or if you have a coil of wire which moves, the voltage from t

generator will be very apparent.

 The references listed at the start of this section descr

of the procedures for the interpretation of SP anomalies. I 

only one interesting pattern with this survey at Petersburg, 

interpretation gave an interesting result.

 I found that the measurements mapped in Fig. B167 c

closely modeled by a small concentration of electrical char

is called an electric monopole. With a computer program, I 

semi-automatic procedure to find the amplitude and location 

monopole charge which best fit the measurements. This best 

found with a monopole having a charge of -0.25 nC (nanocoulom

depth of 3.5 m and located at E525.8 S144.8; this assumes 

relative permittivity (dielectric constant) of the soil is 1

charge is about that caused by an excess of roughly one 

million electrons.

 Fig. B172 shows that the measurements are quite erra

this model does match their general trend; the two-dimensiona

the lower right corner of Fig. B167 also shows that th

approximately matches the measurements.

 The current which could be flowing away from the monopol

can be estimated if the soil is assumed to have a resistivit

ohm-m. This current is: charge / (permittivity * resistiv

the calculated value is 14 mA. This current could be due to 

different effects, including the seepage of water, but a like

is the corrosion of metal, most likely iron. From the meas

of Evans (1926 p. 45), the current above could be caused 

corrosion of iron at the rate of 0.35 g/day (by way of compa

dime in the USA has a mass of 2.5 g). If this corrosion 

steady since 1865, a total mass of 17 kg of iron would have ru

Special Topics

Soil chemistry

 Geochemical exploration can be suitable for locating di

buried ores (Siegel 1974; Hawkes 1957). The concentrations 

chemicals in the soil have also been found to be good indic

the former use of the land by humans (Eidt 1985; Carr 1982).

 As part of the archaeological investigation (Blades 1993

Taylor House area, soil samples were removed from the plow zo

ft (3 m) intervals and the concentrations of magnesium (Fig

calcium (Fig. B173), phosphorus (Fig. B174), and potassiu

B176) were determined, along with the pH of the soil (Fig. B17

 These measurements were interpreted by Brooke Blades 

1993 p. 49) and he notes that some concentrations of phosph

correlated with the locations of former structures in the ar
63

survey area. This finding may agree with the low conc

calcium and phosphorus in that area.

Multiple instruments

 If one geophysical survey is good, are two better? 

One instrument may be ideal for a site, and nothing els

Additional surveys increase the time and cost of the wor

little bit like photography. If you take a black 

photograph, this will have most of the information 

record. If you also take a color photograph, perhaps 

additional information, but you about double the cos

further take an infrared photograph, you might i

information by an additional 25%, but the increase in 

than that.

 Of all of the surveys at archaeological sites which 

exactly half have made significant applications of more t

of geophysical instrument. However, I have never a

geophysical instruments to one site than I have for th

Petersburg.

 If you wish a greater reliability to the findi

survey, you may choose instruments which will generally 

same or similar features; these might be conductivity and 

meters, or perhaps magnetometers and magnetic susceptibi

If you wish to detect the greatest variety of features, y

a selection of instruments such that each of them 

generally different features from the others. David O

specified many geophysical surveys for the National Park 

found that there will be generally little correlation 

findings of a radar and a magnetic survey at many of hi

he finds this combination of surveys to be particularly 

locating a wide variety of features.

 It is usually possible to make fair predicti

suitability of different geophysical instruments for a 

the survey begins. If these estimates are wrong, it may 

to have several different geophysical instruments at a 

preliminary tests with them will suggest which is best.

 The information from each type of geophysical su

first interpreted independently from the findings of 

surveys. Then, correlations between the measurements and 

be evaluated in case these combinations suggests any 

information. Some of these tests with the data from thi

included in Volume C.

 The different geophysical surveys may be done seque

after the other. In some circumstances, several differ

survey can be done together. If there is an on-going exc

site, there may be extra people at some times, and 

geophysical surveys can put them to work. When I have d

like this, I have never found it practical to have mor

geophysical surveys going on at once. Parallel surveys m
64

 With parallel or concurrent geophysical surveys, it 

necessary to carefully test for interaction between the inst

Generally, identical types of geophysical instruments will 

each other's readings if they are near each other. Resisti

conductivity instruments will interact at the greatest d

Magnetometers will interact at a shorter distance. 

penetrating radars will also interfere with each other, 

problem is not as bad as I would have thought.

 If the same types of geophysical instruments are operat

site, it will generally be necessary to measure their 

calibration. This will be most reliably done by measuring 

line of survey with each of the instruments. Then the cor

between pairs of instruments can be determined with a best

straight line. If the measurements of the two instruments ar

Z2, this correlation will reveal the constants a and b in th

equation: Z1 = a * Z2 + b and this will allow the measure

one instrument to be converted to equivalent measuremen

another instrument. While the measurements of both instrumen

also be compared at a single point, this may only reveal the 

b in the equation above.

 Dissimilar geophysical instruments will sometimes also i

with each other. The magnetic field generated by a magnetom

affect a conductivity meter. The wires of a resistivity me

affect a conductivity meter. The iron mass of a radar may 

magnetic survey. Everybody who is walking around will a

seismic survey. The geophones of a seismic survey are magneti

 If you are going to have several instruments operating 

you will have to make careful tests of their interaction 

instruments will be within about 30 m of each other. Prob

best method is to have the first instrument make a se

measurements while it is stationary. The second instru

brought closer and closer to the first instrument. This 

instrument is either turned on and off, or is set to take a s

measurements; the measurements should be taken at the same 

the first instrument and also deliberately at slightly d

times. The first instrument should be closely watched 

unusual change in its measurements. The second instrument s

kept farther away than the distance of this first no

interaction.

 This was a test of the one-way effect of the second in

on the first. A second test should be made of the effect 

first instrument on the second.

 You must also check for any interference between the geo

instruments and the field computers at the site. If walkie

are being used nearby, they are another source of interference

Maintenance, reliability, safety

 If you can do simple electrical repairs, you should k

simple spare parts for your equipment. Connectors and wi
65

cells causing corrosion within the instrument. Some 

instruments have a lithium cell inside which will power 

clock and data memory for the periods when the primary c

of the instrument. It is possible that these lithium cel

a shorter life when the primary power is removed from the 

If a geophysical instrument operates on rechargeable batt

important to have a spare battery pack which can be 

during the day, when the internal battery dies; this 

identical rechargeable battery or possibly an exter

battery.

 Some simple tests may help to reveal if a geophysica

is operating correctly or not. With a resistivity meter

necessary to connect the meter as a two-electrode resi

(connect the instruments wires V+ and I+ together, and al

together); then, just measure the value of a resistor 

1000 ohm) which you keep for this test. You may check 

calibration of several resistivity meters this way als

ground-penetrating radar, it is easy to check if the tr

working by holding a portable radio near the antenna. Y

able to hear the pulsing of the radar over a wide freq

You can test if a conductivity meter is transmitting 

field. All you need is a small coil of wire and an audi

the frequency of most of these instruments is in the aud

range and you will be able to hear the hum. That same e

also test a magnetometer; the energizing signal going t

of a proton magnetometer will cause a click. The sig

sensors of other types of magnetometers may also be audibl

 I generally keep each of my geophysical instruments 

sheet of clear plastic. While this does not make the 

any better, and probably does not make the instrument

longer, it does keep them from getting scratched in bru

accidents. If they do not look like they have been 

handled, other people who use them take better care of the

 If you are making a large number of measurements, 

will be very valuable for recording the data. With a p

your measurements, the condition of your recording is 

visible. Since this is not true for a digital copy on 

will have to make extra care that your data are not lost 

unusable. The best protection is one of making several c

data on separate floppy disks. In high risk situations, 

to keep these disks in separate locations. The dat

recorded in the standard format of ASCII (American Stand

Information Interchange); this will allow the data to 

even if the disk directory is damaged. With computerized 

it is important to be able to transfer the measurements t

in the field; this will allow a partial days's data to 

the memory of the geophysical instrument to be cleared

portable computers are common now, but you should try 

that the one you choose will operate on several source

these may include the internal battery, a car batter
66

surveyors and petroleum geophysicists provide some guid

geophysical surveys at archaeological sites (Tibbs 1987; Prof

Surveyor 1992; International Association of Geophysical Con

1982). Because geophysical surveys can be done in the rain

important to know how to be safe when lightning might strik

(Lee 1977; Berger 1980).

 The greatest hazard of a geophysical survey for passe

usually just tripping on all of the wires and ropes which ar

around the site. While guide ropes usually have to be st

tight, wires can be slackened so that they cause little diffic

 Most of the hazards which are encountered when doing geo

surveys are just caused by common plants (Lampe 1984) and 

(Biery 1977). Poison ivy, whose leaves have a skin irritant

difficulty for geophysical surveys because of all of the 

ropes, and instruments which are set or moved on the ground

field items may pick up the irritating resins, which ar

transferred to the unsuspecting geophysicist, perhaps at 

site. If there is a field of clean grass nearby, it is 

practice to drag the equipment through this before it is packe

 Dangerous animals are very uncommon, and normal pract

all that are needed (Klauber 1982). Non-dangerous animals c

most difficulty. Dogs may try to run off with your ropes 

will nibble on your instruments.

Special survey conditions

 Geophysical surveys are most commonly done in quite c

and pleasant locations. A small number of them are done in 

or difficult circumstances of weather or location. Some re

to procedures which might help you are given here. These e

the particular problems which are encountered in doing t

field work. General references to camping or survival 

listed.

 Surveys can readily be done at remote locations from 

field camps (Dillon 1989; Land 1978; Gordon 1975; Volunt

Technical Assistance 1975; Farkas 1970; John 1965). Transp

of equipment and supplies is the major factor, so the ligh

instruments may have first priority. Electrical power 

necessary for the instruments; if the survey will require a f

of field time, rechargeable batteries may be better than pri

disposable) batteries. Photovoltaic and gasoline-powered 

generators can recharge these batteries.

 If the area of field work is separate from the camp site

be best to leave the geophysical instruments at the wo

overnight. If you do this, you may have to make sure that r

not damage the equipment while you are away. Two layers 

plastic will provide all the protection you need. The equi

set on one sheet, which is wrapped over the top of the pile. 

second sheet is wrapped around that and tucked below the firs

provides protection from both rainfall and also water washi
67

the snow; the antenna sleds are easier to pull. Batterie

very short life; they may have to be kept inside th

clothing. The liquid crystal displays (LCDs) of some 

and computers, will not operate in cold weather. Some t

warmers are based on the heat generated by the rusting of 

in a saline solution; these cannot be used if you 

magnetic survey. Because of all of the clothing which is 

much more difficult for the operator of a magnetom

nonmagnetic in the winter. If the ground is frozen, a 

and a seismic survey may not be practical. If you wish 

lake, it will be easiest to do when the water is fr

winter. Irritating insects are almost never found in the 

 Geophysical surveys in hot weather are little di

other types of field work (Glen 1980; Adolph and Assoc

The primary difficulty is usually dust. If the recorder

are lifted on tables to a height of 1 m or so, this 

provide good protection from some blowing dust. In very 

the LCDs of some instruments will go completely black; 

need to be cooled in the shade. If the soil is very dry, 

surveys will give erratic and unreliable readings; if a h

water is poured on each electrode, adequate measurem

possible.

 In areas with a hot and humid climate, the humidi

causes more difficulty for the equipment than the heat 

Porkert 1974). For longer surveys, fungus may g

instruments or supplies. High voltage difficulties mig

also; radar equipment and computer monitors are potential 

 All of the geophysical surveys can be done in the ra

soil is at the surface, resistivity, radar, and seismic 

be quite messy, for these surveys have equipment on th

the ground. The messiest surveys of all are done wh

freezing mud. If there are thorny bushes at the site

quickly rip most rain coats and other plastic sheets wh

through the bushes. Be sure to keep waterproof wr

available; normal paper will be useless.

 I have never done a geophysical survey in a cave, b

believe that there are any limits past the difficulties o

work. Geophysical surveys can be done in cellars, bu

tunnels if wished.

 Geophysical surveys can also be done at night easi

other types of field work. While I have not tested it 

geomagnetic field is quieter at night (Parkinson 1983 p. 

the site is then shielded from the Sun by the Earth. 

finds interference from electrical machinery or traffic, 

night may be considered; seismic and conductivity surveys 

improved.
68

resistivity of 310 ohm-m when connected to the four electrodes

 Fig. B2: The two seasonal soundings were done on line N

were centered at E0 and E65. The locations are slightly o

the figure in order to distinguish them. The circles m

electrode points at the greatest separation between the electr

 Fig. B3: The resistivity measurements for this map and 

the other resistivity maps here were made with a Gossen Geohm 

resistance meter. The map is rather similar to that measu

the Geonics EM38 conductivity meter. The correction descri

Fig. C72 has been applied to this map. The survey was done 

1992 (10:46 - 16:15), using the pole-pole configura

electrodes; measurements were plotted at the midpoint between 

moving electrodes. Two ropes defined the line of each t

these calibrated ropes were placed along east-west lines 5 ft 

apart. Measurements were made alternately with the array nort

then east-west. For the north-south orientation, the two el

were placed at the same east coordinate on the two ropes. 

east-west orientation, the two electrodes were set along 

rope. For each new measurement, it was only necessary to m

electrode. Except for the correction of the north-south measu

the map has not been smoothed. The reference electrodes wer

locations mapped in Fig. C157; the resistance betwee

electrodes was measured to be 6300 ohm. The direction of t

arrow on this contour map (and all of the other contour maps) 

accurate than the north arrows on the interpretation drawin

is because the drawing of the arrow is pasted on the contour 

there could be an error in its alignment.

 Fig. B4: The lines of two pseudosections are shown 

figure. This is a 3x3 E-W striation smoothed version of Fig. 

 Fig. B5: The northern-most line in the grid was measur

times, one after another. The differences between the sur

primarily due to errors in location of the electrodes.

 Fig. B6: The offset Wenner configuration had its 

electrode at E50 N85, and the electrode spacing was 

Measurements were made at rotational increments of 5 degree

upper diagram plots the measurements with the contrasts exa

by assuming the middle of the circle has a resistivity of 35

The survey was done on 2 April 1993 (12:14 - 13:23) and th

Geohm 3 was used. The broken line offset curve was measured 

array offset from the midpoint in the direction shown; the so

is the average of the two offsets.

 Fig. B7: The resistivity sounding shown in Fig. C74 is 

with 5 circles to mark the greatest electrode spread. The 

the resistivity and EM profiles over a pipe is marked by 

line at the top of the figure. The dotted pattern sh

estimated location of the fort.

 Fig. B8: This survey was done on 8 September 1992 

13:57), using the same procedures as for the Taylor House sur

electrode spacing was 5 ft (1.5 m) and the pole-pole confi

was used. Measurements were made with the array both north-s

east-west, with electrode pairs defining the edges of 5 ft 

There were no bees at the southeast corner of the grid dur
69

average of the averages of the lines to the north and 

were measured with the array east-west. The interpreta

data is given in Fig. A42.

 Fig. B9: The high resistivity along the lines of 

bombproof trenches is more apparent here. The norther

was partly detected, as was the third from the north. 

trench was rather clearly detected, but the fourth tre

detected. There is little correlation with the magnetic s

map has been smoothed from Fig. B8 with an east-west 3

filter.

 Fig. B11: The 325 measurements of this map were m

September 1992 (14:19 - 16:02). The spacing between th

was 5 ft (1.5 m) and the pole-pole array was oriented 

south and east-west. The electrodes were placed at 

multiples in the grid and the measurements were reco

midpoint between the two electrodes. There were 1

anomalies which appear to be measurement errors; these we

resistivity lows and the measurements were replaced by th

the four adjacent points. Traverses were made along north

and the measurements perpendicular to the guide rope

slightly greater than the measurements with the array par

guide ropes. The averages with the array east-west wer

the average of the averages of the adjacent lines wit

north-south.

 Fig. B12: The line of a resistivity pseudosection 

S25. A 3x3 north-south striation smoothing has been ap

original data to create this map.

 Fig. B13: The 84 measurements of this survey wer

September 1992 (17:02 - 17:30). The normal pole-pole ar

with an electrode spacing of 5 ft (1.5 m); traverses 

south. As with the other resistivity surveys at Fort 

average values on each line with the array perpendic

traverse were forced to the average of the adjacent line

the electrodes along the line of the guide ropes. Fr

resistivity maps made at this site, it was found that the 

measurements were always higher with the array perpendi

guide ropes than parallel to the guide ropes, by an aver

this is equivalent to saying that the electrode spacing p

to the two ropes was about 5 cm too small. This is 

error; while it is readily corrected, it would generally 

use a rigid bar to hold the electrode spacing correct w

is practical.

 Fig. B14: The two profiles have a shift between t

about 10 ohm-m or 6%. This is probably caused by a s

between the electrodes, and the measurements with the ar

to the pipe are probably in error. The electrode 

intended to be 5 ft (1.5 m); this was set by two paralle

they must have been slightly too far apart. This survey 

7 September 1992 (16:20 - 16:36), making measurements wi

of 2.5 ft (0.8 m). The Gossen Geohm 3 was used with 

reference points.
70

soundings I use the sequence: 0.1, 0.15, 0.2, 0.3, 0.4, 0.5, 

1.5, 2, 3, 4, 5 m. As you can see, the lines are then rather 

spaced along the horizontal, logarithmic axis of the graph, 

provides the most efficient sequence for analysis. For t

sounding, the measurements were not quite as uniformly spac

the graph. This analysis assumes that the soil interfa

horizontal and abrupt; while most soil interfaces would be 

gradual, the approximation of abrupt changes is generally adeq

 Fig. B17: These are called two-layer curves; t

calculations which assume a simple and standardized upper str

has a thickness of 1 m and a resistivity of 100 ohm-m. The d

curves then plot how a resistivity sounding would look 

infinitely thick lower layer had a different, but constant 

resistivity. The ratio of the resistivity of the lower laye

upper layer is indicated by the numbers at the right hand sid

graph. These graphs were calculated by the procedure given 

(1964 p. 37).

 Fig. B19: The details on this sounding are included w

C159. The offset Wenner configuration was used for this s

With it, five electrodes are set along the line, but four of 

used at a time; these are the four on the left or the four 

right. For the offset configuration, the average of th

resistivity values is calculated. The procedure is further d

in Volume C.

 Fig. B20: The compass was rested on a wooden rod for 

and the apparent angle to a distant stake was measured alon

which passed close to the buried iron object. The compas

Suunto model KB-14, which can be read to a precision of 0.1 

Two separate traverses were made on the line and both show 

deflection. The survey was done on 23 October 1992, between 

9:30. With the monopole model of Fig. C55, it is poss

calculate the deflection of a compass, and this agrees w

measurements. The difference is simply due to a calibration 

the compass.

 Fig. B21: The maximum deflection is found next to 

object.

 Fig. B22: The numbers show the pattern of the changing 

of the Earth's magnetic field. Measurement traverses were m

going toward the north and parallel lines progresses toward t

This survey was done as part of a training course coordin

Steven De Vore, of the National Park Service. The students 

course measured this magnetic map. The magnetometer for th

was loaned by Bob Sonderman and Barbara Little of the 

Capital Area of the National Park Service. The E0 N0 point 

figure has an actual site coordinate of E40 N3. The Freder

A-index on 9 June 1993 was 8; on the next day, it was 25.

 Fig. B27: These two curves show the range of variabili

was measured during this survey. The curve at the top is u

quiet and smooth. The A-indices shown in the figure are 

magnetic observatory at Fredericksburg, Virginia. The base 

curve for 25 June 1992 shows a larger irregularity than usua

is a particularly large deviation at about hour 17. Both cur
71

Petersburg: strength = 53,200 nT, inclination = 

declination = -7 degrees (grid angle).

 Fig. B29: Each calculation area covers a 5 m s

average half-width of each anomaly is about 1 m and the 

object is indeed 1 m below the elevation of the calcul

source is a magnetic dipole with a moment of 0.31 

approximates 1 kg of iron with only remanent magnetizati

Fig. C43, three different types of magnetometers are sh

magnetic latitudes. Note that since the moment is const

latitude, the magnetic anomaly decreases by about h

equator. The anomaly patterns here are the same as those 

cube in Fig. C43; therefore the shape of that sourc

resolved at this sensor elevation.

 Fig. B30: For model #1, the sources are rectangular 

a total moment of 300 Am^2 for each span (this is the tot

the east-west and the north-south segments). For this 

pipe is assumed to have only induced magnetization; the c

the park road has this magnetic pattern. The iron pip

former farm may be steel pipes which appear to sho

remanent magnetization; these match model #2. For that 

directions of magnetization of the dipoles, which were 

ft (3.7 m), were chosen semi-randomly; each dipole has a 

Am^2.

 Fig. B31: The general anomaly of the square slab at 

its typical form. The small square inside is a hole in t

this magnetic slab. The high caused by the hole in the 

in the gradient of the nearby low. These models are cal

45 degree magnetic latitude. The slab is 6 m on a side 

1 m hole in it. Its magnetic susceptibility is 0

thickness is 1/6 m; the top of the slab is 1 m below the 

the overhead object, a dipole with a moment of 3.13 Am^

(about 10 kg iron); this dipole is 3 m above the se

located at the asterisk in the figure.

 Fig. B32: The survey was done on 13 August 1991

period 15:20 - 16:55. The sensor height was 0.85 m and 

traverses were made going to the east. The Gem GSM-19FG 

to make measurements automatically at 1 second interva

traverse speed was adjusted to pass 5 ft measurement po

rate; a buzzer alerted that each measurement was made. 

marks at 5 ft intervals guided the lines of traverse. T

moved every 3 lines, since the 5 ft offset could be est

to a recording error, measurements were not made at the 

each line. Base station measurements were made at 20 s i

the Fredericksburg A-index on this day was 9. A Geometr

856AX proton magnetometer was used for all of the b

readings for these magnetic surveys. For the surveys a

House, the sensor was typically placed against a tree t

N36 with the sensor at an elevation of 2.4 m. The correc

temporal variation in the magnetic field was done by det

time at which each spatial measurement was made and sub

base station measurement from it. The base station mea

estimated by making a linear interpolation between the 
72

anomalies also indicate these very high amplitude anomalies.

 Fig. B33: The spacing between measurements was 2.5 ft 

in both the north-south and the east-west directions. Th

height was 0.8 m. Traverses were made going to both the east 

west. The alignment rope was moved after every 5 traverses, 

offset of 5 ft could be estimated by eye. The survey was do

January 1992, between about 14:15 and 16:00. The Frederick

index on the day of this survey was 18. The base 

magnetometer was in the area of Fort Morton, at E499 S30

trains went by during this survey, but I was between l

measurement then. The survey was probably done with pola

continuously on, and with an external trigger. The blank a

W20 N20 is due to the fact that the contour lines for v

anomalies were not drawn. The broken line contours are at i

of 5 nT. When I mapped the original data, I found a slight a

striations in the map. I determined that these were caus

perspective or parallax error in my location of the magneti

for each measurement; the sensor was far enough in front of m

could not tell its exact location on the ground. I stopped 

short of the desired point. I corrected the map by 

alternate lines of measurement by 1 ft to the east or west.

 Fig. B34: This survey was done during the period of 

June 1992. The Gem GSM-19FG Overhauser magnetometer was 

with an external trigger with the sensor at the front end of 

(2.4 m) long bar; the height of this bar and the sensor was 

Measurement traverses were made going toward the west

Geometrics G-856AX proton magnetometer had its sensor agains

trunk at W132 N36 on the west side of the grid. The interval 

base station measurements was 15 s. The Fredericksburg A-in

the three days were 16, 10, and 15. The survey was halted 

trains were passing on the tracks to the west. A thundersto

at the end of the survey. In the northern part of the map, f

- N120 there were 10 measurement spikes, visible as on

anomalies with an amplitude of about -38 nT (only 2 of the 1

had a positive anomaly). These are almost certainly 

electrical interference from lightning. The errors were re

averaging the 4 adjacent measurements. The base station 

appear to detect these lightning effects; this may be due 

orientation of its sensor.

 Fig. B35: The solid line contours are at intervals o

(from -190 to +190 nT), while the broken line contours are 

intervals (these are at anomaly levels of -15 and -5 nT). Th

enlargement of the western third of Fig. B34. Note that the

overlap on the three maps.

 Fig. B38: Note that the vertical axis is the negative 

anomaly; distant objects always cause the magnetic field 

reduced. This analysis assumes that the measurements are 

near the equator and the elevation of the sensor and the obj

roughly the same. For a total field magnetometer, the magn

the anomaly decreases as the cube of the distance from the 

for a gradiometer it decreases as the fourth power of distanc
73

the same for large distances.

 Fig. B39: The survey was done with the Gem GSM-19F

be operated as a base station, a total field magnetomet

gradiometer. The lower sensor was at an elevation of 1

upper sensor was at 2.0 m. Since I did the survey by mys

the two sensors on a triangular wooden frame which I pull

lines of the survey. The traverse directions were both 

and to the west. The survey was done on 30 June 1992, b

and 14:27. The base station was the G-856AX, with measur

s intervals; the sensor was in the tree at W132 N36. Th

stopped for a passing train (at a distance of 103 m) whic

15 nT anomaly. The Fredericksburg A-index was moderately 

The Gem instrument makes simultaneous measurements 

sensors. The base station measurements allowed the tot

both sensors to be corrected for the temporal change. Th

contours are at intervals of 10 nT, while the broken l

fill in parts of the map with 5 nT intervals. The lin

low along N42.5, and just south of the foundation 

apparently a measurement error. The magnetic low near 

not been fully contoured.

 Fig. B40: This survey was done during the period 

June 1992. Measurement traverses were made going altern

east or west and parallel lines progressed to the north

going traverses were made along lines with line coord

were a multiple of 5. The instrument was the Gem GSM-19F

magnetometer which had its sensor at an elevation of 0.8 

station magnetometer measured the temporal change in 

field at 60 s intervals; it was a Geometrics G-8

magnetometer whose sensor was tied to a tree 2.5 ft south

S300. At this distance, passing trains cause the magne

drop by about 1.4 nT; spatial measurements were halte

passing trains.

 Fig. B42: The symbols mark the estimated loca

magnetic objects; the key to the different types of symb

in Fig. A30. The upper number indicates the estimated 

object in kg, and assumes that it is iron with a magnet

0.3 Am^2/kg. The lower number of the pairs indicates t

depth of the object in meters. Dots without numbe

magnetic objects which were not interpreted. This is usu

it is likely that the object is caused by modern iron 

few cases, the anomalies were not clear enough for int

The lines connecting objects indicate the likely pat

segments of iron pipe. The Fort Morton grid extends acr

road at the upper left corner, and no survey was done ther

 Fig. B44: A total of 374 magnetic anomalies were 

for the analysis of the Fort Morton survey. The total i

estimated to be 3512 kg and the average mass per object 

the average depth of the objects is 0.67 m. Because o

number of anomalies to be interpreted for this map, 

assisted approach was used. The program first located 

all anomalies automatically, and then sorted them in or

magnitude. Starting with the highest amplitude ano
74

the direction to the magnetic low from the local gradient 

field to a resolution of 45 degrees. I would check if this d

was reasonable, and then the program would locate the sourc

anomaly a distance of 1 ft (0.3 m) in that direction. S

magnetic map at Fort Morton was made with a measurement sp

2.5 ft, I did not make a detailed mathematical model for the 

of each anomaly.

 Fig. B46: It is possible that many of the objects 

interpreted depth of 0 could be modern iron trash just 

surface. While visible iron objects were moved from the surv

leaves or grass could cover many objects. These shallow 

appear to be most common in the wooded or unmown areas, just 

is most likely that modern trash would accumulate.

 Fig. B47: The broken line is a curve which shows 

interpretation errors. As an example, assume that an object 

interpreted as having a mass of 1 kg and a depth of 1 m. 

possible that the actual mass and depth are not those values, 

actual value of mass and depth would probably fall along th

line curve. Points along this curve have the same anomaly am

It is always most likely that depths and masses are estimat

too large than too small. See also Fig. A99.

 Fig. B48: While most of the map is contoured with an 

of 1 nT, the high anomaly amplitudes have a 5 nT interval; th

shift in the density of contour lines reveals the change 

contour interval. The survey was done on 7 August 19

measurements spaced by 1 ft (0.3 m). The sensor height was 

and the sensor was carried on the horizontal staff. Traver

made going toward the south, and parallel lines progressed 

east. The survey was stopped for three trains. The spatial 

corrected with the magnetic base station at E499 S301, 

measurements at 30 s intervals. The magnetic field was mo

irregular on the day of this survey, with an A-index o

Fredericksburg, Virginia. A value of 0.5 nT was added 

measurements on line E333 and 0.8 nT was subtracted from line 

correct for a striation. There were 17 one-point anomalies 

amplitude of about 4 nT; these appear to have been ca

lightning noise, and they were smoothed by averaging the 

four readings. The blank line at the bottom of the map is th

of a recording error in the magnetometer.

 Fig. B49: The measurement at each point was replaced 

average of the nine measurements which surround the point; t

3x3 matrix smoothing. The striations in Fig. B48 are accent

the close spacing between measurements. While these stria

not have a serious effect on the interpretation of the da

contour lines are rather jagged; this makes the patterns in 

more difficult to see.

 Fig. B50: The shift between the 1 nT and 5 nT contours 

by the change in the line spacing of the high amplitude an

The sensor elevation was 0.75 m and the Gem GSM-19FG Ov

magnetometer was operated with the sensor on a horizontal sta

measurement spacing of this resurvey was 1 ft (0.3 m); traver

made going toward the north and parallel lines progressed to
75

the magnetic low of the large iron source near E525 S145.

 Fig. B51: The maps on the left plot the measurem

the upper and lower sensor of a total field gradiometer. 

was done with the Gem GSM-19FG operating in its gradiomet

calculated gradient from these two maps is plotted in the 

corner. The contours of the gradient map are at 100 nT/

with additional 10 nT/m contours shown with broken lines. 

gradient map was measured on 21 October 1992, a separate 

map was measured on 27 June 1992 and this is plotted 

right of the figure. Base station magnetometers correct

field maps. On 21 October, the base station was at E4

measurements were at 15 s intervals; the A-index at Fre

Virginia, was 6. For the 27 June survey, the base sta

W132 N36 and measurements were at 15 s intervals; the A-in

 Fig. B52: For each of the three cross-sections, th

at the top of the section. These three sections indic

conductivity reading will change if the depth of a

changes, the thickness of a layer changes, or the conc

density of a material changes laterally.

 Fig. B53: This survey was done on 3 Septembe

measurements were spaced by 1 ft (0.3 m); with this cl

the shape of the curves is quite accurate. The bar was a

height of 1 m. The width of the negative-going anom

background level of about 8.3 mS/m) is 3.6 m; this is ab

spacing of the EM31.

 Fig. B54: This survey was also done on 3 September 

it was finished, the standard deviation of the EM noi

mS/m. The pipe at about E275 S130 was also traversed w

parallel to the pipe and it also revealed a sharp low 

over the pipe. At other sites, the EM31 has indicated 

conductivity over a pipe when the EM31 is parallel to 

this may be due to the lower spatial resolution of 

surveys.

 Fig. B55: This survey was done on 5 September 19

11:25; Fig. B93 is a plot of the noise which was measured 

the start of this survey. The EM38 was at an elevation 

(0.4 m) and the data have not been adjusted for elev

dipoles were vertical and the measurement spacing was 1 ft 

 Fig. B56: The metal plate was iron, with a thicknes

The metal mesh was made of soldered or welded iron wire

holes in the mesh had sides of 0.5 inch (1.3 cm) a

diameter was 1.3 mm. The loop was a shorted one-turn sq

wire; the diameter of the wire was 1.3 mm also. All of t

were insulated from the ground. The measurements wer

October 1992 in my yard, and the soil conductivity is 

about 6 mS/m. The measurement interval was 1 ft (0.3 m

test, the "depth" of the objects is 0.4 times the inter

of the EM31.

 Fig. B57: The half-widths of these anomaly highs a

the same, with an average of 1.1 m; this is less than the 

the objects from the EM38. For this test, the "depth" of 
76

"apparent depth" of the objects below the EM38.

 Fig. B60: The same aluminum foil used in Fig. B58 was 

this test. The EM38 was hung in a tree at an elevation of 0

the foil was moved below the instrument in a grid patte

measurements at intervals of 0.1 m; the area of the te

rectangle of 1.2 by 1.8 m. The dipoles of the EM38 were vert

background value of 2.3 mS/m has been subtracted from all 

measurements. The map should be symmetrical in each quadr

differences are due to errors resulting from noise, tilt of t

and mispositioning. This test was also done in my yard

October 1992. A defect in the contouring program, Surfer, ca

contour line (at -5) to be missing its tick marks.

 Fig. B61: This is an enlargement of a section of Fig. B

object was detected on east-going traverses with both the 180 

the 315 MHz radar antennas; the depth from the radar prof

either 0.4 or 0.5 m. An analysis of the high resolution magn

indicates an object at a depth of 0.45 m at this point.

 Fig. B62: For this very fast survey, the traverses w

quickly, without averaging the measurements with the data log

survey was done on 11 August 1991, between 10:10 and 11:

survey was done immediately after the main survey of this gr

spacing of 2 ft) was completed. After this survey was finis

noise standard deviation was measured to be 0.05 mS/m. The 

was east-west, as was the traverse direction; paralle

progressed to the north. The integration time of the EM3

caused the measurements to be striated; this was removed by 

alternate lines to the east or west by 4 ft (1.2 m). The 

was done by resampling along the lines using a q

interpolation of three adjacent measurements.

 Fig. B63: This survey also had a 5 ft (1.5 m) mea

spacing, but it was done more carefully than the survey pl

Fig. B62. It was done on 9 October 1991, between 13:00 and 1

adjustment or lateral shifting was needed for this surve

traverses were made with westward traverses only, and parall

progressed toward the south. As before, the bar of the E

east-west. After the end of the survey the standard devi

repeated measurements was measured to be 0.03 mS/m.

 Fig. B64: The measurement spacing was 2 ft (0.6 m) 

survey. This survey was done during the period 8 - 9 Augu

Traverses were made going to the east and west. The Metroson

logger (dl-712) was set to sample the EM31 at intervals of 0

to average for a period of about 5 s (or 9 samples) The ba

was about 1 m. On 8 August 1991, the standard devia

stationary repeated measurements was 0.10 mS/m. Paralle

progressed toward the north.

 Fig. B65: This map is quite similar to that of Fig. B

the bar east-west; the measurement spacing was also 2 ft 

survey. The survey was done during the period of 9 - 11 Augu

The sampling was done the same as with the survey shown in F

Traverses were still made to the east and west, but the EM31 

in a wooden pack frame for this perpendicular orientati

instrument height was then about 1.1 m. Parallel lines progr
77

14:32, or about 2.5 hours. The height of the bar was 2.

and it was aligned east-west. Traverses were made east a

parallel lines progressed toward the north. The Metros

sampled at 0.5 s intervals and averaged for about 3 s 

samples per measurement). After the survey was fi

standard deviation of the EM38 noise was 0.03 mS/m, alt

averaging period was used for that test. The measur

converted to approximate the measurements at the 

multiplying them by 1.54. No smoothing has been done to t

 Fig. B70: The measurement spacing was 2 ft (0.6 m)

were made going to the east and west and parallel lines p

the north. The EM38 bar was oriented east-west also; sin

are at the ends of the bar, the line between the coils w

also. The dipoles were vertical and the instrument was 

ft (0.7 m) in the air. The EM38 was encased in a Styro

thermal shell; this was the first survey which I 

insulation. A triangular frame of plastic pipes support

The Metrosonics dl-712 data logger recorded the measurem

programmed to sample the EM38 values at 0.5 s inter

average for about 5 s (9 samples) for each recorded measu

survey was done during the period of 3 - 5 October 199

deviations of the values of stationary measurements were 

3 October 1991 = 0.02 mS/m; 4 October = 0.04 mS/m. The o

showed level shifts of 0.5 mS/m in the apparent conducti

restarted the survey after stopping to download data to a 

at the start of a new day. Three perpendicular test 

W34, and E50) done at the end of the survey provided a co

the level shifts of the EM38 from line to line. A correc

elevation of the instrument was made by multiplying all 

by a factor of 1.54 in order to approximate the readings 

be measured with the instrument on the surface. Excep

corrections, no smoothing has been done to the measure

contour range is 0 - 15 mS/m and the two blank areas 

conductivity.

 Fig. B71: The differences between Fig. B70 and F

primarily the result of measurement and positional er

survey was done the same as that shown in Fig. B70, exc

bar of the EM38 was oriented north-south. The traverse

done in an east-west direction, but the parallel line

toward the south. The survey was done during the peri

October 1991. The measurement noise was 0.03 mS/m on e

days. The east-west line averages were forced to be 

those from the survey shown in Fig. B70.

 Fig. B72: The average of Fig. B70 and Fig. B71 is p

The difference between the two figures was also calcula

revealed primarily noise.

 Fig. B73: A 3x3 matrix smoothing was done on Fig

this figure.

 Fig. B75: Measurements were made at 5 ft (1.5 

along the lines. On 24 January 1991, I made the measurem

multiples of 50 ft (such as S100 and S150); on 16 March 1

lines at intermediate 25 ft intervals, such as S25, S
78

about 5 samples in the recorded average at each point

conductivity of the soil is typically in the range of 6 - 9 m

standard deviation of the temporal noise was measured to be 0

on 30 April and 0.08 mS/m on 1 May 1991. The contours are 

the range of 2 - 17 mS/m; blank areas have apparent cond

which is either more or less than this range.

 Fig. B78: Contour lines are drawn at +0.2 and -0.2 mS/m

there are other ways of isolating small-area anomalies, thi

is suitable for this area.

 Fig. B79: The survey was done 20 - 25 June 1991 w

Geonics EM38. The instrument was raised by 1 ft (0.3 m) a

ground, partly to reduce the effect of small metal objects an

to simplify the survey in the wooded area. The EM38 was t

triangular frame of plastic pipe to hold it at the correct 

The dipoles of the EM38 were vertical; the length of the 

east-west, the same as the traverse direction. The survey 

the south end of the area and parallel lines progressed to

north; the entire 600 ft (181 m) length of the area was sur

one continuous line. The measurements were recorded in a Met

dl-712 data logger. This logger was programmed to sample 

values at intervals of 0.5 s; these samples were average

period of about 5 s at each measurement point, and therefore 

samples were averaged for each recorded measurement. The 

measurements are plotted in the map. The standard deviation 

measurement noise was monitored on two of the survey days: 2

0.25 mS/m (see Fig. B94); 24 June = 0.07 mS/m. This survey 

before a thermal shell was constructed for the EM38, and th

changes in the measurements as the temperature of the in

changed. As with the EM31 survey, four check lines were 

after the survey was finished. These check lines went perpe

to the direction of the original survey, on lines E100, 300, 

700. These check lines were used to correct for the drift 

instrument's readings along each line by forcing the along-

slopes of the original measurements to these lines. Thes

lines also adjusted for the line-to-line drift of the instru

was done for the EM31 survey. The measurements hav

approximately corrected from the elevation of 1 ft down 

surface by multiplying each of them by 1.16. The complexity 

contours prevents line labels in the map; contours are drawn 

range of 8 - 18 mS/m.

 Fig. B80: The many metallic objects caused a large n

abrupt measurement changes; this made the map more diffi

smooth. First, the measurements were truncated to hold them 

range of 0 - 15 mS/m. Then the 740 most anomalous poin

replaced by the average of their 4 neighbors. Finally, a s

smoothing was applied to the data. For this striation smooth

points along the line of traverse are given a weight of 2, w

points along the lines to the north and south are given a w

1; the sum of the 9 numbers is divided by 12, the sum of the w

 Fig. B81: This figure is done the same as Fig. B78. I

difference of Fig. B79 and Fig. B80. Only the two contour 
79

bumblebee nest at the southeast corner prevented reading

3 September 1992, lines E405 - 425 were resurveyed bec

interference found there on the original survey; the 0.1

in the measurements on the two days was corrected. A nor

striation filter was used to smooth the measurements.

 Fig. B83: While the contours are at intervals of 

was not practical to include the conductivity levels on t

because of their complexity. The survey was done on 11 

(9:04 - 15:00) and the standard deviation of the noise 

survey was rather high at 0.24 and 0.37 mS/m. The 

increased during the day, and parallel lines progressed 

where noise is more evident. The EM38 was encased in t

shell and tied to a pair of 2.4 m long boards to make a t

bottom of the instrument was at an elevation of about 1.2

and the dipoles were vertical. Traverses were made to th

measurements were made at 1 ft and 1 s intervals; the 

logger was programmed to sample at 0.5 s intervals an

samples. The logger was about 1.1 m from the nearest 

EM38. The data were corrected to the surface by mul

measurements by 1.25, however, no line-to-line adjus

needed for the data, and the measurements are not smoothed

 Fig. B84: Some of the trends in the data are appa

3x3 striation smoothed map. Both this figure and Fig. 

hachure error on a contour line near E375 S235. The lo

is at a level of 5 mS/m; the strongest low values hav

their middles.

 Fig. B85: The breadth of the lows indicates 

(amplitude and width) of the small-area anomalies; a gre

may also indicate that the object has a larger size and 

depth. This map is a difference: Fig. B83 - Fig. C60. 

line is at a level of -1 mS/m on the map. This map cou

been generated by a high pass filter map of the original 

than subtracting the low pass filtered data from the origi

 Fig. B86: For this map, wherever values were found 

than -0.3 mS/m, the values were replaced by the averag

measurements. A 3x3 striation smoothing was also app

measurements for this map. While the low pass filtered 

C60 rather enlarges the area of the anomalies, the anomal

not been changed much in this map.

 Fig. B87: The survey shown here was done on 3 Se

(11:30 - 13:47); the noise was 0.04 mS/m before the sur

mS/m after the survey. Measurements were made at 1 

intervals along south-going traverses; parallel lines p

the east. The EM31 bar was north-south at a height of ab

the dipoles were vertical. The measurements were 1 s ave

samples, and the traverse speed was 1 ft/s. An earlie

done on 9 August 1992 (16:02 - 18:25), but the noise at 

the survey was 0.13 mS/m and the data were not usab

striation smoothing filter has been applied to t

measurements for the data in the figure.

 Fig. B88: This map averages the measurements of th

surveys with the EM38. Two surveys with the bar of th
80

stationary measurements was found to be 0.09 mS/m at the end 

survey. The measurements in the map have been corrected 

ground surface, and have had line-to-line adjustments for in

drift, but have not been smoothed.

 Fig. B89: A 3x3 matrix smoothing has be done on the data

 Fig. B90: Four different surveys are mapped in the fig

of the surveys were done with the EM31 and two with the EM

EM31 survey was done once with the instrument's bar north-s

once with it east-west. The two surveys are averaged in t

left map, while the difference (east - north) is plotted at t

right. With a single orientation of the EM bar, the low is 

which is elongated in the direction of the bar; the avera

eliminates this elongation. The EM38 magnetic susceptibility 

the right, is rather similar to the EM38 conductivity ma

contour interval of the susceptibility map is 0.25 ppt, whi

susceptibility of 0.00025 in SI units; note that the 

contours of two strong lows are not completed. The spacing 

measurements for all surveys was 1 ft (0.3 m); therefore, eac

created from 961 measurements. The Metrosonics dl-712 dat

recorded all the measurements. It was set to automatical

measurements at 1 s intervals; the logger sampled the data 

intervals and two samples were averaged for each 

measurement. The first EM31 survey was done on 27 Jun

traverses were made going toward the east, and the bar was ea

The second EM31 survey was done on 4 September 1992; traver

made going toward the north and the bar was oriented north-so

EM31 was also operated in its susceptibility mode on 10 Aug

over the iron anomaly; since the map of that data is almost i

to the conductivity map, it is not included here. T

conductivity survey was done on 10 August 1992 (15:39 - 16:

dipoles were vertical, the bar was east-west, and traverses w

to the east. The instrument was carried in its thermal shell 

aid of a shoulder strap, holding it at an elevation of 1 ft a

ground. The measurements were corrected to indicate the value

ground surface. The EM38 susceptibility survey was don

September 1992 (15:29 - 16:45). The dipoles were vertical; 

was east-west, and traverses were made to the west. The rea

apparent conductivity were converted to magnetic susceptibilit

 Fig. B92: The EM31 was operated with its dipoles vertica

 Fig. B95: The noise is measured by the standard devi

repeated measurements, with the instrument stationary.

 Fig. B97: This survey was done on 5 May 1992.

 Fig. B98: This survey was done on 11 December 1991, 

dipoles vertical and the bar east-west. The normal dl-712 o

procedure was used with a 5 s average.

 Fig. B99: These tests were made by three profiles al

N85 at the Taylor House on 21 October 1992.

 Fig. B100: The resistivity has usually been determin

resistivity soundings; in a few cases, the resistivity h

determined by EM measurements or from a resistivity ma

measurement with a single electrode spacing. The average res

is determined from a depth model of the resistivity soundi
81

this antenna was not found to be generally the best fo

not very much effort was made to optimize the range gain 

this antenna. It is likely that the major cause of 

apparent depth with this antenna is simply the fact tha

part of the echo from the cellar floor has a very low a

is not detected in this profile.

 Fig. B102: This is a model 3105 antenna. The 180 

figure indicates the predominant frequency in the spec

radar pulse; this is determined by the width of the black 

 Fig. B103: This is a model 3102 antenna; the manufa

this a 500 MHz antenna, but I have found the predominant 

be about 315 MHz in average soil. The gray echo ban

apparent depth of 1.4 m are caused by reverberations i

system; they do not indicate soil strata.

 Fig. B104: This profile was made with a model 3101 a

 Fig. B105: These profiles are made along line E400 

made on 22 October 1992.

 Fig. B106: This 180 MHz antenna was found to be g

most suitable for the survey at this site.

 Fig. B107: Note that this antenna separates the ech

trenches almost completely from the horizontal echo b

primarily by the surface of the soil. This allows the 

interfaces to be clearly determined. However, note th

from the third trench from the right (at S240) is quite fa

 Fig. B108: The northern trench, at S200, still ca

distinct echo. This must mean that the soil contrast at 

very large and sharp.

 Fig. B109: Both of these profiles are along Taylo

N80 and both are profiles made with the model 3105 (180 M

the survey was done on 2 September 1992.

 Fig. B110: These profiles were made on 28 June 199

N90 at the Taylor House, and both were made with the mod

MHz) antenna.

 Fig. B111: These are profiles of line S200 at 

surveyed with the model 3102 (315 MHz) antenna. During t

6 - 8 March 1992, there was 7.4 cm (2.9 inch) of rain 

there was no rain on March 4, 5, or 9 and the rain ende

morning survey of March 8. The ground was still soggy on 

 The lower frequency model 3105 (180 MHz) antenna s

effects. Similar tests at the Taylor house on these two 

little difference in those profiles.

 Fig. B112: These are profiles of line S200 at Fort 

survey was done with the model 3102 (315 MHz) antenna. 

profile was made at 12:45 pm; an hour earlier there was h

the site. The rain began slowly early in the morning 

about mid-day. While the rainfall at the site is no

Hopewell, there was 1.9 cm of rain on the 6th. There wa

the site on August 7 or 8. The complex radar pattern on 

be caused by lenses of soil which have different moist

these are indicated by the distinct oblique echo li

profile. While all of the Fort Morton depth scales ass

velocity of 0.09 m/ns, the apparent depth of these 
82

calculation. Often the time scale of a radar profile will 

full, or two-way, delay time. If the reflector is larg

extended in a direction which is not perpendicular to the lin

radar traverse, a more complicated analysis is needed; see Vol

 Fig. B116: It is also possible that some reverberation

due to the length of a metallic object. During my 1979 s

this site, I studied the reverberations from metal objects, t

find out if there was any important information in the freq

the reverberations, but I found none.

 Fig. B117: This survey was done only with the low resol

120 MHz, model 3055, radar antenna. The symbols which I used 

the radar echoes were somewhat different for that survey, an

not changed them to make them consistent with the symbols f

later survey. For that survey, the radar profiles we

primarily along east-west lines. After the cellar fl

detected, I made separate short profiles along north-sout

there in order to define its shape better.

 Fig. B118: Only the profiles from the 1991 - 1993 sur

located here. The profiles with the 900 MHz antenna were o

close to the Taylor House cellar. While only one line was 

with the 120 MHz antenna, this was the same antenna which I 

for the 1979 survey here.

 Fig. B119: The average distance between the start of e

14.8 ft. This echo spacing is much smaller than that at Fort 

This survey was done during the period of 29 April - 2 May 1

a total field time of 34 hours was needed.

 Fig. B120: The profiles in this area were measured on 

1992 with the model 3105 antenna. Numbers indicate depths

There were 69 lines profiled, with a total length of 6900 f

are 454 echoes mapped in the figure.

 Fig. B121: This survey was done with the model 3105 

during 29 - 30 April 1992. The numbers show echo de

decimeters. With 41 profile lines, a total length of 6970 

profiled for this map; there are 553 echoes located on it.

 Fig. B123: The survey was done on 2 May 1992 with t

3102 antenna. The numbers indicate the depths of the echoes

For the 69 lines of profile, there were 370 echoes mapped on 

length of 6900 ft.

 Fig. B124: Using the model 3102 antenna, this survey 

entirely on 1 May 1992. The numbers show the depth of the ec

dm. The traverse direction was to the east. There are 41 

lines, for a total length of 6970 ft. There are 501 echoes m

the figure.

 Fig. B125: The two depth calculations should give t

number; any difference indicates an error either in location 

the assumed velocity of the radar pulse.

 Fig. B126: The solid lines mark the profiles which w

with the 180 MHz antenna, while the dashed lines show the 

with the 315 MHz antenna. Some of the lines are slightly off

their true path, so they do not obscure other lines.

 Fig. B127: Survey with a 180 MHz antenna on 29 April 19

profile closely follows the southern wall of the cellar, alth
83

profiles in the area of the Taylor House, independent o

date or the radar antenna. Depth actually means range. 

echoes are plotted at the bottom of the profile and 

indicate their depth, in m. Tick marks at the top of 

indicate 5 ft intervals; these marks were placed by push

on the handle of the radar antenna whenever the midp

antenna passed a 5 ft mark on the guide rope. Double 

indicate 50 ft intervals in the grid system. The trave

the radar antenna was 8 s per 5 ft; this was contr

electronic metronome which beeped at 1 s intervals. Unle

stated, all profiles made with the model 3105 antenna ha

traverse speed. The radar was set to measure depth sca

lines) at a rate of 12.8 scans per s; therefore, the 

between depths scans was 1.5 cm. These scans are faintl

vertical lines in the profiles; only every other scan i

these illustrations. The traversing direction of the r

was to the north here, as is conventional; the left side 

was always measured earlier than the right side.

 Fig. B129: Both traverse directions of the model 

are included in the statistics. There are a total of 539 

west of W10, and 306 echoes from the east side. E

normalized to those sums to give the fractions which are 

echoes within each depth interval of 0.1 m are added.

 Fig. B130: The survey was done on 29 April 1992 

MHz antenna. This echo was similarly detected on line N

on line N62.5, there was a shallow reverberation in thi

echo was not detected at all on the north-going trave

evidence suggests that there is a metal object like a p

which goes along a north-south line in this area. Thi

clearly detected by the 1979 radar survey also, and it is 

in Figure 2 of the publication (Bevan, Orr, and Bl

however, the depth of the object listed there (1.4 m) was 

too large because the low frequency model 3055 antenn

detect the shallow echo at 0.6 m.

 Fig. B131: Note that depths are in cm here; depths 

radar maps are in dm. The radar survey was done during 

April 1993 with the model 3101C antenna. For the 13 li

length of 780 ft was profiled and 48 echoes were mapped.

 Fig. B132: The lines which were surveyed wi

frequency, model 3055 (120 MHz) antenna are slightly of

indicated as a long-short dash line. Note that the pr

was done only with the 180 MHz antenna. Since this ant

315 MHz antennas were both about equally suitable here, 

used.

 Fig. B133: The map is the result of radar profiles 

traverse length of 34,560 ft. There are 1365 echoes 

which indicates that the separation from the start of one 

start of the next averages 25.3 ft. A total of 43 hou

work was needed to generate the profiles of this map.

 Fig. B134: This survey was done with the model 31

antenna during the period of 6 - 8 December 1991. A tot

19,155 ft was profiled on 68 lines, and 785 echoes are 
84

spacing between the profile lines was 10 ft and the direc

traverse was to the south; this direction is backward f

typical and standard direction because it allowed the travers

from the grassy area into the wooded area, and this is muc

than the opposite. Long traverses were broken into two span

S0 to S300 and S300 to S450. The profile spans were int

separately, and the interpretation symbol can change at the 

between the two segments of a traverse.

 Fig. B136: The profiles with the 180 MHz antenna ar

with a solid line, while the 315 MHz profiles are indicated 

broken line.

 Fig. B137: Survey on 26 June 1991. The traversing spee

radar antenna was 12 s per 5 ft; this is 0.13 m/s or 0.46 

Since the scan rate of the antenna was set for 12.8 scans per 

the scan lines were spaced by 1.0 cm; this printout has eve

scan on it, so the spacing between vertical scan lines here is 

 Fig. B138: The bombproof trench is the third from the n

the east of E450, the profile crosses the unmown area of t

the profile shows no difficulty there. The survey was do

December 1991 with the 180 MHz antenna.

 Fig. B139: Survey 6 December 1991 with the 180 MHz anten

 Fig. B140: The iron pipe causes a strong echo at E359. 

6 December 1991 with the 180 MHz antenna.

 Fig. B141: Both profiles were made with the 180 MHz 

the profile of line S70 was made on 6 December 1991, while l

was profiled on 9 December 1991. It is possible that this w

from about E263 S10 to E250 S80. The very distinct echo on t

hand side of the E250 profile is caused by an iron pipe there

 Fig. B142: The distinct echo at E160 is caused by an 

object in front of the fort. A perpendicular profile was 

here. Survey 7 December 1994 with the 180 MHz antenna.

 Fig. B143: There is an indication of reverberation on t

hand half of the road echo. Survey 7 December 1991; 180 MHz a

 Fig. B144: Solid lines indicate the lines profiled with 

MHz antenna, while broken lines show the path of the 315 MHz a

 Fig. B145: This map reveals how the clarity of an echo 

can change along the length of a trench. Some of the echoe

to extend from one trench to another along the north-south 

this is partly caused by the indistinct boundaries on some 

patterns. This map was generated from a total profile le

12,155 ft. A total of 513 echoes are plotted here, indic

average spacing between the starting points of echoes to be 

A total of 25 hours of field work was required to make the 

for this map.

 Fig. B146: The survey was done on 5 August 1992 with t

3102 antenna. For the 42 lines of profile, with a total l

4070 ft, a total of 174 echoes were mapped. The numbers s

depth in dm.

 Fig. B147: This survey was done of 5 and 6 August 1992 

model 3105 antenna. For the 40 lines of profile, a total 

echoes were mapped in the length of 3985 ft. East-going t

were made with the model 3105 antenna later in the day of A
85

the radar echoes. Survey 6 August 1992 with the 315 MHz a

 Fig. B150: The lines for the model 3102 (315 MHz) 

offset from the actual line of traverse so that these 

distinguished from the lines of the 180 MHz antenna.

 Fig. B151: The survey was done during 9 hours of f

8 August 1992. A total profile length of 4900 ft was gen

86 lines. There are 248 echoes mapped in the figure.

 Fig. B152: This survey was done on 8 August 1992 wi

3102 antenna. The spacing between profile lines is 2.5 

were made with east-going traverses. Only three lines we

south-going traverses, and these echoes are mapped here 

numbers show echo depth, dm; the orientation sho

direction.

 Fig. B153: This is also the model 3105 antenna, an

continued on 8 August 1992. The numbers show echo depth, 

 Fig. B154: This survey was done with the model 310

8 August 1992.

 Fig. B155: Survey 8 August 1992 with the 180 MHz a

radar profiles indicate a line of echo arcs along E485

plotted in Fig. B152. At first I thought that this mar

of a buried pipe or wire; since the conductivity and magn

do not detect this, there must be another cause. The li

is probably caused by the bottom corner of the fortifica

this is acting like a corner reflector; corners are a

detected by airborne radars.

 Fig. B156: Some of these apparent pits may also b

the side of the trench, but most are isolated pits. 

December 1991 using the 180 MHz antenna.

 Fig. B157: The field work required about 3 hours 

1992. A total of 26 lines were profiled, with a total l

ft. A total of 66 echoes are mapped, and the numbers 

echoes show their depth in decimeters. Note that the 3l5 

was only used for profiles to the east and the 180 MHz o

to the south.

 Fig. B158: The two profiles are along perpendicul

are made with two different antennas. Note that the d

feature here is about 0.4 m, less than that from the in

of other geophysical data; this suggests that part of 

mass is at a shallow depth, while the major part of the 

begin at a greater depth. Survey 28 June 1992.

 Fig. B159: The survey area of the Iron detail is 

behind the main fortification trench at the fort.

 Fig. B160: The survey was done on 1 December 19

16:37). Only a single pair of these pots were prepar

test, so no adjustment of the differences between pots 

The upper 2 - 3 cm of the grass turf was scraped away, an

pot was set on the surface of the soil. The resistance 

pair of pots was measured to be 9000 ohm, much lower than 

AgCl electrodes. The measurements were made at 30 s in

the voltage dropped by about 2 mV during that period. 

measurement traverse to the east was finished, the point 
86

expose the bare soil. In two locations, a slight excavation 

grass roots was made, and this raised the voltage at the el

The voltage of these Tinker and Rasor electrodes drops by a

mV when sunlight shines into the transparent inspection windo

side of the electrodes.

 Fig. B162: Stainless steel electrodes were driven to a 

about 13 cm and measurements were made at 30 s intervals. 

that 30 s, the voltage usually rose by about 0.15 V; this la

may account for much of the irregularity of the measurement

survey was done 1 December 1992 (15:43 - 15:58).

 Fig. B163: The 25 October 1992 survey was done during 

11:18 with the six Ag-AgCl electrodes; electrode #0 was the r

negative. The resistances between the electrodes were 

relative to electrode #0 before the start of the survey, a

the electrodes were in a KCl bath; these values (in kilohms) 

= 60; #2 = 85; #3 = 55; #4 = 23; #5 = 50. All measurements w

with a Fluke model 8060A digital voltmeter. After the 

electrodes were placed in the soil, the sequence was as 

assuming the measurements move to the right: Measure the vo

the left electrode; move the voltmeter to the next location 

right; core a hole on the right and remove the plug of soil; 

left electrode; wash it in water and place it in the new hol

right. The electrode numbers cycled 1234512345 ... and the e

number was recorded along with the voltage. The inter-e

resistance, with the electrodes in the soil, was measured 

times during the survey; both values were about 110,000 ohms. 

found that the voltages changed by about 3 mV when the el

were moved about the bottom of the cored holes. The voltag

electrode increased by about 4 mV in a minute after it was p

a hole; the measurement sequence above allowed this vol

stabilize before it was recorded. The inter-electrode dif

were adjusted by subtracting constants from the measuremen

each electrode; these constants were: #1 = 37 mV; #2 = 45 

33 mV; #4 = -1 mV; #5 = 24 mV. On 2 December 1992, the meas

were made between 10:06 - 10:52. The procedures were the same 

the earlier survey. However, for this survey, measurements w

at exact one minute intervals, in order to make the temporal 

the electrodes in the holes about the same. It was easier 

the soil corer into the earth in December than it was in Octob

 Fig. B164: The survey was done on 2 December 1992 

12:17). The procedures described with Fig. B163 were applie

that the measurement interval was fixed at 30 s per measuremen

 Fig. B165: The survey was done on 25 October 1992 

15:45). The procedures were the same as for the survey ill

in Fig. B163. The voltages would change by 3 - 10 mV w

electrode was moved across the bottom of the 1 inch (2.5 cm) 

cored holes, which were about 13 cm deep. At E479, the corer 

abruptly, so there is probably the tunnel of a rodent there.

 Fig. B166: These measurements are just as noisy as al

other measurements with the Ag-AgCl electrodes. The survey 

on 2 December 1992 (13:39 - 14:37), making measurements 

intervals with the procedures described with Fig. B163.
87

Possible rodent tunnels were found at two points along l

E510 and E534.

 Fig. B168: This test was made on 25 October 19

10:44) just at the start of the measurements shown in Fi

the moving electrode was at E411.

 Fig. B169: Six Ag-AgCl electrodes were placed to

water basin containing a solution of KCl for these tests

#1 was selected for the reference negative electrode and 

of all of the other electrodes were measured relative 

electrode. A broken line separates two groups of nu

numbers below the line were measured in the field du

surveys; the numbers above the line were measured ins

laboratory and with the electrodes continuously in a bath

survey was started. On 14 October 1992, at 5:30 pm, ther

change in the voltages; I measured the inter-electrode 

before this. Electrodes #1 - #5 are Ag-AgCl double 

reference electrodes; they have a porous Teflon tip a

body. Before this survey, an algae-like growth was remov

of the electrodes. Electrode #0 is a single junction el

a glass body; I bought this surplus. During all of thes

average voltage change of the electrodes was 13 mV.

 Fig. B170: As with Fig. B169, the measurements sho

broken line were measured in the field, while the measur

were measured in the laboratory, where the changes in tem

light were much smaller. The inter-electrode voltages w

with the electrodes grouped together in a bath of coppe

water. While the Geoex electrodes were bought in 1980

and Rasor electrodes were bought in 1987. Unlike 

electrodes, no growths appeared on these electrodes aft

storage; this is probably because CuSO4 is toxic to orga

the Tinker and Rasor and the Geoex electrodes have a copp

dips into the saturated solution of copper sulfate in wate

 Fig. B171: Measurements were recorded with the Met

712 data logger; it averaged samples for an interval of 

samples were taken at either 1 or 15 s intervals.

 Fig. B172: The central point of the SP anomaly is 

be at E525.8 S144.8 and the distances from this point t

original measurements, shown in the upper left part of Fi

plotted along with the measured voltages. While a bipole 

poles at depths of 4.3 and 11 m) gave a poorer 

measurements, the data are so noisy that these model

reliably distinguished.

 Fig. B173: The data for this figure, and also Fig

B175, Fig. B176, and Fig. B177 was kindly sent to m

Blades. No samples were taken in the northwest corner. T

of these samples was measured by the Soil Testing Labor

Agronomy Department of the University of Maryland in July 

not know what are the units of these measurements.

 Fig. B177: High pH means that the soil is more acidi
88

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