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This is the first webinar in a series of 3 webinars on Magnetic surveying for anthropogenic objects.

The theory, data acquisition and data processing. The webinars are jointly delivered by Seequent and Geometrics. Search for man-made objects in the near surface has been an important objective for near surface geophysicists.

Items of interest include Unexploded Ordnance (UXO) from both training activities and warfare, archaeological investigations, and old infrastructure such as pipelines, cables and underground storage tanks where the available maps and drawings are not present or inadequate. Magnetic methods have been one of the primary methods for detecting these objects.

This webinar focuses on the practical implications of magnetic theory for conducting surveys – including the impacts of:

  • Size, shape and orientation of ferrous objects
  • Remnant magnetisation
  • Gradient vs total field measurements


42 min

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Video Transcript

<v ->Hello, and thank you for joining us today.</v>

My name is Gretchen Schmauder and I am a geoscientist.

And the Director of Marketing for Geometrics.

Today we also have Becky Bodger, who’s a geoscientist

for Seequent.
<v ->Hello.</v>

<v ->and Bart Hoekstra, who’s a geophysicist</v>

and the Vice President of sales for Geometrics.

<v ->Hello and thank you all for joining us today.</v>

<v ->Stefan Burns also contributed to today’s webinar.</v>

He created the video you will be seeing here

in a few moments.

This is the first in a series of three webinars

about magnetic geophysics.

Today’s webinar covers magnetic theory.

Future webinars are going to talk about

magnetic data collection,

and then the third will be magnetic data processing.

We really want today’s webinar to be both informative

and interactive.

So to keep people engaged, we have included

a series of questions that will pop up on the screen

as we progress.

We’d ask that you answer these questions

either using the poll that will pop up on your screen

or in the chat window as part of this webinar.

Towards the end of today’s session

we will talk about some of the answers

and hopefully answer any questions you may have.

Today we are focusing primarily on anthropogenic objects

or those objects related to human beings

and their interaction with the earth.

The biggest driver for this

is a search for unexploded ordinance

also known as UXO in both the North Sea

and in Southeast Asia.

For example, the U.S. dropped over 2 million tons

of explosives in Laos in the 1960s and 70s.

The image you see on the left shows a field

with bomb craters still present after nearly 50 years.

On the right-hand side, you can see an unexploded bomb

that is still posing significant risk to human inhabitants.

In the North Sea world war II era bombs

are still littering the sea floor.

With the recent increase in wind farm development,

efficient exploration for UXO is very important.

Other areas driving the need for magnetometers

include pipeline tracking and environmental concerns

such as leaking underground storage tanks.

You can see one of these here in the far left image.

In the middle image, you can see an abandoned wellhead.

These are frequently cut off below the ground surface,

so they are not visible to the eye.

Finally, in the right hand image,

you can see a magnet, where a magnetometer was used

to help characterize an archeological site.

So here is our first question.

Please write your answers in the chat window.

What other anthropogenic objects or hazards can you think of

that we can use magnetic surveys to identify?

Our presentation begins with a brief explanation

of the earth’s magnetic field.

After this Bart and Becky will go into a deeper discussion

of magnetics and magnetic anomalies.

We are here to answer your questions.

So please feel free to use the chat feature to contact us

at any time during today’s presentation.

You can also email us.

So let’s get started.

<v Stefan>Electromagnetism is one</v>

of four fundamental forces that govern our universe.

Like two sides of the same coin

electrical and magnetic fields can not exist

without the other.

Electromagnetic field is created when positive

and negative particles interact

like the nucleus and electrons of an atom.

Magnetic fields hold electrons to their atoms

resulting in molecular bonds,

that hold compounds together and govern chemical reactions.

At a macroscopic scale, some planets like earth

and the gas giants of our solar system

maintain massive magnetic fields of their own.

It’s earth’s magnetic field that protects life

from dangerous high energy solar wind emitted by the sun.

Humans use magnetometry the study of magnetic fields

to understand geology and define anthropogenic objects

humans have left behind.

Hello, my name is Stefan Burns

and welcome to the magnetic surveying

for detection of anthropogenic objects.

If you’ve ever played with a magnet,

you have experienced firsthand

the unique physics of magnetic fields.

Magnets have opposing north and south poles.

As the saying goes, opposites attract.

And in this case, the north and south poles of a magnet

are attracted to each other.

When to north or to south poles are placed near each other,

repulsive force exists.

The poles repel each other at the atomic level.

If you could see magnetic fields,

you would observe that magnets are surrounded in space

by force fields originating at the positive

south magnetic pole

and ending at the negative north magnetic pole.

If you could see an even greater detail,

you would find that this field is created

by the movement of electrons.

Electrons moving from an area of negative charge

to one of positive charge to find as electrical current

is what gives magnets our ability to attract or repel.

When an electric current is created

and electrons are in flow a magnetic field is created.

And this is the case because electricity and magnetism

are linked at the quantum level.

Modern magnetometers can sample the magnetic field

hundreds or thousands of times per second.

Before this, we were still aware of magnetic fields,

thanks to simpler technologies, such as the compass.

When suspended in water, the magnetised pin of a compass

lines up with the magnetic field lines of our planet.

Each end pointing to the earth

oppositely charged magnetic pole.

The earliest recorded use of a compass

is dated to the second century BC

when people use naturally occurring lodestone,

otherwise known as magnetite to locate north.

And in the 11th century,

Chinese texts describe the needs for navigation

amongst the ever-changing conditions at sea.

The north arrow of a compass points to what we refer to

as Earth’s magnetic north pole.

However, you will recall that opposites attract.

So what is really going on

when a compass needle points north?

It turns out that if you represented

the earth’s magnetic field using a bar magnet,

the bar magnet south pole

would lay in the Northern hemisphere.

A compass pointing towards the earth magnetic south pole

ends up pointing towards geographic north

since the geographic north pole and magnetic south pole

lie in close proximity to each other.

Across geologic time, the earth’s magnetic field

has regularly gone through magnetic pole reversals.

In the case of a pole reversal today,

the poles were switched places

and a compass’s north arrow

would still point towards the magnetic south pole

now located in the Southern hemisphere.

And the geographic north pole

would now be home to the magnetic north pole.

On a human timescale, the earth’s magnetic field

wobbles and shifts on a yearly basis

and magnetic south is currently moving away from Canada,

approximately 55 kilometers per year towards Siberia.

Scientists first discovered magnetic pole reversals

just over 100 years ago.

And it wasn’t until the 1950s

that extensive summary mapping projects

showed how well the basaltic ocean floor

records these magnetic reversals.

As seafloor spreading centers deep underwater,

magma spews out along geologic fissures

and cools into a rock known as basalt.

Basalt is faintly magnetic

due to its mafic iron rich composition.

But in a molten state,

iron is not yet permanently magnetized.

As this magma cools,

minerals containing iron forms in a line

to the earth’s magnetic field,

just like tiny compass needles.

This magnetization continues

until the basalt passes through the Curie point

or the temperature at which iron containing minerals

become fully magnetic.

At this point, they are magnetically frozen in space,

and unless reheated will show the alignment

of the earth’s magnetic field at the geologic time

that Curie point was passed.

Recognizing this phenomenon,

geophysicists can analyze the magnetic signature of a rock

along with radiometric age to chronicle the age

and timing of earth’s magnetic cycles.

The record shows that the earth’s magnetic field

can flip quite rapidly and then remain stable

for hundreds of thousands or millions of years

before undergoing another magnetic pole reversal.

Many man-made objects contain magnetic material,

and ferromagnetism allows us to detect these objects

with magnetometers.

Ferromagnetism occurs when electrons spin

in alignment with each other and iron cobalt and nickel

are common ferromagnetic materials.

The greater number of aligned spinning electrons

a ferromagnetic material has, the strongest magnetic field.

Since the advent of the iron age,

many objects created by humans contain ferrous materials,

and therefore can be detected

by measuring the magnetic field near the earth surface.

There are two primary methods

for measuring the magnetic field

that are commonly used for detecting man-made objects.

Magnetic sensors that measure the magnitude

of the magnetic field are referred to as atomic sensors,

more commonly known as cesium,

rubidium, proton, or Overhauser sensors.

They each utilized slightly different physics

to measure the magnetic field vary from each other

in their accuracy, sensitivity, and sample rates.

The local distortions of the earth’s magnetic field

are observed by magnetometers as anomalous signatures,

which can be used to precisely locate anthropogenic objects.

Modern magnetic instrumentation can detect variations

as small as one millionth

of the value of the Earth’s magnetic field.

And this increased sensitivity allows for the detection

of smaller objects at greater depths.

In addition to their incredible resolution,

modern magnetometers also rapidly sample the magnetic field.

And when many sensors are combined into arrays,

large areas of both land and sea can be surveyed

in great detail.

The other primary method for measuring a magnetic field

is to measure the change in the magnetic field

over a distance in a particular orientation.

These are known as gradient sensors

with fluxgate magnetometers being the most common type

of gradient sensor.

In the remainder of this presentation,

we will discuss the factors affecting the size

and shape of magnetic signatures of ferromagnetic objects.

<v Bart>Hello, my name is Bart. Hoekstra</v>

I’m vice president of geophysical sales at Geometrics

and have had a long history of surveying

for detection of metal objects primarily for UXO,

but also other infrastructure related objects

such as underground storage tanks and pipelines.

What I will be talking about is some of the complexities

that occur when we try and measure an anomaly

or the signature of a man-made object.

One of the things about magnetic surveys

is that they’re quite easy to do.

The sensors aren’t that large,

and you can carry them or fly them around,

download the data and create a map.

But the simplicity of this survey

and processing disguises the fact

that sometimes what we measure

is not as simple as we would like it to be.

This can lead to misinterpretation of features

and possibly not recognizing the presence of an object.

One of these complexities arises from something called

remanent magnetization.

Stefan briefly discussed this in his intro,

but what happens with ferromagnetic materials

is that they have mineral grains.

And within each one of these mineral grains,

you can have a different alignment of the electrons

which causes a different alignment

of the south and north magnetic poles.

When the ferromagnetic object that contains the grains

is heated and then cooled below its Curie temperature,

the magnetic grains will align

with the earth’s magnetic field that occurs

at the point in time and space and orientation of the object

within the surrounding magnetic field.

So the upper image shows the effects

or what the remanent magnetization will be

above the Curie temperature,

but once it’s aligned and cooled below,

the Curie temperature,

you can see that all the magnetic domains within each grain

are aligned with the external magnetic field.

But what happens over time

is that some of the remanent magnetization

in these mineral grains becomes weaker

and can go away altogether.

And then you have a combination of remanent magnetization

in some mineral grains and induced magnetization

in other mineral grains.

And the induced magnetization will line up

with the external magnetic field

that is present at the space and time

in orientation where the object is now

which may be different from the magnetic field where it was

when it first cooled below the Curie temperature.

And these two fields can be very different from each other

in both magnitude and direction.

And in some cases, the remanent magnetization,

depending on the strength of it,

can be up to five times the size

of the induced magnetization,

or it could be an almost negative negligible factor

of the, compared to the induced magnetization.

And so the result of this

is that we have a vector sum,

the field you measure will be a vector sum

of the induced magnetization and the remanent magnetization,

and often what is referred to as a Koenigsberger ratio

is the ratio of the magnetic remanent field

versus the induced field.

So in the next series of slides,

I’m going to be talking about

and giving examples of the effects of remanent magnetization

on a measured anomaly for a particular object.

The particular object we are going to

show the results, the model results of

is an oblate spheroid.

And this is kind of a UXO shaped object.

It’s 10 centimeters in diameter, 50 centimeters in length.

And it’s located at a depth of three meters

below the plane of the model measurements.

It’s inclined 60 degrees down

and is oriented 60 degrees from north.

The background field is approximately 50 nanoteslas,

which is a close approximation

of the earth’s magnetic field.

And it’s at an inclination of 30 degrees positive,

which is somewhat different from those that are

used to seeing measurements in the Northern hemisphere,

but you’ll still recognize the anomalies that are modeled.

So now we’re going to show the modeled results

from this target.

And this first slide is,

contains just the induced magnetic field.

On the left-hand side we have the model data

from the object as measured by a total field sensor.

And on the right hand side,

we have the model data as measured

as would be measured from a gradient sensor

with 50 centimeter with, on a gradient.

This first slide shows just the induced field only.

And as you can see, we have a large peak

that’s to the Northeast of the

target and a smaller trough to the south west.

The next animation shows what happens

when we have a remanent field that is the same amplitude

as the induced magnetic field,

and it’s oriented vertically downwards.

As you can see,

it changes the shape of the anomalies significantly,

and we have a much larger trough

now that’s still oriented towards the Southwest

and it’s decreased the peak.

All right, the third animation shows what happens

when we have a remanent magnetization

still the same amplitude, but now it’s oriented in the,

towards the west horizontally.

And as you can see it all,

again, change the shape of the anomaly

and now our smaller trough

is oriented towards the Southeast.

So this is quite a change from the original induced

all the measurement that we have.

The final animation in this slide

is with the induced or the remanent field

oriented towards the west.

And as you can see this looks actually

kind of similar to what we had with just the induced field,

except now that the trough is oriented

towards the Northwest.

This just shows you some of the complexities

that can arise when you have strong remanent magnetization,

and it can distinctly change the shape of your anomaly

and may cause you to misinterpret what you have.

The next set of animations we’re going to model

is using the same object

and the same induced magnetization field,

and use the same set of four parameters

for the remanent magnetization.

But what we’re doing, what we’ve done now

is added noise that you’ll typically see

from a total field survey and from a gradient survey

so you can see how the remanent magnetization

will change possibly the interpretation of the anomaly.

The first animation is with the induced magnetic field only,

and you can see the object quite well.

The next animation has remanent magnetization

that is oriented vertically downwards.

The third animation has the remanent magnetization

that is horizontal, but it’s in the Southern direction.

And the final animation has the remanent magnetic field

oriented horizontally but pointing to the west.

And as you can see from our examples with noise,

in some cases, the remanent magnetization

can make an anomaly from a very metallic object

look quite different than what you’re expecting

from just an induced field anomaly.

And in many cases that may lead you

to not selecting the object as an object of interest

or misinterpreting what that object is.

We’re now going to discuss

some of the different types of measurements

that Stefan discussed in the introduction.

We’re going to talk about two,

basically two kinds of measurements.

The first one is what we call a total field sensor,

which measures just the magnitude of the field

in, at a point in and time.

These total field measurements

are called total field sensors, scalar magnetometers,

or atomic magnetometers.

And then we’re also going to look at

what are called gradient measurements.

And what they look at is the difference

in the magnetic field over a distance

in a specific orientation.

In most cases, gradient measurements are made with

in either the vertical or a horizontal plane.

With vertical gradient measurements,

we measure the difference in the magnetic field

from the top and the bottom of your sensor or sensor array.

For horizontal gradient measurements,

you’re looking at the difference across the width

of your sensor or sensors array.

Gradient measurements can be made with total field sensors

that are separated

by a specified distance, or they can be made

with what are referred to as fluxgate sensors.

On the left-hand side, we have a measurement

that’s made by a fluxgate magnetometer

in this case, the FM18.

And this was done over the NSGG test site located in the UK.

You can see various features on this map.

And in particular,

you can see clearly distinct anomalies

to the, in the central Northern part of the map

that were buried objects as tests for anomaly detection.

And that’s shown on the left-hand side.

On the right hand side,

we have measurements that were made using

two total field sensors separated by about a meter.

And you can see the two maps are quite similar.

There’s some differences.

But in general, you can see the anomalies quite well.

And it’s fairly easy to pick out our objects of interest.

In the next slide, what we’re showing

is the total field measurement made by a single sensor

from the array of G858.

And you can see this map looks different

than the other sensors or other maps that we were showing.

You can clearly see some of the large anomalies

in the Northern half of this map,

which were the buried objects.

And you can see some other anomalies outside that region,

but they tend to be a little bit more subtler.

And that’s likely due to the fact

that they were smaller objects nearer to the center

or near to the surface.

And in general these are not picked up as easily as

the gradient measurements too.

But, this map I would say tends to be a little bit

less noisy.

Where there are no objects

you don’t see much magnetic field variation.

The other thing you’ll note about this odd map is that

there is a gradient, a long wavelength gradient

in the data with the magnetic field decreasing

to the south of this area

that you did not see in a gradient field measurement.

And this is because when you take two sensors

or you’re just measuring the difference between two points,

they will see the same long wavelength gradient

and when you subtract out the field from the top and bottom,

then there is no long-term wavelength difference

in the measurement.

So I just wanted to highlight that and I’ll just go in,

in the next slide.

So in this slide, I’ll discuss some of the reasons why

there is a difference between measuring the magnetic field

with a total field sensor or a scaler sensor

versus a gradient sensor.

The total field signal decreases in amplitude

by a factor of one over R to the third with distance.

The gradient signal decreases

by a factor of one over R to the fourth.

On the chart, on the right hand side,

the decrease in amplitude is,

with distance is represented by this blue curve

for a total field measurement

and a decrease in amplitude of the measurement

for a gradient signal is shown in the red curve.

And this distance and amplitude are normalized

by the amplitude of the anomaly

and also the length of the dipole.

But as you can see at a factor,

a distance of eight compared to the length of the dipole,

the total field measurement in blue

is actually an order of magnitude almost greater

than the amplitude of the gradient measurement.

And so this has a pretty big impact on your ability

to detect objects at a greater range from your sensor.

So in general, a total field measurement

is more effective at detecting objects at greater distances.

But, there are some advantages

to doing gradient measurements.

They tend to filter out low frequency trends

as you saw in your previous map.

The anomaly that you see from a gradient measurement

is sometimes a little bit better defined.

It’s sharper and narrower in width

which makes it easier to determine the location

of an object.

And if you’re interested in shallow objects,

it can also enhance the location and detection ability

of those shallow objects.

One other factor to consider

when you look at gradient measurements

is that they tend to be noisier.

That’s because we’re actually taking the derivative

of the magnetic field in that particular orientation,

either vertically or horizontally

and in all cases derivative measurements

tend to act as a high pass filter.

And noise in most magnetic surveys is,

increases with frequency

and so you will tend to increase noise

in your measured signals.

So, as a question I’d like to ask people,

what kind of measurement the magnetic field measurements

they have made in the past or plan to in the future.

Do you use a total field sensor or atomic sensor,

a gradient sensor such as a fluxgate

or use a gradient array of total field sensors?

Have you done both kinds of surveys

or are you brand new to magnetic surveying

and haven’t done anything?

So please answer the poll in the chat window

and we can discuss the results later on.

Well, that concludes my section of the talk.

And now I’m going to hand off the presentation

to Becky Bodger from Seequent.

I will be back to discuss the conclusions of this webinar,

and also to answer any questions that you may have.

<v Becky>Thanks, Bart.</v>

As mentioned, my name is Becky Bodger

and I’m a geoscientist at Seequent.

In this next section

I’m going to show you a series of examples

that were created using the forward modeling capabilities

available in Oasis montaj with the UXO extension.

The grid on the left is the background that I used

for the forward modeling.

It’s from an actual UXO survey in the North Sea,

and is representative of the type and level of noise

you would get on a real survey.

In most of the examples,

I’m using a 155 millimeter projectile

unless otherwise stated.

And there’s an image on the right

that shows you what that looks like.

So whenever you are working with any potential field data,

for example, gravity or magnetics,

we need to remember that mathematically

magnetic anomalies are non-unique.

Multiple theoretical solutions are possible.

This is true whether we’re talking about geological features

or anthropogenic objects.

I found this image in a paper online

that talks about the ambiguity in potential field modeling.

And I like it because it demonstrates

that the same Mickey mouse shaped deposit

can have three different geophysical signatures.

Why do we continue to use magnetics though,

or any potential field data?

Because there are ways to minimize this issue.

So using a priori information in most cases

especially when we were talking about anthropogenic objects,

there’s history somewhere.

If it’s UXO, we can research

which munitions were dropped by either side

during the various wars or conflicts.

For archeology, hopefully you know some of the history

of the area and what you are looking for,

whether it’s an old burial site or building foundations.

And for geotechnical investigations,

there are often existing infrastructure maps

showing the locations of buried cables and pipelines.

This information is not always available,

or it can be difficult to unearth,

but it’s worth checking what’s available

to aiding your processing and interpretation of the data

before you start.

Cross-referencing multiple types of geophysical data.

So in the case of offshore geophysical surveys,

mag is only part of the story.

Oftentimes surveyors will simultaneously

be collecting multi-team data, side scan, sonar,

seismic, sub-bottom profile data, or even seabed images.

Using the results of all of this data

will really help with processing

and the interpretation of your results.

And in archeological studies, for example,

you might have gravity and resistivity as well.

Finally, use your common sense and be smart.

You’re going to use common sense, your education,

your past experience, all of that,

to apply logic and find the best interpretation.

Here, I attempt to demonstrate

the non uniqueness of this mag anomaly.

While they are not identical,

I hope that we can agree that they are similar enough

to demonstrate the point.

So on the left we have two UXOs.

One is an 81 millimeter projectile

at 2.5 meters below the sensor.

And one is a two and three quarter inch rocket

at two meters below the sensor.

On the right, we have a single UXO,

which is the 155 millimeter that we’ve been looking at

at 3.5 meters.

And you can see that on the left-hand side,

you know, the inflection point

between the dipole’s a little bit slanted.

It’s a little smaller than the one on the right.

If we look at the profile below the grid,

we can see that in profile they’re even harder

to distinguish the differences.

They both have similar positive and negative peaks.

And again, the real only difference that we see here

is the width of the actual dipole.

This example also demonstrates the importance

of griding your data and not only interpreting the results

in, along the profile.

It’s important to visualize it in 2D space

to really see the full picture.

So here’s another question for you guys.

Which inclination of an object

produces the strongest amplitude

whether it’s peak to peak of the dipole

or a single peak amplitude?

Do you think it’s A, vertical, B horizontal,

or C inclined at 45 degrees?

So I’ll give you a few seconds

just to putting your answers and then we’ll carry on.

In this example, I’m going to show you the response

of the object at different inclinations.

So on the right,

the image is just to represent the orientation

we didn’t actually model the UXO that you’re seeing.

So you can see that when it’s horizontal,

it’s a nice perfect dipole.

When you add inclination,

so on the example that we modeled here

was inclined at 45 degrees.

You can see that the negative trough

becomes a lot less negative.

And when it’s perfectly vertical,

you can see that the negative disappears altogether

and you’re left with a monopole.

And again, just remember,

we’re not modeling the size of the UXO in the image,

it’s just there to show you that it’s vertical is possible

especially in the marine environment.

So what’s the answer to the question that I asked?

Here we have the three responses along a profile

through the center of the dipole.

The first is the horizontal,

and we can see that the peak to peak amplitude

is 5.4 nanoteslas.

The second one in the middle here,

is the object inclined at 45 degrees

and it has a peak to peak value of six nanoteslas.

And the vertical object has a positive monopole

total peak value of 6.4 nanoteslas.

So the answer to that last question was C for vertical.

Next, we’re going to look at how the response changes

as the object changes direction or destination.

The top row is the horizontal 155 millimeter projectile

at four meters below the sensor.

And the bottom row is the inclined object at 45 degrees.

Note how the negative part of the dipole

decreases more rapidly as we rotate the inclined object.

The first row is the near vertical object

inclined at 85 degrees.

And the second row is the same object

inclined at negative 60 degrees

which implies the opposite polarity.

So, instead of the north pole up in the air,

in this example, the south pole is up in the air.

An interesting effect in the negative 60 degree example

is also the halo that we see around the more obvious dipole.

This is important

when trying to model the depth of the object,

whether you are using Euler deconvolution

or an inversion style modeling method.

Most methods require you to define the modeling window

in order to select which data to invert.

Since all of these examples use the exact same background

which you can see in the top right-hand corner here,

any differences in color that we see

is part of the signal from the object.

So for accurate modeling, we would ideally want to include

as much of that signal as possible,

which means it would require a much larger modeling window.

Let’s also consider the effect of depth below the sensor

on the inclined examples.

So in the top row, we have the eastward facing

155 millimeter projectile inclined at negative 60.

And on the bottom, we have the same size projectile,

but inclined at 45 degrees.

And we can see the different response we get

at 1.5 meters below the sensor, 2.5, 3.5, 4.5.

And we can see that, we can actually see

that none of these examples really produce

that perfect looking dipole.

And in fact, as the distance between the sensor

and the object increases depending on the example,

the positive in the top example

and the negative in the bottom example,

disappears almost entirely

and leaves us with another parent monopole.

So the last example or complexity

I wanted to talk about today

is simply the difficulty that arises

when you have multiple objects stacked on top

or close to one another.

And this is a common phenomenon in test ranges

or munition dumpsites.

So on the left, I’ve modeled two UXOs.

One is a 105 millimeter projectile

at 3.5 meters below the sensor.

Southward facing.

And one meter away is a second UXO,

a 60 millimeter projectile at two meters below the sensor

and westward facing.

It creates what I think would be called a complex dipole.

On the right-hand side, is another example.

A 155 millimeter at four meters

and a 105 millimeters at two meter depth below sensor.

Both roughly southward facing.

With careful analysis, the example on the right

could probably be accurately classified

as two separate targets,

but the example on the left would be a lot,

would be much more difficult to separate.

And I just wanted to give you a few,

a couple of examples where we see this in real life

and which caused a lot of problems.

So in this example, this is a map

of Lac Saint-Pierre in Canada.

And I just wanted to show you the complexity

and the problem that they’re dealing with here.

And as an example, one of these large blue circles is,

there are over 200 UXOs in that tiny little space.

Another example in Europe is,

this is the port in marked munition dump

off the coast of Zeebrugge Harbor.

This site contains a mix of world war I

and world war II munitions,

as well as a number of shipwrecks.

This is a preliminary magnetic anomaly map.

But now the real work begin is trying to separate the signal

and finding the best method for cleaning up

and monitoring the site.

Because in these examples,

mag alone will not get the job done.

And you’ll, they’ll definitely need to use mag

along with other geophysical methods to solve this problem.

<v Bart>In this section I talked about,</v>

we talked about two factors that can influence

the magnetic data that you measure.

One is the remanent magnetization

that can occur in ferromagnetic objects.

And what we saw is that the presence

orientation and strength of remanent magnetization

can have a very strong impact

on the anomaly amplitude in shape,

and that it can be quite common in ferrous materials.

And that’s sort of highlighted in these two images

on the left-hand side of this slide.

The other thing I discussed is the difference

between a total field and gradient measurement

of the magnetic field

and how they differ and their ability to locate

targets that are deeper

or farther away from the sensor versus shallower,

their noise levels and the impact

of low frequency wavelength, spatial wavelengths signals

on the data sets that you acquire.

<v Becky>So if there’s one key point</v>

I’d like you to take away from my examples,

it’s the presence of these monopoles.

We always assume that the UXO

is going to give us a nice clean dipole,

and as we could see, that’s not the case.

So, I mean, it was just a few minutes ago,

but can you even remember all these examples

that I showed you?

So the first one, it was the

155 inclined object at four meter depth.

The next one was the 45 degree at 4.5 meter depth.

This one was the 155 inclined at 85 degrees.

So that’s almost vertical in the middle.

And then this one was the negative 60 at four meters.

So quite at a, quite a depth.

And then that final one

was the same negative 60 at 4.5 meters.

So I think that was a really interesting

observation from those examples.

<v Gretchen>Thank you again for joining us today.</v>

If you have questions after this presentation,

please feel free to email us

at the address listed on the side.

Don’t forget we will be having future webinars

on magnetic data collection and magnetic data processing

later on this year.

We have not determined the dates for this yet,

but we are tentatively scheduling them

for November and December.

We will be adding this recording

to our Geometrics YouTube channel,

and it will also be available on both

the Seequent and Geometrics websites.

Thank you again for your time

and we hope our presentation was useful and informative.

We look forward to hearing from you.