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Это первый вебинар в серии из 3-х вебинаров по магниторазведке антропогенных объектов.

Теория, сбор данных и обработка данных. Вебинары проводятся совместно Seequent и Geometrics. Поиск искусственных объектов вблизи поверхности всегда был важной задачей геофизиков.

Объекты, представляющие интерес, включают неразорвавшиеся боеприпасы (НРБ), оставшиеся после учений и боевых действий, археологические предметы и старую инфраструктуру, такую как трубопроводы, кабели и подземные резервуары-хранилища в случаях, когда карты и чертежи отсутствуют или расходятся с реальным положением дел. Магнитная разведка всегда была одним из основных методов обнаружения этих объектов.

Этот вебинар посвящен практическим последствиям магнитной теории для проведения съемок, включая влияние следующих факторов:

  • Размер, форма и ориентация железных предметов
  • Остаточная намагниченность
  • Градиентометрия и измерение общей напряженности поля

Продолжительность

42 минуты

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Расшифровка видеозаписи

[00:00:00.730]<br />
<encoded_tag_open />v -<encoded_tag_closed />Hello, and thank you for joining us today.<encoded_tag_open />/v<encoded_tag_closed /><!— wpml:html_fragment </p> —>
<p>[00:00:03.740]<br />
My name is Gretchen Schmauder and I am a geoscientist.</p>
<p>[00:00:07.200]<br />
And the Director of Marketing for Geometrics.</p>
<p>[00:00:10.220]<br />
Today we also have Becky Bodger, who’s a geoscientist</p>
<p>[00:00:13.640]<br />
for Seequent.<br />
<encoded_tag_open />v -<encoded_tag_closed />Hello.<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:00:15.260]<br />
<encoded_tag_open />v -<encoded_tag_closed />and Bart Hoekstra, who’s a geophysicist<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:00:18.130]<br />
and the Vice President of sales for Geometrics.</p>
<p>[00:00:22.260]<br />
<encoded_tag_open />v -<encoded_tag_closed />Hello and thank you all for joining us today.<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:00:27.240]<br />
<encoded_tag_open />v -<encoded_tag_closed />Stefan Burns also contributed to today’s webinar.<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:00:31.400]<br />
He created the video you will be seeing here</p>
<p>[00:00:33.610]<br />
in a few moments.</p>
<p>[00:00:37.410]<br />
This is the first in a series of three webinars</p>
<p>[00:00:40.500]<br />
about magnetic geophysics.</p>
<p>[00:00:43.110]<br />
Today’s webinar covers magnetic theory.</p>
<p>[00:00:47.610]<br />
Future webinars are going to talk about</p>
<p>[00:00:51.054]<br />
magnetic data collection,</p>
<p>[00:00:53.350]<br />
and then the third will be magnetic data processing.</p>
<p>[00:01:00.090]<br />
We really want today’s webinar to be both informative</p>
<p>[00:01:04.030]<br />
and interactive.</p>
<p>[00:01:07.020]<br />
So to keep people engaged, we have included</p>
<p>[00:01:11.010]<br />
a series of questions that will pop up on the screen</p>
<p>[00:01:14.990]<br />
as we progress.</p>
<p>[00:01:17.150]<br />
We’d ask that you answer these questions</p>
<p>[00:01:19.900]<br />
either using the poll that will pop up on your screen</p>
<p>[00:01:23.800]<br />
or in the chat window as part of this webinar.</p>
<p>[00:01:28.800]<br />
Towards the end of today’s session</p>
<p>[00:01:31.580]<br />
we will talk about some of the answers</p>
<p>[00:01:34.540]<br />
and hopefully answer any questions you may have.</p>
<p>[00:01:40.910]<br />
Today we are focusing primarily on anthropogenic objects</p>
<p>[00:01:45.500]<br />
or those objects related to human beings</p>
<p>[00:01:48.280]<br />
and their interaction with the earth.</p>
<p>[00:01:51.600]<br />
The biggest driver for this</p>
<p>[00:01:53.200]<br />
is a search for unexploded ordinance</p>
<p>[00:01:55.690]<br />
also known as UXO in both the North Sea</p>
<p>[00:01:58.940]<br />
and in Southeast Asia.</p>
<p>[00:02:01.540]<br />
For example, the U.S. dropped over 2 million tons</p>
<p>[00:02:05.760]<br />
of explosives in Laos in the 1960s and 70s.</p>
<p>[00:02:10.400]<br />
The image you see on the left shows a field</p>
<p>[00:02:12.890]<br />
with bomb craters still present after nearly 50 years.</p>
<p>[00:02:18.070]<br />
On the right-hand side, you can see an unexploded bomb</p>
<p>[00:02:20.660]<br />
that is still posing significant risk to human inhabitants.</p>
<p>[00:02:26.000]<br />
In the North Sea world war II era bombs</p>
<p>[00:02:28.630]<br />
are still littering the sea floor.</p>
<p>[00:02:33.470]<br />
With the recent increase in wind farm development,</p>
<p>[00:02:37.470]<br />
efficient exploration for UXO is very important.</p>
<p>[00:02:42.220]<br />
Other areas driving the need for magnetometers</p>
<p>[00:02:44.830]<br />
include pipeline tracking and environmental concerns</p>
<p>[00:02:48.200]<br />
such as leaking underground storage tanks.</p>
<p>[00:02:51.300]<br />
You can see one of these here in the far left image.</p>
<p>[00:02:55.720]<br />
In the middle image, you can see an abandoned wellhead.</p>
<p>[00:03:00.740]<br />
These are frequently cut off below the ground surface,</p>
<p>[00:03:04.470]<br />
so they are not visible to the eye.</p>
<p>[00:03:10.290]<br />
Finally, in the right hand image,</p>
<p>[00:03:12.380]<br />
you can see a magnet, where a magnetometer was used</p>
<p>[00:03:15.450]<br />
to help characterize an archeological site.</p>
<p>[00:03:20.120]<br />
So here is our first question.</p>
<p>[00:03:23.220]<br />
Please write your answers in the chat window.</p>
<p>[00:03:27.170]<br />
What other anthropogenic objects or hazards can you think of</p>
<p>[00:03:31.670]<br />
that we can use magnetic surveys to identify?</p>
<p>[00:03:51.160]<br />
Our presentation begins with a brief explanation</p>
<p>[00:03:54.030]<br />
of the earth’s magnetic field.</p>
<p>[00:03:56.150]<br />
After this Bart and Becky will go into a deeper discussion</p>
<p>[00:04:00.010]<br />
of magnetics and magnetic anomalies.</p>
<p>[00:04:03.810]<br />
We are here to answer your questions.</p>
<p>[00:04:06.050]<br />
So please feel free to use the chat feature to contact us</p>
<p>[00:04:09.420]<br />
at any time during today’s presentation.</p>
<p>[00:04:12.520]<br />
You can also email us.</p>
<p>[00:04:14.677]<br />
So let’s get started.</p>
<p>[00:04:18.030]<br />
<encoded_tag_open />v Stefan<encoded_tag_closed />Electromagnetism is one<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:04:19.390]<br />
of four fundamental forces that govern our universe.</p>
<p>[00:04:22.780]<br />
Like two sides of the same coin</p>
<p>[00:04:24.640]<br />
electrical and magnetic fields can not exist</p>
<p>[00:04:27.060]<br />
without the other.</p>
<p>[00:04:28.690]<br />
Electromagnetic field is created when positive</p>
<p>[00:04:31.160]<br />
and negative particles interact</p>
<p>[00:04:32.980]<br />
like the nucleus and electrons of an atom.</p>
<p>[00:04:35.350]<br />
Magnetic fields hold electrons to their atoms</p>
<p>[00:04:38.050]<br />
resulting in molecular bonds,</p>
<p>[00:04:39.710]<br />
that hold compounds together and govern chemical reactions.</p>
<p>[00:04:43.330]<br />
At a macroscopic scale, some planets like earth</p>
<p>[00:04:46.260]<br />
and the gas giants of our solar system</p>
<p>[00:04:48.410]<br />
maintain massive magnetic fields of their own.</p>
<p>[00:04:51.590]<br />
It’s earth’s magnetic field that protects life</p>
<p>[00:04:54.150]<br />
from dangerous high energy solar wind emitted by the sun.</p>
<p>[00:04:57.990]<br />
Humans use magnetometry the study of magnetic fields</p>
<p>[00:05:01.640]<br />
to understand geology and define anthropogenic objects</p>
<p>[00:05:04.680]<br />
humans have left behind.</p>
<p>[00:05:06.910]<br />
Hello, my name is Stefan Burns</p>
<p>[00:05:08.730]<br />
and welcome to the magnetic surveying</p>
<p>[00:05:10.740]<br />
for detection of anthropogenic objects.</p>
<p>[00:05:15.210]<br />
If you’ve ever played with a magnet,</p>
<p>[00:05:16.730]<br />
you have experienced firsthand</p>
<p>[00:05:18.190]<br />
the unique physics of magnetic fields.</p>
<p>[00:05:20.930]<br />
Magnets have opposing north and south poles.</p>
<p>[00:05:23.740]<br />
As the saying goes, opposites attract.</p>
<p>[00:05:26.080]<br />
And in this case, the north and south poles of a magnet</p>
<p>[00:05:28.780]<br />
are attracted to each other.</p>
<p>[00:05:30.710]<br />
When to north or to south poles are placed near each other,</p>
<p>[00:05:34.000]<br />
repulsive force exists.</p>
<p>[00:05:35.800]<br />
The poles repel each other at the atomic level.</p>
<p>[00:05:38.980]<br />
If you could see magnetic fields,</p>
<p>[00:05:40.960]<br />
you would observe that magnets are surrounded in space</p>
<p>[00:05:43.550]<br />
by force fields originating at the positive</p>
<p>[00:05:46.410]<br />
south magnetic pole</p>
<p>[00:05:48.100]<br />
and ending at the negative north magnetic pole.</p>
<p>[00:05:51.530]<br />
If you could see an even greater detail,</p>
<p>[00:05:53.860]<br />
you would find that this field is created</p>
<p>[00:05:55.640]<br />
by the movement of electrons.</p>
<p>[00:05:57.890]<br />
Electrons moving from an area of negative charge</p>
<p>[00:06:00.470]<br />
to one of positive charge to find as electrical current</p>
<p>[00:06:04.160]<br />
is what gives magnets our ability to attract or repel.</p>
<p>[00:06:07.750]<br />
When an electric current is created</p>
<p>[00:06:09.470]<br />
and electrons are in flow a magnetic field is created.</p>
<p>[00:06:12.780]<br />
And this is the case because electricity and magnetism</p>
<p>[00:06:15.550]<br />
are linked at the quantum level.</p>
<p>[00:06:18.150]<br />
Modern magnetometers can sample the magnetic field</p>
<p>[00:06:20.700]<br />
hundreds or thousands of times per second.</p>
<p>[00:06:23.520]<br />
Before this, we were still aware of magnetic fields,</p>
<p>[00:06:26.410]<br />
thanks to simpler technologies, such as the compass.</p>
<p>[00:06:29.470]<br />
When suspended in water, the magnetised pin of a compass</p>
<p>[00:06:32.650]<br />
lines up with the magnetic field lines of our planet.</p>
<p>[00:06:35.490]<br />
Each end pointing to the earth</p>
<p>[00:06:36.930]<br />
oppositely charged magnetic pole.</p>
<p>[00:06:39.500]<br />
The earliest recorded use of a compass</p>
<p>[00:06:41.370]<br />
is dated to the second century BC</p>
<p>[00:06:43.830]<br />
when people use naturally occurring lodestone,</p>
<p>[00:06:46.450]<br />
otherwise known as magnetite to locate north.</p>
<p>[00:06:49.560]<br />
And in the 11th century,</p>
<p>[00:06:50.830]<br />
Chinese texts describe the needs for navigation</p>
<p>[00:06:53.490]<br />
amongst the ever-changing conditions at sea.</p>
<p>[00:06:56.650]<br />
The north arrow of a compass points to what we refer to</p>
<p>[00:06:59.640]<br />
as Earth’s magnetic north pole.</p>
<p>[00:07:01.620]<br />
However, you will recall that opposites attract.</p>
<p>[00:07:04.370]<br />
So what is really going on</p>
<p>[00:07:05.750]<br />
when a compass needle points north?</p>
<p>[00:07:07.980]<br />
It turns out that if you represented</p>
<p>[00:07:09.750]<br />
the earth’s magnetic field using a bar magnet,</p>
<p>[00:07:12.540]<br />
the bar magnet south pole</p>
<p>[00:07:14.170]<br />
would lay in the Northern hemisphere.</p>
<p>[00:07:16.750]<br />
A compass pointing towards the earth magnetic south pole</p>
<p>[00:07:19.790]<br />
ends up pointing towards geographic north</p>
<p>[00:07:22.210]<br />
since the geographic north pole and magnetic south pole</p>
<p>[00:07:25.260]<br />
lie in close proximity to each other.</p>
<p>[00:07:28.280]<br />
Across geologic time, the earth’s magnetic field</p>
<p>[00:07:30.870]<br />
has regularly gone through magnetic pole reversals.</p>
<p>[00:07:33.750]<br />
In the case of a pole reversal today,</p>
<p>[00:07:36.130]<br />
the poles were switched places</p>
<p>[00:07:37.780]<br />
and a compass’s north arrow</p>
<p>[00:07:39.360]<br />
would still point towards the magnetic south pole</p>
<p>[00:07:41.870]<br />
now located in the Southern hemisphere.</p>
<p>[00:07:44.530]<br />
And the geographic north pole</p>
<p>[00:07:45.830]<br />
would now be home to the magnetic north pole.</p>
<p>[00:07:49.410]<br />
On a human timescale, the earth’s magnetic field</p>
<p>[00:07:52.090]<br />
wobbles and shifts on a yearly basis</p>
<p>[00:07:54.630]<br />
and magnetic south is currently moving away from Canada,</p>
<p>[00:07:57.950]<br />
approximately 55 kilometers per year towards Siberia.</p>
<p>[00:08:02.330]<br />
Scientists first discovered magnetic pole reversals</p>
<p>[00:08:04.900]<br />
just over 100 years ago.</p>
<p>[00:08:06.630]<br />
And it wasn’t until the 1950s</p>
<p>[00:08:08.370]<br />
that extensive summary mapping projects</p>
<p>[00:08:10.550]<br />
showed how well the basaltic ocean floor</p>
<p>[00:08:12.690]<br />
records these magnetic reversals.</p>
<p>[00:08:15.120]<br />
As seafloor spreading centers deep underwater,</p>
<p>[00:08:17.700]<br />
magma spews out along geologic fissures</p>
<p>[00:08:20.140]<br />
and cools into a rock known as basalt.</p>
<p>[00:08:22.950]<br />
Basalt is faintly magnetic</p>
<p>[00:08:24.500]<br />
due to its mafic iron rich composition.</p>
<p>[00:08:27.150]<br />
But in a molten state,</p>
<p>[00:08:28.320]<br />
iron is not yet permanently magnetized.</p>
<p>[00:08:30.560]<br />
As this magma cools,</p>
<p>[00:08:31.940]<br />
minerals containing iron forms in a line</p>
<p>[00:08:34.140]<br />
to the earth’s magnetic field,</p>
<p>[00:08:35.660]<br />
just like tiny compass needles.</p>
<p>[00:08:37.900]<br />
This magnetization continues</p>
<p>[00:08:39.730]<br />
until the basalt passes through the Curie point</p>
<p>[00:08:42.240]<br />
or the temperature at which iron containing minerals</p>
<p>[00:08:44.660]<br />
become fully magnetic.</p>
<p>[00:08:46.460]<br />
At this point, they are magnetically frozen in space,</p>
<p>[00:08:49.600]<br />
and unless reheated will show the alignment</p>
<p>[00:08:51.780]<br />
of the earth’s magnetic field at the geologic time</p>
<p>[00:08:54.023]<br />
that Curie point was passed.</p>
<p>[00:08:56.190]<br />
Recognizing this phenomenon,</p>
<p>[00:08:57.690]<br />
geophysicists can analyze the magnetic signature of a rock</p>
<p>[00:09:01.090]<br />
along with radiometric age to chronicle the age</p>
<p>[00:09:04.050]<br />
and timing of earth’s magnetic cycles.</p>
<p>[00:09:06.810]<br />
The record shows that the earth’s magnetic field</p>
<p>[00:09:09.150]<br />
can flip quite rapidly and then remain stable</p>
<p>[00:09:11.760]<br />
for hundreds of thousands or millions of years</p>
<p>[00:09:14.290]<br />
before undergoing another magnetic pole reversal.</p>
<p>[00:09:17.810]<br />
Many man-made objects contain magnetic material,</p>
<p>[00:09:20.670]<br />
and ferromagnetism allows us to detect these objects</p>
<p>[00:09:23.670]<br />
with magnetometers.</p>
<p>[00:09:25.595]<br />
Ferromagnetism occurs when electrons spin</p>
<p>[00:09:27.730]<br />
in alignment with each other and iron cobalt and nickel</p>
<p>[00:09:31.240]<br />
are common ferromagnetic materials.</p>
<p>[00:09:33.760]<br />
The greater number of aligned spinning electrons</p>
<p>[00:09:36.210]<br />
a ferromagnetic material has, the strongest magnetic field.</p>
<p>[00:09:40.200]<br />
Since the advent of the iron age,</p>
<p>[00:09:41.870]<br />
many objects created by humans contain ferrous materials,</p>
<p>[00:09:44.860]<br />
and therefore can be detected</p>
<p>[00:09:46.340]<br />
by measuring the magnetic field near the earth surface.</p>
<p>[00:09:50.150]<br />
There are two primary methods</p>
<p>[00:09:51.520]<br />
for measuring the magnetic field</p>
<p>[00:09:53.150]<br />
that are commonly used for detecting man-made objects.</p>
<p>[00:09:56.410]<br />
Magnetic sensors that measure the magnitude</p>
<p>[00:09:58.660]<br />
of the magnetic field are referred to as atomic sensors,</p>
<p>[00:10:01.840]<br />
more commonly known as cesium,</p>
<p>[00:10:03.770]<br />
rubidium, proton, or Overhauser sensors.</p>
<p>[00:10:07.600]<br />
They each utilized slightly different physics</p>
<p>[00:10:09.630]<br />
to measure the magnetic field vary from each other</p>
<p>[00:10:12.120]<br />
in their accuracy, sensitivity, and sample rates.</p>
<p>[00:10:16.090]<br />
The local distortions of the earth’s magnetic field</p>
<p>[00:10:18.490]<br />
are observed by magnetometers as anomalous signatures,</p>
<p>[00:10:21.310]<br />
which can be used to precisely locate anthropogenic objects.</p>
<p>[00:10:25.290]<br />
Modern magnetic instrumentation can detect variations</p>
<p>[00:10:28.480]<br />
as small as one millionth</p>
<p>[00:10:29.930]<br />
of the value of the Earth’s magnetic field.</p>
<p>[00:10:32.237]<br />
And this increased sensitivity allows for the detection</p>
<p>[00:10:35.620]<br />
of smaller objects at greater depths.</p>
<p>[00:10:38.430]<br />
In addition to their incredible resolution,</p>
<p>[00:10:40.600]<br />
modern magnetometers also rapidly sample the magnetic field.</p>
<p>[00:10:44.000]<br />
And when many sensors are combined into arrays,</p>
<p>[00:10:46.820]<br />
large areas of both land and sea can be surveyed</p>
<p>[00:10:49.650]<br />
in great detail.</p>
<p>[00:10:51.140]<br />
The other primary method for measuring a magnetic field</p>
<p>[00:10:53.780]<br />
is to measure the change in the magnetic field</p>
<p>[00:10:55.990]<br />
over a distance in a particular orientation.</p>
<p>[00:10:59.340]<br />
These are known as gradient sensors</p>
<p>[00:11:01.240]<br />
with fluxgate magnetometers being the most common type</p>
<p>[00:11:03.980]<br />
of gradient sensor.</p>
<p>[00:11:05.890]<br />
In the remainder of this presentation,</p>
<p>[00:11:07.720]<br />
we will discuss the factors affecting the size</p>
<p>[00:11:09.980]<br />
and shape of magnetic signatures of ferromagnetic objects.</p>
<p>[00:11:15.160]<br />
<encoded_tag_open />v Bart<encoded_tag_closed />Hello, my name is Bart. Hoekstra<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:11:17.470]<br />
I’m vice president of geophysical sales at Geometrics</p>
<p>[00:11:21.440]<br />
and have had a long history of surveying</p>
<p>[00:11:23.820]<br />
for detection of metal objects primarily for UXO,</p>
<p>[00:11:27.960]<br />
but also other infrastructure related objects</p>
<p>[00:11:30.690]<br />
such as underground storage tanks and pipelines.</p>
<p>[00:11:35.640]<br />
What I will be talking about is some of the complexities</p>
<p>[00:11:38.510]<br />
that occur when we try and measure an anomaly</p>
<p>[00:11:42.430]<br />
or the signature of a man-made object.</p>
<p>[00:11:47.180]<br />
One of the things about magnetic surveys</p>
<p>[00:11:49.310]<br />
is that they’re quite easy to do.</p>
<p>[00:11:51.800]<br />
The sensors aren’t that large,</p>
<p>[00:11:53.840]<br />
and you can carry them or fly them around,</p>
<p>[00:11:56.456]<br />
download the data and create a map.</p>
<p>[00:11:59.940]<br />
But the simplicity of this survey</p>
<p>[00:12:02.160]<br />
and processing disguises the fact</p>
<p>[00:12:05.030]<br />
that sometimes what we measure</p>
<p>[00:12:06.790]<br />
is not as simple as we would like it to be.</p>
<p>[00:12:10.010]<br />
This can lead to misinterpretation of features</p>
<p>[00:12:12.980]<br />
and possibly not recognizing the presence of an object.</p>
<p>[00:12:17.330]<br />
One of these complexities arises from something called</p>
<p>[00:12:21.430]<br />
remanent magnetization.</p>
<p>[00:12:25.630]<br />
Stefan briefly discussed this in his intro,</p>
<p>[00:12:28.920]<br />
but what happens with ferromagnetic materials</p>
<p>[00:12:31.750]<br />
is that they have mineral grains.</p>
<p>[00:12:34.180]<br />
And within each one of these mineral grains,</p>
<p>[00:12:36.640]<br />
you can have a different alignment of the electrons</p>
<p>[00:12:39.300]<br />
which causes a different alignment</p>
<p>[00:12:41.100]<br />
of the south and north magnetic poles.</p>
<p>[00:12:44.950]<br />
When the ferromagnetic object that contains the grains</p>
<p>[00:12:48.160]<br />
is heated and then cooled below its Curie temperature,</p>
<p>[00:12:52.650]<br />
the magnetic grains will align</p>
<p>[00:12:54.870]<br />
with the earth’s magnetic field that occurs</p>
<p>[00:12:57.670]<br />
at the point in time and space and orientation of the object</p>
<p>[00:13:02.310]<br />
within the surrounding magnetic field.</p>
<p>[00:13:05.460]<br />
So the upper image shows the effects</p>
<p>[00:13:10.300]<br />
or what the remanent magnetization will be</p>
<p>[00:13:12.980]<br />
above the Curie temperature,</p>
<p>[00:13:14.650]<br />
but once it’s aligned and cooled below,</p>
<p>[00:13:18.690]<br />
the Curie temperature,</p>
<p>[00:13:19.640]<br />
you can see that all the magnetic domains within each grain</p>
<p>[00:13:23.220]<br />
are aligned with the external magnetic field.</p>
<p>[00:13:29.780]<br />
But what happens over time</p>
<p>[00:13:31.960]<br />
is that some of the remanent magnetization</p>
<p>[00:13:34.130]<br />
in these mineral grains becomes weaker</p>
<p>[00:13:36.460]<br />
and can go away altogether.</p>
<p>[00:13:39.300]<br />
And then you have a combination of remanent magnetization</p>
<p>[00:13:42.630]<br />
in some mineral grains and induced magnetization</p>
<p>[00:13:45.840]<br />
in other mineral grains.</p>
<p>[00:13:48.400]<br />
And the induced magnetization will line up</p>
<p>[00:13:50.620]<br />
with the external magnetic field</p>
<p>[00:13:52.720]<br />
that is present at the space and time</p>
<p>[00:13:55.130]<br />
in orientation where the object is now</p>
<p>[00:13:57.370]<br />
which may be different from the magnetic field where it was</p>
<p>[00:14:02.040]<br />
when it first cooled below the Curie temperature.</p>
<p>[00:14:06.230]<br />
And these two fields can be very different from each other</p>
<p>[00:14:09.880]<br />
in both magnitude and direction.</p>
<p>[00:14:12.675]<br />
And in some cases, the remanent magnetization,</p>
<p>[00:14:15.840]<br />
depending on the strength of it,</p>
<p>[00:14:17.930]<br />
can be up to five times the size</p>
<p>[00:14:20.900]<br />
of the induced magnetization,</p>
<p>[00:14:23.090]<br />
or it could be an almost negative negligible factor</p>
<p>[00:14:26.230]<br />
of the, compared to the induced magnetization.</p>
<p>[00:14:33.650]<br />
And so the result of this</p>
<p>[00:14:35.720]<br />
is that we have a vector sum,</p>
<p>[00:14:38.190]<br />
the field you measure will be a vector sum</p>
<p>[00:14:41.520]<br />
of the induced magnetization and the remanent magnetization,</p>
<p>[00:14:46.320]<br />
and often what is referred to as a Koenigsberger ratio</p>
<p>[00:14:51.050]<br />
is the ratio of the magnetic remanent field</p>
<p>[00:14:55.790]<br />
versus the induced field.</p>
<p>[00:14:58.630]<br />
So in the next series of slides,</p>
<p>[00:15:00.870]<br />
I’m going to be talking about</p>
<p>[00:15:02.840]<br />
and giving examples of the effects of remanent magnetization</p>
<p>[00:15:08.090]<br />
on a measured anomaly for a particular object.</p>
<p>[00:15:15.446]<br />
The particular object we are going to</p>
<p>[00:15:19.500]<br />
show the results, the model results of</p>
<p>[00:15:22.040]<br />
is an oblate spheroid.</p>
<p>[00:15:25.090]<br />
And this is kind of a UXO shaped object.</p>
<p>[00:15:28.750]<br />
It’s 10 centimeters in diameter, 50 centimeters in length.</p>
<p>[00:15:32.070]<br />
And it’s located at a depth of three meters</p>
<p>[00:15:35.570]<br />
below the plane of the model measurements.</p>
<p>[00:15:39.640]<br />
It’s inclined 60 degrees down</p>
<p>[00:15:42.800]<br />
and is oriented 60 degrees from north.</p>
<p>[00:15:47.940]<br />
The background field is approximately 50 nanoteslas,</p>
<p>[00:15:52.600]<br />
which is a close approximation</p>
<p>[00:15:55.060]<br />
of the earth’s magnetic field.</p>
<p>[00:15:56.190]<br />
And it’s at an inclination of 30 degrees positive,</p>
<p>[00:16:00.180]<br />
which is somewhat different from those that are</p>
<p>[00:16:02.920]<br />
used to seeing measurements in the Northern hemisphere,</p>
<p>[00:16:06.090]<br />
but you’ll still recognize the anomalies that are modeled.</p>
<p>[00:16:14.880]<br />
So now we’re going to show the modeled results</p>
<p>[00:16:17.330]<br />
from this target.</p>
<p>[00:16:18.270]<br />
And this first slide is,</p>
<p>[00:16:20.120]<br />
contains just the induced magnetic field.</p>
<p>[00:16:23.820]<br />
On the left-hand side we have the model data</p>
<p>[00:16:26.780]<br />
from the object as measured by a total field sensor.</p>
<p>[00:16:31.330]<br />
And on the right hand side,</p>
<p>[00:16:32.690]<br />
we have the model data as measured</p>
<p>[00:16:35.770]<br />
as would be measured from a gradient sensor</p>
<p>[00:16:39.510]<br />
with 50 centimeter with, on a gradient.</p>
<p>[00:16:45.180]<br />
This first slide shows just the induced field only.</p>
<p>[00:16:50.150]<br />
And as you can see, we have a large peak</p>
<p>[00:16:53.110]<br />
that’s to the Northeast of the</p>
<p>[00:16:58.420]<br />
target and a smaller trough to the south west.</p>
<p>[00:17:10.050]<br />
The next animation shows what happens</p>
<p>[00:17:13.450]<br />
when we have a remanent field that is the same amplitude</p>
<p>[00:17:17.940]<br />
as the induced magnetic field,</p>
<p>[00:17:20.940]<br />
and it’s oriented vertically downwards.</p>
<p>[00:17:22.830]<br />
As you can see,</p>
<p>[00:17:23.740]<br />
it changes the shape of the anomalies significantly,</p>
<p>[00:17:28.010]<br />
and we have a much larger trough</p>
<p>[00:17:30.490]<br />
now that’s still oriented towards the Southwest</p>
<p>[00:17:34.040]<br />
and it’s decreased the peak.</p>
<p>[00:17:38.520]<br />
All right, the third animation shows what happens</p>
<p>[00:17:41.880]<br />
when we have a remanent magnetization</p>
<p>[00:17:45.070]<br />
still the same amplitude, but now it’s oriented in the,</p>
<p>[00:17:50.120]<br />
towards the west horizontally.</p>
<p>[00:17:52.400]<br />
And as you can see it all,</p>
<p>[00:17:54.480]<br />
again, change the shape of the anomaly</p>
<p>[00:17:56.460]<br />
and now our smaller trough</p>
<p>[00:17:59.300]<br />
is oriented towards the Southeast.</p>
<p>[00:18:03.320]<br />
So this is quite a change from the original induced</p>
<p>[00:18:07.465]<br />
all the measurement that we have.</p>
<p>[00:18:11.060]<br />
The final animation in this slide</p>
<p>[00:18:13.500]<br />
is with the induced or the remanent field</p>
<p>[00:18:18.140]<br />
oriented towards the west.</p>
<p>[00:18:23.320]<br />
And as you can see this looks actually</p>
<p>[00:18:25.410]<br />
kind of similar to what we had with just the induced field,</p>
<p>[00:18:29.190]<br />
except now that the trough is oriented</p>
<p>[00:18:32.880]<br />
towards the Northwest.</p>
<p>[00:18:37.080]<br />
This just shows you some of the complexities</p>
<p>[00:18:39.440]<br />
that can arise when you have strong remanent magnetization,</p>
<p>[00:18:43.150]<br />
and it can distinctly change the shape of your anomaly</p>
<p>[00:18:46.360]<br />
and may cause you to misinterpret what you have.</p>
<p>[00:18:51.760]<br />
The next set of animations we’re going to model</p>
<p>[00:18:54.350]<br />
is using the same object</p>
<p>[00:18:56.760]<br />
and the same induced magnetization field,</p>
<p>[00:18:59.588]<br />
and use the same set of four parameters</p>
<p>[00:19:03.250]<br />
for the remanent magnetization.</p>
<p>[00:19:05.440]<br />
But what we’re doing, what we’ve done now</p>
<p>[00:19:07.910]<br />
is added noise that you’ll typically see</p>
<p>[00:19:10.470]<br />
from a total field survey and from a gradient survey</p>
<p>[00:19:14.600]<br />
so you can see how the remanent magnetization</p>
<p>[00:19:18.020]<br />
will change possibly the interpretation of the anomaly.</p>
<p>[00:19:22.320]<br />
The first animation is with the induced magnetic field only,</p>
<p>[00:19:27.710]<br />
and you can see the object quite well.</p>
<p>[00:19:31.640]<br />
The next animation has remanent magnetization</p>
<p>[00:19:35.780]<br />
that is oriented vertically downwards.</p>
<p>[00:19:40.440]<br />
The third animation has the remanent magnetization</p>
<p>[00:19:44.673]<br />
that is horizontal, but it’s in the Southern direction.</p>
<p>[00:19:48.933]<br />
And the final animation has the remanent magnetic field</p>
<p>[00:19:53.171]<br />
oriented horizontally but pointing to the west.</p>
<p>[00:19:57.840]<br />
And as you can see from our examples with noise,</p>
<p>[00:20:00.330]<br />
in some cases, the remanent magnetization</p>
<p>[00:20:03.200]<br />
can make an anomaly from a very metallic object</p>
<p>[00:20:06.640]<br />
look quite different than what you’re expecting</p>
<p>[00:20:08.970]<br />
from just an induced field anomaly.</p>
<p>[00:20:12.660]<br />
And in many cases that may lead you</p>
<p>[00:20:16.130]<br />
to not selecting the object as an object of interest</p>
<p>[00:20:21.660]<br />
or misinterpreting what that object is.</p>
<p>[00:20:26.184]<br />
We’re now going to discuss</p>
<p>[00:20:27.370]<br />
some of the different types of measurements</p>
<p>[00:20:29.340]<br />
that Stefan discussed in the introduction.</p>
<p>[00:20:33.640]<br />
We’re going to talk about two,</p>
<p>[00:20:35.250]<br />
basically two kinds of measurements.</p>
<p>[00:20:37.020]<br />
The first one is what we call a total field sensor,</p>
<p>[00:20:40.680]<br />
which measures just the magnitude of the field</p>
<p>[00:20:45.270]<br />
in, at a point in and time.</p>
<p>[00:20:49.010]<br />
These total field measurements</p>
<p>[00:20:50.470]<br />
are called total field sensors, scalar magnetometers,</p>
<p>[00:20:54.220]<br />
or atomic magnetometers.</p>
<p>[00:20:57.290]<br />
And then we’re also going to look at</p>
<p>[00:20:58.970]<br />
what are called gradient measurements.</p>
<p>[00:21:01.590]<br />
And what they look at is the difference</p>
<p>[00:21:03.800]<br />
in the magnetic field over a distance</p>
<p>[00:21:07.560]<br />
in a specific orientation.</p>
<p>[00:21:10.490]<br />
In most cases, gradient measurements are made with</p>
<p>[00:21:14.020]<br />
in either the vertical or a horizontal plane.</p>
<p>[00:21:18.640]<br />
With vertical gradient measurements,</p>
<p>[00:21:20.700]<br />
we measure the difference in the magnetic field</p>
<p>[00:21:23.560]<br />
from the top and the bottom of your sensor or sensor array.</p>
<p>[00:21:29.160]<br />
For horizontal gradient measurements,</p>
<p>[00:21:31.510]<br />
you’re looking at the difference across the width</p>
<p>[00:21:34.450]<br />
of your sensor or sensors array.</p>
<p>[00:21:38.300]<br />
Gradient measurements can be made with total field sensors</p>
<p>[00:21:41.930]<br />
that are separated</p>
<p>[00:21:43.480]<br />
by a specified distance, or they can be made</p>
<p>[00:21:46.060]<br />
with what are referred to as fluxgate sensors.</p>
<p>[00:21:51.200]<br />
On the left-hand side, we have a measurement</p>
<p>[00:21:53.280]<br />
that’s made by a fluxgate magnetometer</p>
<p>[00:21:55.720]<br />
in this case, the FM18.</p>
<p>[00:21:58.740]<br />
And this was done over the NSGG test site located in the UK.</p>
<p>[00:22:05.500]<br />
You can see various features on this map.</p>
<p>[00:22:08.280]<br />
And in particular,</p>
<p>[00:22:09.460]<br />
you can see clearly distinct anomalies</p>
<p>[00:22:12.370]<br />
to the, in the central Northern part of the map</p>
<p>[00:22:16.110]<br />
that were buried objects as tests for anomaly detection.</p>
<p>[00:22:21.720]<br />
And that’s shown on the left-hand side.</p>
<p>[00:22:25.260]<br />
On the right hand side,</p>
<p>[00:22:27.010]<br />
we have measurements that were made using</p>
<p>[00:22:30.140]<br />
two total field sensors separated by about a meter.</p>
<p>[00:22:34.860]<br />
And you can see the two maps are quite similar.</p>
<p>[00:22:37.660]<br />
There’s some differences.</p>
<p>[00:22:39.730]<br />
But in general, you can see the anomalies quite well.</p>
<p>[00:22:43.640]<br />
And it’s fairly easy to pick out our objects of interest.</p>
<p>[00:22:49.800]<br />
In the next slide, what we’re showing</p>
<p>[00:22:51.730]<br />
is the total field measurement made by a single sensor</p>
<p>[00:22:56.020]<br />
from the array of G858.</p>
<p>[00:22:58.530]<br />
And you can see this map looks different</p>
<p>[00:23:01.320]<br />
than the other sensors or other maps that we were showing.</p>
<p>[00:23:05.900]<br />
You can clearly see some of the large anomalies</p>
<p>[00:23:09.540]<br />
in the Northern half of this map,</p>
<p>[00:23:13.200]<br />
which were the buried objects.</p>
<p>[00:23:15.200]<br />
And you can see some other anomalies outside that region,</p>
<p>[00:23:18.900]<br />
but they tend to be a little bit more subtler.</p>
<p>[00:23:21.620]<br />
And that’s likely due to the fact</p>
<p>[00:23:23.540]<br />
that they were smaller objects nearer to the center</p>
<p>[00:23:27.880]<br />
or near to the surface.</p>
<p>[00:23:29.870]<br />
And in general these are not picked up as easily as</p>
<p>[00:23:35.620]<br />
the gradient measurements too.</p>
<p>[00:23:37.620]<br />
But, this map I would say tends to be a little bit</p>
<p>[00:23:42.130]<br />
less noisy.</p>
<p>[00:23:44.420]<br />
Where there are no objects</p>
<p>[00:23:46.070]<br />
you don’t see much magnetic field variation.</p>
<p>[00:23:49.380]<br />
The other thing you’ll note about this odd map is that</p>
<p>[00:23:53.010]<br />
there is a gradient, a long wavelength gradient</p>
<p>[00:23:59.748]<br />
in the data with the magnetic field decreasing</p>
<p>[00:24:02.200]<br />
to the south of this area</p>
<p>[00:24:05.260]<br />
that you did not see in a gradient field measurement.</p>
<p>[00:24:08.380]<br />
And this is because when you take two sensors</p>
<p>[00:24:12.380]<br />
or you’re just measuring the difference between two points,</p>
<p>[00:24:17.482]<br />
they will see the same long wavelength gradient</p>
<p>[00:24:21.090]<br />
and when you subtract out the field from the top and bottom,</p>
<p>[00:24:26.500]<br />
then there is no long-term wavelength difference</p>
<p>[00:24:31.020]<br />
in the measurement.</p>
<p>[00:24:32.750]<br />
So I just wanted to highlight that and I’ll just go in,</p>
<p>[00:24:37.300]<br />
in the next slide.</p>
<p>[00:24:39.100]<br />
So in this slide, I’ll discuss some of the reasons why</p>
<p>[00:24:42.760]<br />
there is a difference between measuring the magnetic field</p>
<p>[00:24:46.480]<br />
with a total field sensor or a scaler sensor</p>
<p>[00:24:49.920]<br />
versus a gradient sensor.</p>
<p>[00:24:54.570]<br />
The total field signal decreases in amplitude</p>
<p>[00:24:59.830]<br />
by a factor of one over R to the third with distance.</p>
<p>[00:25:05.890]<br />
The gradient signal decreases</p>
<p>[00:25:08.210]<br />
by a factor of one over R to the fourth.</p>
<p>[00:25:11.210]<br />
On the chart, on the right hand side,</p>
<p>[00:25:14.580]<br />
the decrease in amplitude is,</p>
<p>[00:25:18.440]<br />
with distance is represented by this blue curve</p>
<p>[00:25:22.780]<br />
for a total field measurement</p>
<p>[00:25:24.520]<br />
and a decrease in amplitude of the measurement</p>
<p>[00:25:27.580]<br />
for a gradient signal is shown in the red curve.</p>
<p>[00:25:33.790]<br />
And this distance and amplitude are normalized</p>
<p>[00:25:37.140]<br />
by the amplitude of the anomaly</p>
<p>[00:25:39.900]<br />
and also the length of the dipole.</p>
<p>[00:25:43.690]<br />
But as you can see at a factor,</p>
<p>[00:25:46.170]<br />
a distance of eight compared to the length of the dipole,</p>
<p>[00:25:51.160]<br />
the total field measurement in blue</p>
<p>[00:25:54.010]<br />
is actually an order of magnitude almost greater</p>
<p>[00:25:58.450]<br />
than the amplitude of the gradient measurement.</p>
<p>[00:26:05.750]<br />
And so this has a pretty big impact on your ability</p>
<p>[00:26:10.760]<br />
to detect objects at a greater range from your sensor.</p>
<p>[00:26:16.240]<br />
So in general, a total field measurement</p>
<p>[00:26:19.130]<br />
is more effective at detecting objects at greater distances.</p>
<p>[00:26:24.710]<br />
But, there are some advantages</p>
<p>[00:26:27.610]<br />
to doing gradient measurements.</p>
<p>[00:26:29.560]<br />
They tend to filter out low frequency trends</p>
<p>[00:26:31.910]<br />
as you saw in your previous map.</p>
<p>[00:26:34.530]<br />
The anomaly that you see from a gradient measurement</p>
<p>[00:26:37.810]<br />
is sometimes a little bit better defined.</p>
<p>[00:26:40.980]<br />
It’s sharper and narrower in width</p>
<p>[00:26:43.570]<br />
which makes it easier to determine the location</p>
<p>[00:26:47.560]<br />
of an object.</p>
<p>[00:26:50.700]<br />
And if you’re interested in shallow objects,</p>
<p>[00:26:54.860]<br />
it can also enhance the location and detection ability</p>
<p>[00:26:58.220]<br />
of those shallow objects.</p>
<p>[00:27:00.750]<br />
One other factor to consider</p>
<p>[00:27:02.280]<br />
when you look at gradient measurements</p>
<p>[00:27:04.150]<br />
is that they tend to be noisier.</p>
<p>[00:27:07.537]<br />
That’s because we’re actually taking the derivative</p>
<p>[00:27:10.160]<br />
of the magnetic field in that particular orientation,</p>
<p>[00:27:14.390]<br />
either vertically or horizontally</p>
<p>[00:27:16.670]<br />
and in all cases derivative measurements</p>
<p>[00:27:20.020]<br />
tend to act as a high pass filter.</p>
<p>[00:27:22.510]<br />
And noise in most magnetic surveys is,</p>
<p>[00:27:26.950]<br />
increases with frequency</p>
<p>[00:27:29.411]<br />
and so you will tend to increase noise</p>
<p>[00:27:34.160]<br />
in your measured signals.</p>
<p>[00:27:36.930]<br />
So, as a question I’d like to ask people,</p>
<p>[00:27:40.230]<br />
what kind of measurement the magnetic field measurements</p>
<p>[00:27:43.620]<br />
they have made in the past or plan to in the future.</p>
<p>[00:27:48.050]<br />
Do you use a total field sensor or atomic sensor,</p>
<p>[00:27:53.535]<br />
a gradient sensor such as a fluxgate</p>
<p>[00:27:55.760]<br />
or use a gradient array of total field sensors?</p>
<p>[00:28:00.420]<br />
Have you done both kinds of surveys</p>
<p>[00:28:02.280]<br />
or are you brand new to magnetic surveying</p>
<p>[00:28:05.170]<br />
and haven’t done anything?</p>
<p>[00:28:06.790]<br />
So please answer the poll in the chat window</p>
<p>[00:28:10.150]<br />
and we can discuss the results later on.</p>
<p>[00:28:32.960]<br />
Well, that concludes my section of the talk.</p>
<p>[00:28:35.560]<br />
And now I’m going to hand off the presentation</p>
<p>[00:28:37.880]<br />
to Becky Bodger from Seequent.</p>
<p>[00:28:40.870]<br />
I will be back to discuss the conclusions of this webinar,</p>
<p>[00:28:45.040]<br />
and also to answer any questions that you may have.</p>
<p>[00:28:49.837]<br />
<encoded_tag_open />v Becky<encoded_tag_closed />Thanks, Bart.<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:28:51.110]<br />
As mentioned, my name is Becky Bodger</p>
<p>[00:28:52.730]<br />
and I’m a geoscientist at Seequent.</p>
<p>[00:28:55.960]<br />
In this next section</p>
<p>[00:28:56.793]<br />
I’m going to show you a series of examples</p>
<p>[00:28:58.690]<br />
that were created using the forward modeling capabilities</p>
<p>[00:29:01.660]<br />
available in Oasis montaj with the UXO extension.</p>
<p>[00:29:05.330]<br />
The grid on the left is the background that I used</p>
<p>[00:29:07.660]<br />
for the forward modeling.</p>
<p>[00:29:08.910]<br />
It’s from an actual UXO survey in the North Sea,</p>
<p>[00:29:12.220]<br />
and is representative of the type and level of noise</p>
<p>[00:29:15.050]<br />
you would get on a real survey.</p>
<p>[00:29:17.080]<br />
In most of the examples,</p>
<p>[00:29:18.370]<br />
I’m using a 155 millimeter projectile</p>
<p>[00:29:21.700]<br />
unless otherwise stated.</p>
<p>[00:29:23.480]<br />
And there’s an image on the right</p>
<p>[00:29:25.840]<br />
that shows you what that looks like.</p>
<p>[00:29:31.880]<br />
So whenever you are working with any potential field data,</p>
<p>[00:29:34.880]<br />
for example, gravity or magnetics,</p>
<p>[00:29:37.000]<br />
we need to remember that mathematically</p>
<p>[00:29:38.840]<br />
magnetic anomalies are non-unique.</p>
<p>[00:29:40.940]<br />
Multiple theoretical solutions are possible.</p>
<p>[00:29:43.890]<br />
This is true whether we’re talking about geological features</p>
<p>[00:29:47.430]<br />
or anthropogenic objects.</p>
<p>[00:29:50.480]<br />
I found this image in a paper online</p>
<p>[00:29:52.400]<br />
that talks about the ambiguity in potential field modeling.</p>
<p>[00:29:55.800]<br />
And I like it because it demonstrates</p>
<p>[00:29:57.390]<br />
that the same Mickey mouse shaped deposit</p>
<p>[00:29:59.610]<br />
can have three different geophysical signatures.</p>
<p>[00:30:02.390]<br />
Why do we continue to use magnetics though,</p>
<p>[00:30:04.450]<br />
or any potential field data?</p>
<p>[00:30:06.190]<br />
Because there are ways to minimize this issue.</p>
<p>[00:30:10.760]<br />
So using a priori information in most cases</p>
<p>[00:30:13.660]<br />
especially when we were talking about anthropogenic objects,</p>
<p>[00:30:16.480]<br />
there’s history somewhere.</p>
<p>[00:30:18.720]<br />
If it’s UXO, we can research</p>
<p>[00:30:20.610]<br />
which munitions were dropped by either side</p>
<p>[00:30:23.130]<br />
during the various wars or conflicts.</p>
<p>[00:30:25.330]<br />
For archeology, hopefully you know some of the history</p>
<p>[00:30:28.100]<br />
of the area and what you are looking for,</p>
<p>[00:30:30.060]<br />
whether it’s an old burial site or building foundations.</p>
<p>[00:30:33.440]<br />
And for geotechnical investigations,</p>
<p>[00:30:35.660]<br />
there are often existing infrastructure maps</p>
<p>[00:30:37.980]<br />
showing the locations of buried cables and pipelines.</p>
<p>[00:30:41.290]<br />
This information is not always available,</p>
<p>[00:30:43.470]<br />
or it can be difficult to unearth,</p>
<p>[00:30:45.320]<br />
but it’s worth checking what’s available</p>
<p>[00:30:47.190]<br />
to aiding your processing and interpretation of the data</p>
<p>[00:30:50.050]<br />
before you start.</p>
<p>[00:30:53.780]<br />
Cross-referencing multiple types of geophysical data.</p>
<p>[00:30:57.450]<br />
So in the case of offshore geophysical surveys,</p>
<p>[00:31:00.500]<br />
mag is only part of the story.</p>
<p>[00:31:02.410]<br />
Oftentimes surveyors will simultaneously</p>
<p>[00:31:05.400]<br />
be collecting multi-team data, side scan, sonar,</p>
<p>[00:31:08.650]<br />
seismic, sub-bottom profile data, or even seabed images.</p>
<p>[00:31:13.110]<br />
Using the results of all of this data</p>
<p>[00:31:15.260]<br />
will really help with processing</p>
<p>[00:31:17.470]<br />
and the interpretation of your results.</p>
<p>[00:31:20.240]<br />
And in archeological studies, for example,</p>
<p>[00:31:22.760]<br />
you might have gravity and resistivity as well.</p>
<p>[00:31:28.560]<br />
Finally, use your common sense and be smart.</p>
<p>[00:31:31.770]<br />
You’re going to use common sense, your education,</p>
<p>[00:31:34.320]<br />
your past experience, all of that,</p>
<p>[00:31:36.320]<br />
to apply logic and find the best interpretation.</p>
<p>[00:31:40.760]<br />
Here, I attempt to demonstrate</p>
<p>[00:31:42.390]<br />
the non uniqueness of this mag anomaly.</p>
<p>[00:31:44.710]<br />
While they are not identical,</p>
<p>[00:31:46.620]<br />
I hope that we can agree that they are similar enough</p>
<p>[00:31:48.840]<br />
to demonstrate the point.</p>
<p>[00:31:50.780]<br />
So on the left we have two UXOs.</p>
<p>[00:31:52.890]<br />
One is an 81 millimeter projectile</p>
<p>[00:31:54.970]<br />
at 2.5 meters below the sensor.</p>
<p>[00:31:57.400]<br />
And one is a two and three quarter inch rocket</p>
<p>[00:31:59.490]<br />
at two meters below the sensor.</p>
<p>[00:32:01.440]<br />
On the right, we have a single UXO,</p>
<p>[00:32:04.200]<br />
which is the 155 millimeter that we’ve been looking at</p>
<p>[00:32:07.560]<br />
at 3.5 meters.</p>
<p>[00:32:09.590]<br />
And you can see that on the left-hand side,</p>
<p>[00:32:13.929]<br />
you know, the inflection point</p>
<p>[00:32:15.550]<br />
between the dipole’s a little bit slanted.</p>
<p>[00:32:17.610]<br />
It’s a little smaller than the one on the right.</p>
<p>[00:32:20.230]<br />
If we look at the profile below the grid,</p>
<p>[00:32:22.320]<br />
we can see that in profile they’re even harder</p>
<p>[00:32:24.890]<br />
to distinguish the differences.</p>
<p>[00:32:26.780]<br />
They both have similar positive and negative peaks.</p>
<p>[00:32:30.730]<br />
And again, the real only difference that we see here</p>
<p>[00:32:33.250]<br />
is the width of the actual dipole.</p>
<p>[00:32:38.360]<br />
This example also demonstrates the importance</p>
<p>[00:32:40.710]<br />
of griding your data and not only interpreting the results</p>
<p>[00:32:44.650]<br />
in, along the profile.</p>
<p>[00:32:46.490]<br />
It’s important to visualize it in 2D space</p>
<p>[00:32:48.770]<br />
to really see the full picture.</p>
<p>[00:32:51.120]<br />
So here’s another question for you guys.</p>
<p>[00:32:53.521]<br />
Which inclination of an object</p>
<p>[00:32:55.990]<br />
produces the strongest amplitude</p>
<p>[00:32:58.020]<br />
whether it’s peak to peak of the dipole</p>
<p>[00:33:00.090]<br />
or a single peak amplitude?</p>
<p>[00:33:02.290]<br />
Do you think it’s A, vertical, B horizontal,</p>
<p>[00:33:04.990]<br />
or C inclined at 45 degrees?</p>
<p>[00:33:09.130]<br />
So I’ll give you a few seconds</p>
<p>[00:33:10.150]<br />
just to putting your answers and then we’ll carry on.</p>
<p>[00:33:26.060]<br />
In this example, I’m going to show you the response</p>
<p>[00:33:28.220]<br />
of the object at different inclinations.</p>
<p>[00:33:32.570]<br />
So on the right,</p>
<p>[00:33:34.210]<br />
the image is just to represent the orientation</p>
<p>[00:33:36.920]<br />
we didn’t actually model the UXO that you’re seeing.</p>
<p>[00:33:40.890]<br />
So you can see that when it’s horizontal,</p>
<p>[00:33:42.640]<br />
it’s a nice perfect dipole.</p>
<p>[00:33:45.350]<br />
When you add inclination,</p>
<p>[00:33:46.820]<br />
so on the example that we modeled here</p>
<p>[00:33:48.780]<br />
was inclined at 45 degrees.</p>
<p>[00:33:50.800]<br />
You can see that the negative trough</p>
<p>[00:33:52.910]<br />
becomes a lot less negative.</p>
<p>[00:33:55.630]<br />
And when it’s perfectly vertical,</p>
<p>[00:33:58.630]<br />
you can see that the negative disappears altogether</p>
<p>[00:34:01.850]<br />
and you’re left with a monopole.</p>
<p>[00:34:04.770]<br />
And again, just remember,</p>
<p>[00:34:05.890]<br />
we’re not modeling the size of the UXO in the image,</p>
<p>[00:34:08.550]<br />
it’s just there to show you that it’s vertical is possible</p>
<p>[00:34:11.780]<br />
especially in the marine environment.</p>
<p>[00:34:14.560]<br />
So what’s the answer to the question that I asked?</p>
<p>[00:34:17.500]<br />
Here we have the three responses along a profile</p>
<p>[00:34:20.140]<br />
through the center of the dipole.</p>
<p>[00:34:22.370]<br />
The first is the horizontal,</p>
<p>[00:34:24.240]<br />
and we can see that the peak to peak amplitude</p>
<p>[00:34:27.750]<br />
is 5.4 nanoteslas.</p>
<p>[00:34:31.410]<br />
The second one in the middle here,</p>
<p>[00:34:32.730]<br />
is the object inclined at 45 degrees</p>
<p>[00:34:35.830]<br />
and it has a peak to peak value of six nanoteslas.</p>
<p>[00:34:39.600]<br />
And the vertical object has a positive monopole</p>
<p>[00:34:46.140]<br />
total peak value of 6.4 nanoteslas.</p>
<p>[00:34:50.600]<br />
So the answer to that last question was C for vertical.</p>
<p>[00:34:57.420]<br />
Next, we’re going to look at how the response changes</p>
<p>[00:35:00.170]<br />
as the object changes direction or destination.</p>
<p>[00:35:03.560]<br />
The top row is the horizontal 155 millimeter projectile</p>
<p>[00:35:07.721]<br />
at four meters below the sensor.</p>
<p>[00:35:10.640]<br />
And the bottom row is the inclined object at 45 degrees.</p>
<p>[00:35:15.600]<br />
Note how the negative part of the dipole</p>
<p>[00:35:17.490]<br />
decreases more rapidly as we rotate the inclined object.</p>
<p>[00:35:22.430]<br />
The first row is the near vertical object</p>
<p>[00:35:25.090]<br />
inclined at 85 degrees.</p>
<p>[00:35:26.930]<br />
And the second row is the same object</p>
<p>[00:35:29.960]<br />
inclined at negative 60 degrees</p>
<p>[00:35:33.580]<br />
which implies the opposite polarity.</p>
<p>[00:35:36.240]<br />
So, instead of the north pole up in the air,</p>
<p>[00:35:40.140]<br />
in this example, the south pole is up in the air.</p>
<p>[00:35:45.938]<br />
An interesting effect in the negative 60 degree example</p>
<p>[00:35:50.040]<br />
is also the halo that we see around the more obvious dipole.</p>
<p>[00:35:54.120]<br />
This is important</p>
<p>[00:35:55.130]<br />
when trying to model the depth of the object,</p>
<p>[00:35:57.783]<br />
whether you are using Euler deconvolution</p>
<p>[00:36:00.770]<br />
or an inversion style modeling method.</p>
<p>[00:36:03.540]<br />
Most methods require you to define the modeling window</p>
<p>[00:36:06.810]<br />
in order to select which data to invert.</p>
<p>[00:36:09.390]<br />
Since all of these examples use the exact same background</p>
<p>[00:36:12.600]<br />
which you can see in the top right-hand corner here,</p>
<p>[00:36:15.270]<br />
any differences in color that we see</p>
<p>[00:36:17.840]<br />
is part of the signal from the object.</p>
<p>[00:36:20.220]<br />
So for accurate modeling, we would ideally want to include</p>
<p>[00:36:23.060]<br />
as much of that signal as possible,</p>
<p>[00:36:25.130]<br />
which means it would require a much larger modeling window.</p>
<p>[00:36:30.400]<br />
Let’s also consider the effect of depth below the sensor</p>
<p>[00:36:34.090]<br />
on the inclined examples.</p>
<p>[00:36:35.840]<br />
So in the top row, we have the eastward facing</p>
<p>[00:36:38.560]<br />
155 millimeter projectile inclined at negative 60.</p>
<p>[00:36:42.750]<br />
And on the bottom, we have the same size projectile,</p>
<p>[00:36:45.020]<br />
but inclined at 45 degrees.</p>
<p>[00:36:47.170]<br />
And we can see the different response we get</p>
<p>[00:36:49.580]<br />
at 1.5 meters below the sensor, 2.5, 3.5, 4.5.</p>
<p>[00:36:55.780]<br />
And we can see that, we can actually see</p>
<p>[00:36:58.200]<br />
that none of these examples really produce</p>
<p>[00:37:00.010]<br />
that perfect looking dipole.</p>
<p>[00:37:01.920]<br />
And in fact, as the distance between the sensor</p>
<p>[00:37:04.400]<br />
and the object increases depending on the example,</p>
<p>[00:37:07.680]<br />
the positive in the top example</p>
<p>[00:37:09.420]<br />
and the negative in the bottom example,</p>
<p>[00:37:11.450]<br />
disappears almost entirely</p>
<p>[00:37:13.100]<br />
and leaves us with another parent monopole.</p>
<p>[00:37:17.500]<br />
So the last example or complexity</p>
<p>[00:37:19.710]<br />
I wanted to talk about today</p>
<p>[00:37:21.450]<br />
is simply the difficulty that arises</p>
<p>[00:37:23.370]<br />
when you have multiple objects stacked on top</p>
<p>[00:37:26.260]<br />
or close to one another.</p>
<p>[00:37:27.660]<br />
And this is a common phenomenon in test ranges</p>
<p>[00:37:30.770]<br />
or munition dumpsites.</p>
<p>[00:37:32.460]<br />
So on the left, I’ve modeled two UXOs.</p>
<p>[00:37:35.060]<br />
One is a 105 millimeter projectile</p>
<p>[00:37:37.540]<br />
at 3.5 meters below the sensor.</p>
<p>[00:37:40.650]<br />
Southward facing.</p>
<p>[00:37:42.210]<br />
And one meter away is a second UXO,</p>
<p>[00:37:45.133]<br />
a 60 millimeter projectile at two meters below the sensor</p>
<p>[00:37:48.180]<br />
and westward facing.</p>
<p>[00:37:50.360]<br />
It creates what I think would be called a complex dipole.</p>
<p>[00:37:54.310]<br />
On the right-hand side, is another example.</p>
<p>[00:37:56.510]<br />
A 155 millimeter at four meters</p>
<p>[00:37:59.080]<br />
and a 105 millimeters at two meter depth below sensor.</p>
<p>[00:38:03.170]<br />
Both roughly southward facing.</p>
<p>[00:38:05.200]<br />
With careful analysis, the example on the right</p>
<p>[00:38:08.100]<br />
could probably be accurately classified</p>
<p>[00:38:10.300]<br />
as two separate targets,</p>
<p>[00:38:12.340]<br />
but the example on the left would be a lot,</p>
<p>[00:38:15.300]<br />
would be much more difficult to separate.</p>
<p>[00:38:19.000]<br />
And I just wanted to give you a few,</p>
<p>[00:38:20.420]<br />
a couple of examples where we see this in real life</p>
<p>[00:38:23.330]<br />
and which caused a lot of problems.</p>
<p>[00:38:25.270]<br />
So in this example, this is a map</p>
<p>[00:38:27.450]<br />
of Lac Saint-Pierre in Canada.</p>
<p>[00:38:30.730]<br />
And I just wanted to show you the complexity</p>
<p>[00:38:32.470]<br />
and the problem that they’re dealing with here.</p>
<p>[00:38:34.950]<br />
And as an example, one of these large blue circles is,</p>
<p>[00:38:39.950]<br />
there are over 200 UXOs in that tiny little space.</p>
<p>[00:38:44.720]<br />
Another example in Europe is,</p>
<p>[00:38:47.820]<br />
this is the port in marked munition dump</p>
<p>[00:38:49.850]<br />
off the coast of Zeebrugge Harbor.</p>
<p>[00:38:52.190]<br />
This site contains a mix of world war I</p>
<p>[00:38:54.520]<br />
and world war II munitions,</p>
<p>[00:38:55.940]<br />
as well as a number of shipwrecks.</p>
<p>[00:38:58.050]<br />
This is a preliminary magnetic anomaly map.</p>
<p>[00:39:00.790]<br />
But now the real work begin is trying to separate the signal</p>
<p>[00:39:04.130]<br />
and finding the best method for cleaning up</p>
<p>[00:39:06.290]<br />
and monitoring the site.</p>
<p>[00:39:07.890]<br />
Because in these examples,</p>
<p>[00:39:09.680]<br />
mag alone will not get the job done.</p>
<p>[00:39:11.550]<br />
And you’ll, they’ll definitely need to use mag</p>
<p>[00:39:13.960]<br />
along with other geophysical methods to solve this problem.</p>
<p>[00:39:22.090]<br />
<encoded_tag_open />v Bart<encoded_tag_closed />In this section I talked about,<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:39:23.420]<br />
we talked about two factors that can influence</p>
<p>[00:39:26.063]<br />
the magnetic data that you measure.</p>
<p>[00:39:28.730]<br />
One is the remanent magnetization</p>
<p>[00:39:30.580]<br />
that can occur in ferromagnetic objects.</p>
<p>[00:39:33.320]<br />
And what we saw is that the presence</p>
<p>[00:39:35.750]<br />
orientation and strength of remanent magnetization</p>
<p>[00:39:39.838]<br />
can have a very strong impact</p>
<p>[00:39:41.970]<br />
on the anomaly amplitude in shape,</p>
<p>[00:39:44.800]<br />
and that it can be quite common in ferrous materials.</p>
<p>[00:39:49.200]<br />
And that’s sort of highlighted in these two images</p>
<p>[00:39:52.610]<br />
on the left-hand side of this slide.</p>
<p>[00:39:55.440]<br />
The other thing I discussed is the difference</p>
<p>[00:39:57.900]<br />
between a total field and gradient measurement</p>
<p>[00:40:01.130]<br />
of the magnetic field</p>
<p>[00:40:02.970]<br />
and how they differ and their ability to locate</p>
<p>[00:40:06.960]<br />
targets that are deeper</p>
<p>[00:40:08.210]<br />
or farther away from the sensor versus shallower,</p>
<p>[00:40:11.600]<br />
their noise levels and the impact</p>
<p>[00:40:14.950]<br />
of low frequency wavelength, spatial wavelengths signals</p>
<p>[00:40:19.340]<br />
on the data sets that you acquire.</p>
<p>[00:40:22.800]<br />
<encoded_tag_open />v Becky<encoded_tag_closed />So if there’s one key point<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:40:25.098]<br />
I’d like you to take away from my examples,</p>
<p>[00:40:26.660]<br />
it’s the presence of these monopoles.</p>
<p>[00:40:29.240]<br />
We always assume that the UXO</p>
<p>[00:40:31.300]<br />
is going to give us a nice clean dipole,</p>
<p>[00:40:33.330]<br />
and as we could see, that’s not the case.</p>
<p>[00:40:36.090]<br />
So, I mean, it was just a few minutes ago,</p>
<p>[00:40:38.130]<br />
but can you even remember all these examples</p>
<p>[00:40:40.660]<br />
that I showed you?</p>
<p>[00:40:41.810]<br />
So the first one, it was the</p>
<p>[00:40:43.680]<br />
155 inclined object at four meter depth.</p>
<p>[00:40:48.320]<br />
The next one was the 45 degree at 4.5 meter depth.</p>
<p>[00:40:54.930]<br />
This one was the 155 inclined at 85 degrees.</p>
<p>[00:41:01.710]<br />
So that’s almost vertical in the middle.</p>
<p>[00:41:03.720]<br />
And then this one was the negative 60 at four meters.</p>
<p>[00:41:07.630]<br />
So quite at a, quite a depth.</p>
<p>[00:41:10.220]<br />
And then that final one</p>
<p>[00:41:13.230]<br />
was the same negative 60 at 4.5 meters.</p>
<p>[00:41:18.600]<br />
So I think that was a really interesting</p>
<p>[00:41:21.155]<br />
observation from those examples.</p>
<p>[00:41:26.070]<br />
<encoded_tag_open />v Gretchen<encoded_tag_closed />Thank you again for joining us today.<encoded_tag_open />/v<encoded_tag_closed /></p>
<p>[00:41:28.710]<br />
If you have questions after this presentation,</p>
<p>[00:41:31.044]<br />
please feel free to email us</p>
<p>[00:41:32.930]<br />
at the address listed on the side.</p>
<p>[00:41:35.530]<br />
Don’t forget we will be having future webinars</p>
<p>[00:41:37.950]<br />
on magnetic data collection and magnetic data processing</p>
<p>[00:41:41.820]<br />
later on this year.</p>
<p>[00:41:43.660]<br />
We have not determined the dates for this yet,</p>
<p>[00:41:46.062]<br />
but we are tentatively scheduling them</p>
<p>[00:41:48.350]<br />
for November and December.</p>
<p>[00:41:51.150]<br />
We will be adding this recording</p>
<p>[00:41:52.760]<br />
to our Geometrics YouTube channel,</p>
<p>[00:41:54.700]<br />
and it will also be available on both</p>
<p>[00:41:56.800]<br />
the Seequent and Geometrics websites.</p>
<p>[00:42:00.370]<br />
Thank you again for your time</p>
<p>[00:42:01.660]<br />
and we hope our presentation was useful and informative.</p>
<wpml_invalid_tag original=»PHA+» />[00:42:05.170]<br />
We look forward to hearing from you.

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