Seeing Underground

8/18/2019 Seeing Underground

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Geophysics foundations:

Seeing underground: Introduction

Outline

This five page article was written for those involved in the earth sciences who have no background in

geophysics. It is intended to explain very briefly how applied geophysics can contribute unique and important

information that helps solve a wide range of practical problems in the earth sciences and engineering. The

article was adapted from Geophysical Inversion: New Ways of Seeing the Earth's Subsurface, by Francis

Jones and oug !ldenburg, in Innovation, !ctober "##$, %ssocation of &rofessional 'ngineers and

(eoscientists of )ritish *olumbia.

Importance of Earth's subsurface

The surface of the earth has provided the setting for most human endeavours throughout the history of civili+ation, and these

activities have been profoundly affected by the largely invisible characteristics of the immediate subsurface. uman developmen

has depended heavily on resources obtained from both near surface -as in construction materials and from hundreds to

thousands of metres deep -as in metalliferous ores and petroleum based products. /e also use water from subsurface aquifers

deposit much of our waste within the near subsurface, and build structures that must interface safely with these shallow regions

Physical properties vs rock type and structure

In relation to these activities, subsurface characteristics of particular interest to earth scientists include the location, distribution

and structure of rock types, grain si+e distribution, and material strength, porosity and permeability, to name a few. The earth0s

inherent complexity can make it difficult or impossible to infer these characteristics from direct observation. Therefore they often

must be inferred from the distribution of more fundamental physical properties such as density, electrical conductivity, acoustic

impedance and others. These basic properties can be measured via geophysical surveys that record the earth0s response to

various types of natural or manmade signals. The following table lists physical properties that are most commonly related to

geological materials and1or structures, and geophysical survey types that can map variations of these physical properties.

Common physical properties ssociated geophysical survey techni!ues

'lectrical resistivity -or conductivity * resistivity, all electromagnetic methods

2agnetic susceptibility %ll magnetic survey methods

ensity (ravity, and seismic reflection or refraction

%coustic wave velocity 3eismic reflection or refraction

!ther physical properties that can be usefully mapped include chargeability, natural radioactivity,dielectric permitivity, and porosity.

"emand for improved modeling

3ubsurface structures are usually interpreted either in terms of ob4ects, layers, linear features, or

complex distributions. This type of information, obtained remotely and non5invasively using

geophysical surveys, is routinely used in geotechnical, exploration and environmental activities to

characteri+e geological structures, estimate ore reserves, map contaminant plumes, etc. /hat is

involved in obtaining such information6 First, field work is done -Figure " which involves making

many careful measurements along survey lines on the ground or from aircraft. Traditionally,

interpretations of these measurements are often made from graphs or maps of raw or processeddata, resulting in qualitative or crudely quantitative information about the locations, depths, and types of materials under groun

In the face of ongoing demand for increasingly quantitative information, however, sophisticated techniques are now being used t

numerically estimate the distribution of the earth0s physical properties. These modelling procedures give geoscientists a more co

effective, reliable and accurate means of extracting as much information as possible from conventional survey data. They also

make it possible to present the rather technical information in more visual and meaningful ways to managers, shareholders,

regulatory agencies and other interest groups.

%fter reading this article, it should become evident that the application of geophysics to problems involving earth0s subsurface is

non5trivial process. % seven step framework can be used to help understand each aspect of this process. This framework is not

referenced often in the article, but there is a one page summary referenced elsewhere which should be examined.


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Geophysics foundations:Seeing underground:

Geophysics prime

(eophysical surveys are performed when information about the earth0s subsurface

is desired but direct sampling through expensive and invasive techniques such as

drilling or trenching is insufficient, impractical or ill5advised. % survey may target a

whole earth scale, within the top few metres of the subsurface, or anywhere in

between.

#easuring physical properties

uring a geophysical survey, energy is put into the earth and responses are

recorded at the surface, in the air or in boreholes. 7esulting data reveal

information about the earth because the behaviour of the energy within the ground

is controlled by the distribution of the earth0s physical properties. For instance, one

basic physical property is magnetic susceptibility, which describes a rock0s ability to

become magneti+ed. This physical property provides information on rock type and

structures because the rock0s magnetic susceptibility relates directly to mineral

type, and to the chemical alteration processes involved in its deposition. % second

important physical property is electrical conductivity, which quantifies a material0s capacity to carry electrical current. Figure 8

illustrates one way that a geophysical survey can be carried out to provide information about the subsurface distribution of

electrical conductivity.

$igure %: %n example of how the distribution of a physical property -electrical conductivity in this casecan be measured

to provide information about geologic materials. *lick buttons to reveal corresponding images.

& The physical properties under this surface are unknown. % geophysical survey 5 * resistivity in this case

5 is used to generate data.

% *urrent is in4ected into the ground, and resulting voltages are measured as electrode geometry varies. In this case,

voltages get smaller as electrodes are separated further and further apart.


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'vidently, the application of geophysics to problems involving earth0s subsurface is a non5trivial process. % seven step framework

can be used to help understand each aspect of this process. This framework is outlined in a one page appendix.

(raditional interpretation

Traditionally, useful information was extracted from geophysical field results by examining maps or line profiles of raw or filtered

survey data. 3uch images are useful for estimating locations and quantities of buried materials, and to help choose locations for

more invasive -and expensive techniques such as drilling. For example, large scale maps of magnetic of magnetic or gravity da

often show geologic structure, or identify an anomalous region that might be associated with a desired target. %s an example

Figure 9 shows the magnetic data acquired at the )athurst region of :ew )runswick. The ma4or features observed are related to

geologic structure.

) Inversion of this data set produces an estimate of a ;layered earth; or " model of the relevant physical property 5electrical conductivity.

* Interpretation converts the model into geologic information.

$igure ), Tetatouche %ntiform 5

Total 2agnetic Field, from ;%irborne (eophysical

3urvey of the )athurst 2ining *amp;,(eological

3urvey of *anada website,

http>?.

http://gdcinfo.agg.nrcan.gc.ca/app/bathmaghttp://gdcinfo.agg.nrcan.gc.ca/app/bathmag


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istorically, in mineral exploration, the identification of an anomalous region was often the endpoint of the analysis, and the imag

was used to plan the location of a drill hole. @nfortunately, the success rate was generally poor. %t best, data maps provide some

information about the lateral extent of a body but little information about what is happening at depth. Auantitative analysis, in

particular inversion, is required to obtain 9 information. The mineral exploration example in this article expands on this.

!ther geoscience professionals also need to obtain quantitative information from data sets that are difficult to interpret without

inversion. The geotechnical example in this article illustrates both traditional images of data and quantitative models generated b

inversion of this data.

Inversion

The problem of using recorded data to estimate a reasonable earthmodel -i.e. a quantitative distribution of one or more physical

properties is known as the geophysical inverse problem. The ad4acent

cartoon illustrates that the pertinant question being addressed is ;what

subsurface physical property distribution could have caused the data

that were observed at the surface6; 'arlier inversion solutions involved

characteri+ing the earth by a few prisms or layers and then numerically

finding geometrical and physical properties of these simplified earth

models.

ue to the earth0s extreme complexity, useful models often need to have many parameters, usually more than the number of

data. This means that the problem of finding a model -i.e. estimating values for every parameter is one in which there are mor

unknowns that data. 3uch problems do not have unique solutions, and this nonuniqueness is exacerbated when data are noisy o

inaccurate. Formal inversion methods address these issues using well defined mathematical techniques. %n appendix explainsinversion in a little more detail.

In the remainder of this article, some benefits of applying rigorous inversion can be seen by comparing the information in 9 an

8 models obtained by inversion, to the traditional map and pseudosection plots of the raw data.

pg. 8 of B


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Geophysics foundations:Seeing underground:

Mineral exploration exampl

Carge quantities of magnetic field measurements are routinely gathered over mineral and petroleum exploration prospects using

airborne techniques. 7esulting magnetic anomaly maps can provide information about geological trends because rocks containing high

proportions of the mineral magnetite have a higher magnetic susceptibility, and will affect the local behaviour of the earth0s magnetic

field.

+egional and local magnetic surveys

Figure 9 -supplied courtesy of &lacer ome 'xploration provides an example of regional information from an area surrounding the 2t

2illigan copper porphyry deposit, located in central )ritish *olumbia. (eological trends can be decerned using this type of data,

however, exploration for a specific deposit requires more detailed information about local subsurface distributions of rock types. Figure

9b shows anomalous strengths of the earth0s magnetic field for a small region of one ore body. 'vidently there is a range of different

rock types below the surface, but details of location, depth and magnetic susceptibility are difficult to determine directly using

conventional methods.

a $igure ) Total magnetic field strength map for the 2t 2illigan region, gathered by airborne magnetic survey

techniques.

b *lick the button to see a ground based magnetic anomaly map for the small outlined region over one ore body.The large scale regional magnetic field has been removed from this local map to emphasi+e the signature of

anomalous subsurface magnetically susceptible rocks.

Inversion to obtain )" details

The goal of inverting this data set was to produce detailed 9 models of magnetic susceptibility to help geologists develop a more

complete understanding of the rocks associated with the ore deposit. The first step was to reduce the dense data set from the small

region -Figure 9a to a more manageable ",>8# evenly spaced data points and to divide the model region into "?#,>>> cells. Then a

desirable model type was chosen. In this instance, the process was set up with two criteriaD namely to find a model that was -i as clo

as possible to a uniform earth with +ero susceptibility, and -ii included structure that was smooth in all three spatial dimensions.

In addition, the numerical procedure for finding plausible subsurface models of susceptibility was constrained so that data predicted

from the model would match observed field measurements to a degree specified by assuming a noise level -on measurements of BE

The resulting model was a 9 volume represented by the "?#,>>> cells, each with a magnetic susceptibility recovered by the inversio

,isuali-ing results

There are several ways to usefully present volumetric information of this kind. *ontour plots of hori+ontal or vertical slices through the

volume, as shown in Figure , provide quantitative details at any required location. %lternatively, for a more general impression of the

model, a 9 iso5surface image can be created. This is shown in Figure B, which suggests there is a well defined volume of magnetical

susceptible rocks associated with this deposit. This model correlates well with one of the known principal local rock units -2)G

monsonite stock and with locations of minerali+ation.


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$igure *: The model of magnetic susceptibiility recovered by the inversion of ground5based magnetic data is illustrated

by plotting slices from the volume under the survey area. The left panel is a hori+ontal slice at $>m depthD the rightpanels are three vertical slices taken along lines at #?>>, #B>>, and #>> metres north. (ray lines indicate the slice

locations.

Corroboration .ith independent geophysical results

Few geophysical surveys are used alone with no other independent information. %t 2t 2illigan many types geophysical surveys were

performed on the ground, from airborne platforms, and from within boreholes. For example, a similar inversion procedure was used to

interpret * electrical measurements gathered over the same area. The 9 iso5surface image of Figure ? shows a model of the

distribution of chargeability -the capacity for material to hold an electrical charge, a physical property related essentially to metal or

clay content and grain si+e. The apparent anti5correlation between magnetic susceptibility and chargeability at 2t 2illigan is evident

only after careful inversion of two unrelated geophysical data sets. This example illustrates that conducting inversions on multiple type

of data sets can provide an enhanced understanding of the surveyed regionD in this case it provides insight about subsequent alteratio

of the rocks that occurred after the initial formation of the mineral deposit.

$igure /: The same magnetic susceptibility distribution modelshown in the previous figure is plotted here as a 9 isosurface

of constant susceptibility. %ny surface between +ero and the

maximum susceptibility recovered could be chosen for the plot.The best choice for illustrating geologically relevant features

depends upon estimating the true susceptibility of rocks,

perhaps from borehole or outcropping samples.

$igure 0: %n isosurface plot of chargeability, which is usuallyrelated to the presence of sulphide ores, graphite, or clay

minerals. The chargeability model was obtained by carrying out

a 9 inversion of induced polari+ation data collected alongparallel survey lines over the deposit region. *omparison with

the 9 model of magnetic susceptibility shows that low

chargeability is correlated with high susceptibility. etailedcorrelation of the two inversion results provided information

that contributed to an enhanced understanding of how the orebody was deposited.


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Geophysics foundations:Seeing underground:Geotechnical exampl

(eotechnical work also requires quantitative, accurately located information about the subsurface. Figure Ha. below shows initial

unprocessed results of a * electrical survey over calcine tailings at the 3ullivan 2ine in southern )*. Cateral locations of

conductive material can be interpreted directly. owever, for this application, there was a need to characteri+e the extent and

depth of the calcine material -which has a higher electrical conductivity than host rocks partly to determine the quantity of calcine

and partly to constrain the possible subsurface paths along which ground water could travel.

1imitations of standard data presentation

The standard form of presentation shown in the top panel of figure H, known as a pseudosection, distorts the actual distribution of

subsurface physical properties. :ote that no vertical axis scale is provided. /ithout formal inversion there is no way to identify the

position and value of electrically conductive or resistive materials that gave rise to the observed data.

%lso, with resistivity surveys it is important to estimate the depth of investigation because the ability to resolve geology at depth

depends upon survey geometry and subsurface conductivity as well as the current source power. Traditionally -prior to

development of formal inversion techniques, geophysicists used ad5hoc rules to identify the depths at which interpretations

became unreliable.

$igure 2:

a (top) 7aw * resistivity data from a survey overcalcine tailings are plotted in pseudosection format.

7esistivity values are apparent rather than trueintrinsic resistivities, and the pattern is determined

by the plotting convention. *ircles indicate plottingpoints for recorded data values. Cateral surface

distribution of highly conductive -i.e. low resistivitycalcine is recogni+able, but details of the thickness

and geometry of the conductive +one are obscured.

b (otto!) The conductivity model recovered by 8inversion of data in the top panel. 'ach rectangular

cell has the value of it0s conductivity determined bythe inversion algorithm. The location and volume of

high conductivity material is clearly defined. The

variability at the surface is due to a thin resistive

cover of course bouldery fill overlying the area.&ortions of the 8 model that are not sensitive to the survey are hatched out.

:ote that conductivity -which has units of 3eimens per metre is the inverse of resistivity -quoted in units of !hm5m.

"epth of investigation

% geophysical survey provides information about a limited volume of the earth. In the inversion our mathematical model usually

extends beyond those limits. The value of a physical parameter outside the area of illumination is determined only by parameters

in the inversion and does not present reliable information. To prevent over5interpretation of the inversioin results it is best to

remove those regions from the final images that are to be displayed. The hatching in Figure Hb accomplishes this goal. It is

evident that the geophysical survey provides no information outside of the limits of the survey electrodes and also there is a

maximum depth to which the data are sensitive. The maximum depth depends upon the greatest separation of the current and

potential electrodes and also upon the level of signal strength compared to noise.

"iscussion

There is a well5defined region of high electrical conductivity -ie low resistivity, in red colours near the surface and a region of

lower conductivity -blues that appears at the surface. The low conductivity coincides with a known bedrock outcrop and this adds

confidence about the interpretability of the image.

Interpretation of a precise depth for the interface between conductive material and bedrock would be greatly aided by a single

borehole drilled to a depth of roughly B> metres anywhere within the high conductivity region. This would also help to identify the

value of conductivity at which the physical interface should be interpreted.


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Seeing underground:Conclusions

(eophysical surveys are non5invasive techniques for obtaining information about subsurface

materials and their distribution or structure. The results of surveys can often be used

directly, or after some filtering prior to presentation as graphs or maps.

The survey data, perhaps with some filtering, can sometimes be used to answer the

question of interest. (enerally, however, the information is insufficient and more

quantitative analysis is required. The data need to be inverted to generate a distribution of

the physical property. The inherent nonuniqueness of the inverse problem is a complicating

factor and this has motivated the development of different inversion approaches. Irrespective of details, the application of forma

inversion techniques to conventional geophysical data has contributed decisive information in the resolution of mineral exploratio

and geotechnical problems.

2ineral exploration, petroleum, and engineering organi+ations now routinely apply modern inversion techniques to geophysical

surveys, such as gravity, magnetics, resistivity and others. Instead of applying ad5hoc methods to the interpretation of raw or

filtered data, geoscientists can now produce a range of acceptable subsurface models based upon rigorous and well defined

criteria. The value added through the provision of well constrained, easily visuali+ed 8 and 9 models of subsurface physical

properties means that geophysical surveying can be more cost effective, allowing decision makers to act with more confidence in

assessing the risks and costs of pro4ects requiring subsurface information.

© UBC Earth and Ocean Sciences, F. Jones


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Seeing underground: Appendix - Inversion Outlin

The problem of estimating a reasonable earth model -i.e. a quantitative distribution of one or more physical properties based upo

recorded data is known as the geophysical inverse problem. 'ver since computers became standard tools for geophysical work,

various methodologies for performing geophysical inversion have been developed. There are two broad classes of inversion<

;&arametric; methods and ;(enerali+ed; inversion methods.

Forward modelling< calculating data based upon a

known earth model.

Inversion< estimating a model based upon measured dataand some understanding of the setting.

Parametric methods

These inversion methods involve finding a model of the earth which is described using only a few parameters. In

fact, the solutions re"uire that there be fewer parameters than there are data values so that the problem is

formally ;over5determined; -see glossary. % few examples of parametric models are<

3uried ob4ect< parameters could be depth to a sphere -or cylindar, a radius or radius and length, and the

physical property contrast between the ob4ect and host rocks.

1ayered earth< parameters are layer thicknesses and physical property values.

buried sheet< parameters might be depth to the top of sheet, it0s dip, strike, thickness, and the physicalproperty contrast between the sheet and host rocks.

Inversion usually involves searching for the model -i#e# a set of parameters which generates a data set that best

matches the field measurements. The inversion algorithm ad4usts model parameters to improve the match

between calculated and measured data sets. This is generally an iterative process.

Generali-ed inversion methods

This second class of inversion methods allows the earth0s model to be more realistically complex, which means that

more parameters than data points are permitted. 3uch problems are mathematically referred to as

;under5determined;. 2ost solutions to this more general form of the geophysical inversion problem involve three steps, which ca

be explained briefely as follows<

7epresent the earth with many parameters so that complex distributions of physical properties can be simulated. In

practice, the earth is divided into many thousands of cells of fixed geometry, each with a constant but unknown value of

the relevant physical property.esign an adaptable mathematical function of this earth model called a !odel ob$ective function.

This function0s value depends upon the model. *hange the model and the function0s value changes.The inversion process will involve ad4usting parameters making the model in order to produce a minimum value for

this ob4ective function. ifferent types of functions will require different models to produce a minimum value. For

example, one sensible model ob4ective function measures how spatially ;smooth; the earth0s structure is. /hen themodel causing a ;minimum; value is found, this will be the ;smoothest; model possible. This might be a sensible

choice because large scale features of the subsurface are usually more important than fine scale details.

ow does the geophysical data contribute6 The carefully designed model ob4ective function might be minimi+ed using ageologically unreasonable model of the earth. owever, an acceptable model must be able to cause the measured fielddata. This is a second constraint which allows the inversion process to find reasonable models of the earth.

3o, inversion using optimi+ation methods have two requirements< - i ad4ust the model until it0s ;ob4ective function; takes on a

minimum value -ii sub$ect to the constraint that the model can cause the measured data.

The earth model is a fixed distribution of cells, each with an ad4ustable value of

the physical property. 2easured data

are shown on top.

%n acceptible model can cause the data,and simultaneously produces a

minimum value for the ;model ob4ective

function;.

In practice a number of inversions, with different reasonable ob4ective functions, should be carried out so the interpreter has som

insight about the range of earth models that can acceptably reproduce the field data. 'rror statistics about the data will determin

how closely the reproduced data matches the real measured data. The fact that these error statistics are often poorly known is a

second good reason for performing several inversions before settling upon a preferred model.

© UBC Earth and Ocean Sciences, F. Jones.

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Seeing Underground