Geophysics of Exploration for Water

Introduction

Edited by P. Vass

Geophysics

Geophysics belongs to the group of Earth Sciences, and it• integrates the knowledge primarily coming from geology,

mathematics and physics,• uses and develops physical measurement methods,

mathematical tools, data processing and analysing techniques, as well as special software packages to solve the problems concerning the Earth’s interior and the surrounding space environment.

The two scientific areas of geophysics (by the ranges and objectives of the investigations):

• (global) Earth physics,• applied geophysics.

Applied geophysics

Earth physics

It deals with large-scale or global problems and observations (e.g. the investigation of Earth’s shape, its gravitational and magnetic fields, its internal structure and composition etc.)

Applied geophysics

Its main objective is to help in solving different practical problemsconnecting to the subsurface by means of geophysical measurements.

It tries to provide useful information for mining industry, hydrological, environmental, geotechnical and archaeological surveys.

The investigation depth is limited to the uppermost part of the crust, which can reach a few 1000 metres below the surface.

A special area of applied geophysics is the so-called near-surface geophysics, which focusses on the investigation of small-scale features in the shallow (tens of meters) subsurface.

Exploration geophysics

Applied geophysics can be divided into further fields according to the specialities of problems to be solved:• exploration geophysics (this is the most significant field),• hydrogeophysics,• engineering geophysics• environmental geophysics• archaeological geophysics• forensic geophysics.

Exploration geophysics

Its primary objective is to discover the subsurface geological objectswith which the deposits or reservoirs of raw materials and energy sources may associate under favourable conditions (e.g. ores, coal, natural oil and gas, evaporites, and geothermal energy).

It is also used for obtaining qualitative and quantitative informationabout the targets of interest, and the surrounding rock formations.

Applied Geophysics

Hydrogeophysics

is a cross-disciplinary area of hydrology and geophysics.

It uses geophysical methods to find water resources, to provide quantitative information about hydrogeological parameters of the subsurface structures, and to monitor processes connected to water resources, contamination, and ecological studies.

By means of combining the geophysical and hydrogeological methods the cost of research and the application of invasive (or destructive) methods (drilling and taking soil samples which often disturb the study site) can be decreased or minimized.

Geophysical methods

The main groups of geophysical methods by their physical principles:• gravity method,• magnetic method,• geothermal method,• electric methods,• electromagnetic (EM) methods,• seismic and acoustic (or sonic) methods,• radioactive and nuclear methods.

Each method is based on the fact that the spatial and/or temporal variations of the measurable physical quantities are related to the changes in the properties of material of the subsurface media.These changes are principally connected to the boundaries of different rocks or rock-fluid systems.

Geophysical methodsname of the property of

material influencing

the measured quantity

name of the measured

quantity

name of the method

mass density or bulk density

(for porous media)

change in gravity gravity

magnetic permeability intensity of magnetic field magnetic

concentrations of

radioactive isotopes, or

other elements (e.g.

hydrogen)

count rate of gamma

photons, or neutrons

radioactive and nuclear

thermal conductivity temperature or temperature

change

geothermal

(electrical) resistivity electric voltage geoelectric

dielectric constant+

(electrical) resistivity

+ magnetic permeability

the magnitudes and phase

angles of electric- and

magnetic-field components

electromagnetic

mass density or bulk density

+ elastic moduli

(travel) time seismic and acoustic

Surface geophysicsThree branches of applied geophysics according to the site of measurements:• surface geophysics (airborne and marine geophysics are also

included),• borehole geophysics,• mining geophysics.

Major objectives of the surface geophysics:

• detecting the physical effects of the wanted geological objects

(which may be perspective from the point of view of the

exploration),

• determining the position and dimensions (horizontal and

vertical) of the indicated object,

• delimiting the structural units of the given geological

environment, and detecting the major structural directions,

• supporting the selection of drill sites in each phase of the

exploration.

Borehole geophysics

Main objectives of the borehole geophysics:

• investigating the physical properties of the formations traversed by the borehole,

• determining some technical parameters of the borehole,

• performing the geological correlation among the neighbouring boreholes by comparing the measured logs,

• providing information for data processing and interpretation of surface geophysical measurements (e.g. seismic measurements),

• supporting the reinterpretation of results derived from former geophysical measurements to create a more precise and reliable model of the subsurface.

Mining geophysics

Main objectives of the mining geophysics:

• determining the microtectonic lines which traverse the subsurface pits and shafts,

• estimating the quality of ore, and ore content of the rocks by means of in situ measurements,

• forecasting some dangerous events (e.g. water inflow, downfall, minefire, inflow of fire damp).

Range of investigation:

surface geophysics from the surface to max. n x 1000 m

mining geophysics typically n x 10 m, max. 100 or 200 m from the wall of the shaft

borehole geophysics from the borehole wall to max. 2-3 m

Workflow and stages

A typical workflow of an applied geophysical survey includes the following steps in sequence:• survey design,• data acquisition, • data processing, • data analysis and interpretation, • and generating reports.

Exploration stagesThe entire process of an exploration concerning a larger area is divided into different stages, which follow one after another:• reconnaissance,• prospecting,• general exploration,• detailed exploration.

Exploration stages

Reconnaissance

It includes a regional survey.Its main purpose is to select smaller areas being worthy of further investigations from the initial large area.

Typically performed activities and applied methods:

• geological evaluation of airborne an satellite images • systematic geological mapping on smaller scales

(e.g. 1:100 000 or 1:50 000),• regional geochemical sampling (the space interval is wide),• airborne geophysical surveys (magnetic, electromagnetic,

radiometric).• regional surface geophysical surveys (wide spaced gravity,

magnetic, radioactive measurements, the less expensive methods are applied).

Exploration stages

(Preliminary) Prospecting

The areas selected in the previous stage are studied more extensively (it covers a few km2 or tens of km2) to determine the target area for further investigations.

Typically performed activities and applied methods:

• geological mapping on larger scales (e.g. 1:25 000 or 1:12 500),

• geochemical sampling (survey grid with smaller intervals),

• surface geophysical surveys (survey grid with smaller intervals, e.g. geoelectric, electromagnetic and seismic surveys),

• excavating shallower pits and/or trenches to expose the interesting rock formation (if it is near to the surface).

Exploration stages

General or preliminary explorationMore detailed investigations of the target area delimited in the previous stage (less than a km2 or a few km2) are performed to collect as much information as possible.

Typically performed activities and applied methods:• detailed large-scaled geological mapping (e.g. 1:2000 or 1:1000),• additional surface geophysical surveys if more precise information

is required about the subsurface (e.g. 3D seismic, geoelectric, ground penetrating radar),

• excavating additional pits and trenches to collect rock samples,• drilling shallow holes on a suitably designed pattern for ore

exploration (core sampling, borehole geophysical logging etc.),• drilling the first few exploratory holes on suitably chosen sites for

fluid exploration (core sampling, mud logging, well logging etc.) • detailed petrological and petrophysical studies.• estimation of ore and fluid reserves.

Exploration stages

Detailed exploration

In this stage the presence of an ore body or fluid reservoireconomically exploitable from the subsurface cannot be queried, but additional information is needed for planning the effective mining activity and production.

Additional boreholes (called appraisal wells in hydrocarbon exploration) are drilled at closer intervals to accurately determine the shape, size, and position of the reservoir or the ore body.

Borehole geophysics is of great importance in this stage.In the case of ore exploration, the first few exploratory openings can be excavated.All the collected data are processed, analysed and evaluated to create a precise model of the reservoir or ore body.A more reliable estimation of the reserves is also performed.

Exploration stages

The design of the next stage is always based on the information coming from the previous one.

So, the amount of collected data and obtained information about the subsurface gradually grow as the exploration goes ahead.

More and more exact methods with higher and higher resolutionare generally applied in the subsequent stages.

Of course, this process entails the increase of costs and timeexpended on the exploration.

In the beginning, the less expensive but quick mapping methodsare used for getting an overall view of the main geological features (e.g. gravity and magnetic methods).

The application of more expensive methods with higher resolutionare limited to the areas where the results obtained from the previous measurements are hopeful (positive results).

Exploration stagesThe interpretation of the measured data is an iterative process during the exploration, since the results coming from the newer methods and later measurements enable the experts to make the geological-geophysical model of subsurface more precise.

It means that the exploration entails the periodical re-interpretation of the gradually expanding set of data and observations. The re-interpretation usually produce new information.

In such a way, we can gradually focus the further more detailed investigations on the more perspective parts of a larger area in order to save money and other resources.

The general exploration stage can be started only if the prospecting stage has been closed with a positive result.

And the necessary condition of beginning the detailed explorationis the positive result of the general exploration.

Exploration stages

If we observe this rule consistently, the detailed exploration cannot yield a negative result any longer (because the presence of economically exploitable raw material cannot be questionable in this stage).

A negative result would indicate that a serious professional mistake was made in the previous exploration stage.

So, after the detailed exploration all the necessary information should be available for opening the mining activity on the delimited area.

In fact, the exploration is not over after the mining activity has been started, because

the field development, the design of the production,and solving the actual mining problems.

quasi continuously require additional information of the geologist and geophysicist.

Drilling investigations

Although drilling boreholes is the only way of getting direct information about the deeper subsurface, its high cost strongly limits the number of boreholes during the exploration phases.

Mainly the early phases of the exploration are critical from the point of view of drilling, because of the higher level of risk.

The less information we have the higher the level of risk and uncertainty.

The selection of the first few exploratory drill sites (typically 1 or 2) is based on the joint interpretation of geological and geophysical maps and sections.

The most important aim of exploratory drilling is to obtain as much information as possible by means of different measurements.

Drilling investigationsThe main techniques of subsurface data acquisition:

• drilling data acquisition (ROP, WOH, WOB, bit depth, torque, RPM etc. are recorded as a function of rig time)

• wellsite mud logging (cuttings descriptions, gas analysis of the mud returning to the surface, hydrocarbon indications in cuttings etc.),

• core acquisition or coring (the detailed core analysis with the measurements is performed in laboratories),

• well logging or borehole logging (wide variety of logging probes [tools] and methods are available to measure the different physical

properties of rock formations, as well as to collect rock and fluid samples from the formations in borehole circumstances)

• well testing (used in fluid exploration and production, collecting fluid samples for analysis, measuring the initial pressure, flow rate, pressure change, and temperature during the test production etc.)

Drilling investigations

A very important requirement for an exploratory borehole to traverse the entire depth interval of the possible deposit(s) or reservoir(s). In such a way the vertical delimitation of the investigated object can be implemented.

The results coming from a suitably designed and correctly implemented data acquisition program provide very useful information for refining the geological-geophysical model and designing the next stage of the exploration.

In the detailed exploration stage, further boreholes are drilled to discover the details of the investigated geological structure and the reservoir or mineral deposit. The analysis and interpretation of the collected datasets assist the 3D delimitation of the structure.

The information coming from these boreholes (called appraisal wells in petroleum industry) is very important from the point of view of mineral resource assessment.

Borehole logging

After the important geological and petrophysical details of the perspective area have been revealed (the level of uncertainty has reduced), the application of the most expensive data acquisition methods (e.g. coring) decreases (because of the economy).

The significance of borehole logging does not decrease, but the special (more expensive) logging methods are less used, and the set of measured logs is limited to a standard suite of logs in the wells drilled for production.

The suite of standard logs is suitable to help in answering the essential questions (e.g. lithology determination, identification of perspective zones, estimation of some petrophysical parameters characterizing these zones).

Borehole loggingSome types of well logs do not provide geological and petrophysical information, but are used to check the condition of a well or the production characteristics during the well completion and production.

Borehole logging was initially developed for the hydrocarbon exploration (1927, Conrad et Marcel Schlumberger), and the widest range of logging methods with the most expensive logging tools are still used in this field.

Due to the development in electronics and computer technology, the conventional logging methods were introduced to the other fields of exploration and mining by the 1970s and 80s.

Actually, borehole logging has become a generally used data acquisition technique in the areas where either shallow or deep boreholes are drilled for getting detailed information about the subsurface (solid mineral exploration) or producing fluid from the subsurface (hydrocarbon, water and geothermal exploration).