Geophysical Methods

Vane Shear

Determine untrained shear strengths of saturated clays

Hydrocarbons (R)

Extremely high

GPR works well in what conditions

dry conditions but water will "absorb" the GPR signal in most shallow cases.

Resistivity Logs are broken into two configurations

"short and Long" 16-inch spacing and 32 (or 64) inch spacing

SPT Hardness for Silts and clays

0-2 Very Soft 3-4 soft 5-8 medium stiff 9-15 stiff 16-30 very stiff 31-50 Hard Over 50 Very Hard

Sand SPT Hardness

0-4 Very Loose 5-10 Loose 11-30 medium Dense 31-50 Dense Over 50 Very Dense

Sand diameter

1/16 to 2 mm

Silt diamter mm

1/256 to 1/16mm

Gravel diameter

2 to 4 mm

Pebble Diameter mm

4 to 64 mm

Cobble Diameter (MM)

64 to 256mm

Lidar stands for

Light Detection and Ranging

Manometer

Liquid column instrument to determine minor changes in atmospheric changes

Wet Sand Resistivity

Low Resistivity

GPR is measured in what?

MHZ (Megahertz)

Landsat

Maps soils, geology and the effects of precipitation and evaporation upon the occurrence and character of groundwater

What does Resistivity Measure

Measures the electrical resistivity (inverse of conductivity) of the surrounding formation - pore fluids ➢Effectively averages resistivity along the field of measure - so small beds may be missed in long spacing - "averaged out"

Magnetism is used to locate what?

Method is best used to locate lateral changes in the magnetic permittivity of subsurface materials. ◈Used in mineral exploration and for location of buried metallic tanks or unexploded ordnance ◈Also used to map the magnetic basement in petroleum exploration

Types of unidisturbed samplers (6)

Modified California Sampler (ASTM D-3550) Shelby Tube Osterberg Hvorslev Pitcher barrel Denison

ASTM (D6938)

Nuclear Density Gauge

SLAR - Side Looking airborne Radar

Often used in areas with cloud cover Can be used day or night bu shadows may obscure photos

How does a caliper work?

Operates as a spring loaded caliper that rubs the borehole edges. Caliper opens and closes as it is lowered into the borehole.

Gravitiational method is used to map what

Used to map sedimentary basins, with respect to the basement rocks that surround them (e.g., sandstone against metamorphic rocks) ◈Also useful to map faults that bring together materials of different density (e.g., limestone against basalt)

Inclinometer

Used to measure landslides and slope movement

Dry Sand Resistivity

Very High Resistivity

Wet Clay Resistivity

Very Low Resistivity

SW

Well-graded sands and gravelly sands, little or no fines

Spectral - Near Infrared is best used the application of

Applications in forestry, agriculture, anything where density and health of vegetation is important.

Near Infrared (NIR) application

It's application - primarily to identify healthy vs. unhealthy plant material from satellite or airborne

Clay diameter

< 1/256 mm

What is refraction good for

good for determining depth to stratigraphic layers and the water table

Resistivity Higher Salinity Content

=lower resistivity (higher conductivity) -important in oil drilling where saline water often underlies oil reserves

Boulder Diameter (MM)

>256mm

InSAR (Interferometric Synthetic Aperture Radar)

A remote sensing method that is used to study ground deformation, particularly in subsidence, volcanic and fault studies.

LIDAR is good for

Ability to "see" through vegetation ■Able to map ground surface in jungles, never seen before topographic maps ➢Ability to detect very subtle changes ■Glacial advance, retreat, or growth/shrinkage ■Uplift or subsidence ➢Ability to scan large areas ■Periodic scanning of mining or landfills, comparison to the previous scans - determine net removal or addition of material

Data Logger

An electronic logging device that measures temperature pressure humidity and water levels

Radon (Gamma)

Gamma= High

Petroleum has what kind of resistivity?

High resistivity (Low conductivity)

SP

Poorly graded sand

NDOP

Provides complete coverage of the U.S. The intention is to update digital orthoimagery every 3 to 10 years

rock Quality Designation

RQD

Specific Capacity (SC) Equations

SC= Pumping Rate (Q)/ Drawdown (ds)

Coal (SP , R, and Gamma)

SP= Generally Low R= High Gamma= Low

Sandstone/Sand (SP , R, and Gamma)

SP= High R= Moderate Gamma = Low

Limestone (SP , R, and Gamma)

SP= Low R= High Gamma = Low

Saltwater (SP , R)

SP= Low to moderate R= High

Freshwater (SP, R)

SP=Low R= High

Shale (SP , R, and Gamma)

SP=Low R=Low Gamma=High

ASTM (D1556)

Sand Cone

Scale for plane scaling

Scale= F/H F=Focal length H= flying Height

SRTM - Shuttle Radar Topography

Shows visual actions of the earths surface show changes due to flooding, erosion, landslides, earthquakes, weather. Military uses for mission planning, rehearsal, and simulation

Goal of a Resistivity Log

To determine the electrical resistivity of fluid in the pore space of a formation.

Goal of a Gamma Logs

To identify shale or clay layers/formations. ❖Detect naturally occurring gamma radiation, particularly from the decay of potassium.

Goal of caliper logs

To record the borehole diameter as a function of depth in order to identify areas that have sloughed/caved in or areas of swelling.

with refraction does density have to increase with depth?

Yes if there is a less dense layer it will become invisible

What is a reasonable interpretation of the feature from approximately 125-150 feet below ground surface? a) aquitard b) capillary fringe c) pay zone d) aquifer

a- aquitard. High gamma for high clay/shale and low resistivity is also consistent with clay/shale. Given the answer to #9 above, this is likely an aquitard.

A colleague has asked for your help interpreting a downhole resistivity log, both short and long spacing are present. From 310 to 370 feet below ground surface, both the long and short resistivity traces overlap (are the same). Given the options below, what would be a reasonable interpretation of both long and short traces being the same for this60-foot interval? a)a thick sequence of CH b)a thick sequence of GP c)a thick sequence of ML d)a thick sequence of CL

b- a thick sequence of GP. Poorly sorted gravel should be porous and when the traces overlap it is likely that drilling fluid has penetrated the formation and both are measuring the same thing.

What gamma signal would you expect to see between 50-120 feet below ground surface? a) steady, around moderate API b) increasing API c) decreasing API d) no signal

b- increasing API. Increasing clay content will have an increasing gamma signal, with units of API.

What could explain the behavior of the caliper log trace from the interval between 50-120 feet below ground surface? a) decreasing gravel b) increasing clay c) shale layer d) loosely consolidated SP

b- increasing clay. This log is exaggerated to make the point - clay can swell in a borehole and decrease the diameter. And although it is exaggerated, it is exactly the kind that you may encounter on the exam because exam writers will be making up "homemade" logs for the test.

What is a reasonable interpretation for the behavior of the logs in the interval between approximately 50-125 feet below ground surface? a) SW at 50 feet grading to SP at 125 feet b) GP at 50 feet transitioning to ML at 125 feet c) CL at 50 feet grading to SW at 125 feet d) ML at 50 feet grading to CH at 125 feet

c- CL at 50 feet grading to SW at 125 feet. Clay will have a high gamma response and in general will have low resistivity (50 feet). This is what a fining upward sequence would look like as you progress lower gamma has less of a response and resistivity is increasing

Your client, a landfill operator, has retained you to submit their annual report. The report requires reporting of the amount of material (in cubic yards) that has been added to the landfill cell during the last reporting period. What method would be the most appropriate for determining the amount of material added to the landfill? a)gravity survey b)scale tickets c)LIDAR survey d)ground penetrating radar survey

c- LIDAR survey. LIDAR is a great tool for 3-D modeling changes to the surface of the earth.

The forest service is trying to determine the amount of trees that have been impacted by the invasive bark beetle, which has resulted in the death of a significant amount of trees in the Black Hills of South Dakota and subsequent slope instability. What geophysical method would you recommend to determine this? a)LIDAR survey b)gamma survey c)NIR survey d)airborne DEM survey

c- NIR survey. Because healthy plant tissue reflects the near infrared (NIR) band much more than does unhealthy, it makes a great tool for identifying unhealthy/dead vegetation.

You are a geologist investigating a previously industrial facility situated on around 15acres. The primary building is 120,000 square feet with several smaller buildings and shops. You observe 2-inch pipes sticking out of the ground and suspect there may be a buried tank. What method would you recommend to your client to identify the presence or absence of a shallow buried tank? a)seismic reflection b)test pits with a backhoe c)ground penetrating radar d)gamma survey

c- ground penetrating radar. GPR is good for identifying shallow subsurface structures like tanks, barrels, pipes, etc.

What is a reasonable interpretation of the interval from approximately 150-175 feet below ground surface? a) hydrocarbon bearing zone b) dry clay c) saturated sand d) shale

c- saturated sand. The low gamma and low resistivity give this away. Start with gamma to identify clay/shale or not then look at resistivity - hydrocarbon is very high resistivity, water will be low

What conditions are most conducive to a seismic refraction? a)decreasing density with depth b)alternating density from low to high c)alternating density from high to low d)increasing density with depth

d- increasing density with depth. Any layer that is not MORE dense than the one above it will be "invisible" to seismic refraction. Refraction relies on increasing density with depth, which is usually an ok assumption. But not always.

Resistivity survey further spacing means

deeper penetration

Resistivity Survey

electric current is passed through electrodes that are hammered into the ground and the electrical resistivity between the electrodes is measured

Gravity Method

elies on very precise measurements of the pull of gravity at any given location The pull of gravity varies as the density of the materials in the subsurface varies ◈Method is best used to locate lateral changes in the density of subsurface materials

LIDAR Surveys used for

for creating 3-D models and often for comparing the same LIDAR scan at two different times (months or years apart) to determine differences. Examples are -landfills, glacier advance/retreat, mudslides. Also can "see" through vegetation.

GPR is good for

hallow subsurface, commonly used to locate buried infrastructure (tanks, barrels, pipes).

Spectral - Near Infrared is good for?

healthy vegetation reflects more NIR spectra than does unhealthy vegetation, than does soil. Therefore you are able to easily differentiate between the endmembers of soil to healthy vegetation.

Dry Clay Resistivity

high Resistivity

What industry uses reflection mostly

oil and gas exploration

GPR

radar signals are transmitted into the subsurface and bounce back off density changes. Very similar to reflection but radar not seismic waves.

Magnetism is

relies on measuring variations in the strength of the magnetic field by magnetism induced by subsurface materials with high iron content (either metallic iron or the mineral magnetite)

Refraction

seismic waves are induced (hitting the ground with a sledge hammer) and geophones strung out in a line (50 m long?) record the arrival of waves. Seismic waves travel across bedding planes (any plane with a change in density) and are refracted back to the surface.

Reflection

seismic waves are induced at the surface and geophones record the waves that are REFLECTED back to the surface by a change in density. All that is needed is a change in density to REFLECT. Geophones remain stationary while shot point changes away from the geophones

Resistivity survey closer spacing means

shallower penetration