In the Woods May 2015

WoodsHighlights of employee work in research, technology, techniques, and

the application of science to natural resource management

Page 1 Produced by the Forest Resources Division - Forest Informatics and Planning

In the

Issue 1- Vol. 2 ♦ May 2015

About This Article

This article is second in a series to fos-

ter awareness of the great work being

done by DNR's State Lands employ-

ees. In these articles, we will explore

innovative ideas, scientific research,

new techniques, and accomplishments

to inform and inspire.

General questions about this article

can be sent to Cathy Chauvin at 360-

902-1385 or Cathy.Chauvin@dnr.

wa.gov. For technical questions on

geophysical techniques, please con-

tact Recep (Ray) Cakir at 360-902-1460

or [email protected]. Ques-

tions specific to geology and forest

roads can be directed to John Jenkins

at 360-827-0204 or John.Jenkins2@

dnr.wa.gov. Many thanks to Ray Cakir,

John Jenkins, Venice Goetz, Casey

Hanell, and Timothy Walsh for their

assistance on this article.

Peering Into the Earth Using geophysical techniques to find rock for forest roads

by Cathy Chauvin, Editor and Publisher, Forest Resources Division

Fresh off the stump and replete with water, fresh-cut timber is heavy stuff. A fully loaded logging truck can tip the scales at 68,000 pounds, which is the weight equivalent of approximately 17 average-size cars.

To support this kind of weight, forest roads must be built strong with good, hard rock. How hard? Generally, the best rock has few fractures and requires blasting for extraction.

DNR obtains the rock it needs from existing rock pits located across state trust lands, but new sources often are needed. The farther rock must be hauled from the pit, the more it costs to build the road. Haul rock far enough, and a promising timber sale becomes infeasible.

Finding good rock can be a challenge, especially in steep, heavily forested terrain. Current methods include field reconnaissance and use of geolog-ic maps to locate deposits and predict which direction they run beneath the surface. Promising areas may be explored with test pits and drilling.

Unfortunately, rock deposits are seldom uniform, thick in some places and thin in others, soft and fractured here but harder there. It can be

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Washington State Department of Natural ResourcesPage 2

Text Box 1. Project Team Membersdifficult to know exactly where to dig or what type of equipment to use. All of this begs the question: surely, there must be a better way?

As it turns out, there is.

Looking for Answers

In early 2013, managers in DNR’s South Puget Sound and Pacific Cascade region offices approached DNR’s Geology and Earth Resources Division for a new way to approach this problem. The challenge intrigued Recep (Ray) Cakir, a hazards geophysicist in the division’s Geo-logic Hazards group. Ray organized a team of specialists (Text Box 1) to answer the following question: was there a cost-effective, practical geophysical technique that could be used to locate rock for forest roads, character-ize rock quality, determine the thickness of the overbur-den soils, and identify any concerns with groundwater, all without breaking the ground surface? To answer this question, the team tested a combination of geologic re-connaissance and the following geophysical techniques: active and passive seismic, electromagnetic induction, electrical resistivity, and ground penetrating radar (GPR). These techniques will be explained in this article.

Test Sites and Equipment

The testing was conducted at three sites on state trust lands: the BB Pit timber sale on Tiger Mountain, Perry Creek rock pit in Capitol State Forest, and the Eastern Panhandle timber sale in the Elochoman block near Cathlamet, WA. The Perry Creek rock pit was the prima-ry test site at which all techniques were tested; selected techniques were tested at the other sites. Since DNR had only seismic equipment in-house, some equipment was rented and some was provided by the geotechnical sup-pliers on the team (Text Box 2).

At each of these sites, the primary rock types were basalt, basalt breccia, and andesite. Basalt is a dark, fine-grained rock that forms when molten rock cools on the

DNR

Geology and Earth Resources Division

Recep Cakir, Joseph Schilter, Terran Gufler, Timothy Walsh, and Patricia Newman (intern)

Forest Resources Division

John Jenkins, Venice Goetz, and Casey Hanell, Earth Sciences Program; Laura Cummings, Pacific Cascade Region; and Ana Shafer, South Puget Sound Region

Geometrics, Inc.Koichi Hayashi

Northwest Geophysics, Inc.Matt Benson

Text Box 2. Sites and Methods Tested

Perry Creek rock pitAll techniques tested

Geometrics, Inc. provided an "OHMmapper" for electric resistivity and also participated in testing.

Northwest Geophysics, Inc. provided a "Ge-onics EM31-MK2" for electromagnetic induction.

BB Pit and Eastern Panhandle timber salesAll techniques were tested except electric re-sistivity and electromagnetic induction; rental equipment was used for GPR.

ground. Andesite is similar to basalt, but has less iron and magnesium and more silica. Basalt breccia forms when the outer layer of a mass of molten rock cracks as it cools because the inner layer is still flowing.

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Geophysical Techniques Explained

Active and Passive Seis-mic Techniques. When some-thing shakes the earth, such as an earthquake, an explosion, or a heavy weight falling to the ground, waves of seismic energy travel out from the point of disturbance. By mea-suring the velocity (speed) of these waves, inferences can be made about the composition of the ground. Generally speaking, the higher the velocity, the harder the rock. The types of waves discussed in this sec-tion are shown in Figure 1.

Seismic techniques can be active or passive. The active technique involves hitting the ground with a hammer, dropping a heavy weight, or setting off explosives to gener-ate seismic waves. By contrast, the passive technique involves measuring the small seismic waves or micro-tremors caused by day-to-day life: waves on the beach, water running in a stream, wind blowing across a field, traffic rolling past. With both techniques, one or more ground sensors (“geophones”) are placed directly on the ground. The geophone(s) feed data through a cable to a seismograph that records the arrival of the seismic waves, and the data collected are analyzed using computers.

The team tested two types of active seismic techniques: P wave refraction and multichannel analysis of surface waves (MASW). P wave refraction examines P waves, or primary waves. P waves are the fastest of the seismic waves (Figure 1) and are sometimes called compression

waves because they push and pull the earth as they pass. When a P wave hits an area of density contrast, for ex-ample the boundary between a layer of rock and a layer of soil, some of its energy travels a distance along that boundary and then bends or refracts back to the surface, where its arrival is sensed by the geophones (Figure 2 on the following page). The remainder of that energy con-tinues to travel down until it hits another area of density contrast and refracts upward. The velocity of each layer is calculated using the time it takes the wave to arrive and the distance the wave traveled to reach the geophone. An example of a cross-section built with data from this technique is shown in Figure 3.

Figure 1. Types of Seismic Waves

Body waves move through the interior of the earth; surface waves move primarily near the surface.

Direction of wave

Direction of wave

Direction of wave

Particle motion - counter-clockwise

Particle motion - push and pull

Majority of distortionat surface

Ground surface

Ground surface

Rayleigh wave (surface wave)

P wave (body wave); also called primary or compression wave

Particle motion - up and down Ground surface

S wave (body wave); also called shear wave or secondary wave

Drawings adapted from http://folk.uio.no/valeriem/spice/Frame/surfacew/index.htmland http://www.colorado.edu/physics/phys2900/homepages/Marianne.Hogan/waves.html

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Once collected, P wave val-ues must be translated into information a manager can use to make decisions. For example, if the P wave ve-locity is 2.4 meters per sec-ond, how hard is the rock? Does it require blasting? To answer these questions, the team created a classifica-tion system that correlates P wave velocity values to hardness characteristics of western Washington rock.

The starting point for this work was the basalt portion of the D8R/D8T Caterpillar Chart (Figure 4). Devel-oped in a study sponsored by Caterpillar, Inc., the Cater-pillar Charts define a range of P wave velocity values for rock that is rippable (can be excavated manually, usually

GeophonesSeismograph

Shot

Uniform layer 1

Uniform layer 2

Refracted wave

Refracted wave

Signal(arrives

�rst)

Time in miliseconds

Distance in meters

SignalSignal

Signal(arrives

last)

Uniform layer 3 Velocity of layer 1 (V1) > Velocity of layer 2 (V2) > Velocity of layer 3 (V3)

Drawing adapted from http://asstgroup.com/techniques.html

Figure 2. P wave Refraction Technique

Figure 3. P wave Refraction Technique Cross-section

Categories correspond with classification system in Figure 5. Hardest rock is shown in blue.

with a ripping head mounted on a tractor, dozer, or other heavy equipment [Figure 5]), non-rippable (re-quires blasting), and marginal (hard, but not hard enough to require blasting) using different types and sizes of equipment. The “D8R/D8T” chart (Figure 4) is geared toward a D8R/D8T Caterpillar dozer, which is one of the larger Caterpillar dozers. Using this type of dozer,

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Figure 5. Ripping Head Mounted on an Excavator

Type Mineability Characteristics P wave velocities Comments

I

II

III

Hard: Non-rippable

Intermediate*: Rippable, depending on size of excavator and fracture characteristics

Soft: Diggable with a shovel

Very hard to hard (pings with hammer); fresh to slightly weathered; massive or very wide fracture spacing: tight or slightly open and clean or very thin �lling

2.4-3.6

0.8-1.8; 0.8-2.4

0-0.8

Medium hard to hard; moderately weath-ered; close to moderately close fracture spacing; moderately thin to moderately thick �lling; coarse breccia

Extremely soft to soft; intensely weathered or decomposed rock; very close to close fracture spacing and moderately thick to thick �lling. Note: category includes soil overburden (colluvium and topsoil)

Rock mineable with blasting; typically clean with few �nes** generated (gray area at lower veloci-ties may be rippable)

Gray area exists between low end of rippable and diggable; low end of rippable may be too soft and have too many �nes

Waste material, unusable rock or soil overburden

Caterpillar chart: Caterpillar Inc., 2010, Caterpillar performance handbook (40th ed): Caterpillar, Inc., Peoria, Ill., 1, 442 p.

*Corresponds to marginal and rippable on Caterpillar Chart**Rock dust

Seismic velocityMeters per second x1000

Feet per second x1000

BasaltGranite

Trap rock

TOPSOILCLAYGLACIAL TILLIGNEOUS ROCKS

Rippable Marginal Non-rippable

1 2 3 4 5 6 7 8 9 10

0 1 2 3 4

11 12 13 14

Hardness increases with velocity

Soft/diggable with a shovel

Figure 4. Excerpt from the D8R/D8T Caterpillar Chart (top) and the Team’s Classification System (bottom).

for example, basalt with P wave velocities between 0.8 and approximately 2.4 is rippable; basalt with P wave velocities above 2.4 is not.

The team’s classification system (Figure 4) assigns ranges of P wave values from the D8R/D8T Caterpillar Chart to one of three categories: hard, which is the same as “non-rippable” on the Caterpillar chart; “intermediate,” which combines the Caterpillar chart’s “rippable” and “marginal” categories; and “soft,” for rock that is soft enough to dig with a shovel and therefore useless for road construction. Each category includes a description of the physical characteristics of the rock.

This classification system likely is the first to correlate P wave velocity values from a Caterpillar Chart to local rock hardness characteristics. It may be expanded in fu-ture studies, as will be explained at the end of this article.

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The MASW technique (Figure 6) involves examining Rayleigh waves, which are a type of seismic surface wave. Rayleigh waves cause the ground to ripple and heave like the ocean, and this motion causes particles at the surface to move in counter-clockwise circles as the wave moves past (Figure 1).

Because Rayleigh waves are influenced by the shear strength or stiffness of the rock, their velocities can be used to derive S wave (shear wave) velocities. Those derived S wave velocities are then used to create a cross-section of the subsurface. S waves are a type of body wave that moves through the ground like a waving flag (Figure 1) and are also called secondary waves, because they are the second waves to arrive at a seismograph or geophone (P waves arrive first).

The team also tested two passive seismic techniques, one performed with an array of geophones and another performed with a single, three-component seismograph (three sensors built in one) called a Tromino seismo-graph. Passive techniques usually measure surface waves.

Electromagnetic Induction and Electri-cal Resistivity Techniques. Both of these techniques use the electrical properties of the ground to make inferences about its composition. One technique measures conductivity and the other measures resistivity.

The electromagnetic induction technique measures the ability of the ground to conduct electricity. In this technique, a transmitter coil sends an alternating current

GeophonesSeismograph

Shot

Direction of wave


Signal(arrives

�rst)

Time in miliseconds

Distance in meters

SignalSignal

Signal

Signal(arrives

last)

Figure 6. Multichannel Analysis of Surface Waves (MASW) Technique

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May 2015 Page 7

(AC) into the ground (Figure 7). That current generates an electro-magnetic field (electromagnetic fields are generated when electricity moves from one place to another, for example along the cord of a lamp when the lamp is turned on). This field, called the primary electro-magnetic field, causes the receiver coil to react by generating a secondary electromagnetic field. By measuring the size of the secondary field—in other words, determining how much the receiver coil responds to the primary field—it is possible to infer the conductivity of the ground.

The transmitter and receiver coils often are mounted at opposite ends of a long pole, which is carried across the ground. The drawback to this technique is depth: the shorter the pole, the shallower the depth. For example, if the distance between the centers of the coils is three me-ters, then the depth of the survey is approximately 3 meters.

The electrical resistivity technique measures the ground's resistance to transmitting an electrical current. In this technique, transmitter dipoles (a pair of equally and oppositely charged poles) induce an electrical current directly into the ground, and receiver dipoles are placed nearby. The difference in electrical potential between the trans-mitter and receiver dipoles is then measured and used to calculate resistivity.

At the Perry Creek site, the team used an instrument called an OHMMapper, which consists of transmit-

ConsoleTransmitter

coil

Receivercoil

Primary �eldSecondary �eld


Figure 7. Electromagnetic Induction Technique

Dipole cable

Transmitter diodes Receiver diodes

Dipole cable Weight

Non-conductivetow-link cable

Fiber-opticisolator cable

Current

Console

Drawing adapted from http://terraplus.ca/products/resistivity/ohmmapper.aspx

Figure 8. Electric Resistivity Technique

ter and receiver dipoles mounted on a cable or rope that is dragged along the ground (Figure 8). Similar to electromagnetic induction, the depth of penetration is determined by how far apart the transmitter and receiver diodes are placed.

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GPR Technique. The GPR technique makes use of radio waves, which are a type of electromagnetic wave. Electromagnetic waves can be arranged in a spectrum depending on their frequency (Figure 9). Frequency is the number of waves that pass a fixed point in a given amount of time, and is often measured in hertz, kilo-hertz, megahertz (MHz), or gigahertz.

With the GPR technique, high-frequency (for example, 12.4 to 1,500 MHz) radio waves are pulsed into the ground by a radar antenna on the GPR unit. When radio waves encounter an interface between areas with different dielectric (insulating) properties, such as soil horizons (layers of soil that differ from layers above and below), soil/rock interfaces, or man-made objects, a portion of the wave is reflected up and detected by the GPR unit (Figure 10). From hun-dreds of these measurements it is possible to map the properties of the subsurface.

The depth of the survey is affected by wave frequen-cy. Pulses of lower-frequency radio waves penetrate more deeply but provide less detail than pulses of higher-frequency waves.

Commonly, the GPR unit is pushed or pulled along the ground. Distance is measured by a wheel on the back of the unit, or by an additional GPS unit with a built-in antenna for positioning. Other types of GPR units can be mounted on a car or even a helicopter for aerial surveys. The team used a ground-based unit for this study.

Results

Of all the techniques tested, GPR provided the most detailed information for the depths at which most rock would be extracted, approximately the first 10 meters beneath the surface. In addition, GPR was the easiest technique to perform, although the instrument requires contact with the ground and may be difficult to deploy in some forested areas. The GPR technique had the

added benefit of being fully described in the American Society of Testing Materials (ASTM) standards, which makes its results more legally defensible.

Active seismic techniques (P wave refraction and MASW) provide good detail at the right depths; how-ever, results were less detailed than GPR (Figure 10). Of the two active seismic techniques, P wave refraction was better, because P wave velocities can easily be tied to rippability of various rock types and overburden material such as topsoil. The P wave refraction technique also is fully described in the ASTM standards.

Passive seismic techniques provided information for depths of 100-200 meters, but did not provide enough detail for the first 10 meters.

Transmitted wave

Interface between layers

Re�ected wave

Drawing adapted from http://www.c�hd.gov/resources/agm/engApplications/SubsurfaceChartacter/634DetectUnexplodedOrdnance.cfm

Wheel measures distance

Figure 10. GPR Technique

Radio wavesM

icrowavesInfra-redVisible lightUltra-violetX-rays

Gamm

a rays

Low frequency High frequency

Drawing adapted from http://www.darvill.clara.net/emag/

Figure 9. Electromagnetic spectrum

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Electromagnetic induction and electrical resistivity provided detailed conductivity and resistivity informa-tion (respectively) for the shallow soil layers, but did not penetrate deeply enough to provide adequate informa-tion for locating rock sources. However, both of these techniques provided more detailed groundwater infor-mation than either GPR or P wave refraction.

For the most complete and accurate information, it is best to combine techniques. The team recommended that DNR use both GPR and P wave refraction tech-niques to image the subsurface, determine the thickness of overburden soil, and locate good rock. At sites where groundwater conditions are a concern, these methods could be combined with electromagnetic induction or electrical resistivity techniques.

Figure 10. Comparison of P wave and GPR Technique Cross Sections

The GPR technique provides the most detail.

Validation

Up to this point, results were promising but theoretical. To validate the results, the team compared the GPR and active seismic data with results obtained through drilling at the Perry Creek site. The drilling results demonstrated the accuracy of these techniques.

For further validation, the team will conduct drilling at the other two sites (BB Pit timber sale on Tiger Moun-tain and Eastern Panhandle timber sale in the Elocho-man block). The team also will measure the groundwater depth at Perry Creek.

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Next Steps

The next step is to begin using these techniques (P wave refraction and GPR) in the field to locate rock for for-est roads. DNR recently purchased a GPR unit, which will be useful not only for rock source surveys, but for other applications such as locating underground pipes or culverts, surveying for cultural resources, or develop-ing more detailed geologic maps. Managers interested in using P wave refraction and GPR to locate rock sources are encouraged to contact Casey Hannell (360-902-1657 or [email protected]).

As these techniques are implemented, the team will continue to refine its classification system to make it more specific to western Washington geology. One goal is to expand the classification system to include more

geophysical parameters, including S wave velocities and electric properties. This information will be highly useful in defining boundaries between rocks of different quali-ties.

Over time, geophysical surveys and the new classifica-tion system should not only provide greater certainty for developing new rock sources, but significantly reduce the amount of drilling needed. Geophysical surveys are far less expensive than drilling, which could increase revenue to our trust beneficiaries.

This effort also demonstrated that working across divi-sion lines can be an effective way to solve one of DNR’s daily challenges. And that, indeed, is a better way to find rock for forest roads.

About the Geology and Earth Resources Division

The Geology and Earth Resources Division consists of geologists and support staff who serve the state’s gov-ernment and citizens by providing critical information and education about the geology of Washington State. Division geologists actively engage in geologic resource identification, regulation, and mapping. The Division is responsible for monitoring, assessing, and researching the causes of earthquakes, landslides, and volcanoes, and publishing information on geologic hazards to help government and private sectors reduce the human and financial effects of natural disasters.

About the Forest Resources Division's Earth Sciences Program

The Forest Resource Division's Earth Sciences Program provides technical and scientific support for state trust lands management activities in the fields of geology, geomorphology, and hydrology. Program staff work with foresters and engineers to assess the potential ef-fects of management activities on potentially unstable slopes, soil erosion, and hydrology, and to develop mea-sures to mitigate adverse impacts. Their work includes conducting landslide risk assessments for individual timber sales, performing hydrologic change analyses, developing landscape-scale landslide hazard zonation maps, locating suitable rock sources for constructing and maintaining forest roads, and carrying out earth scienc-es-related research and monitoring.

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For More Information

Basic Information About Seismic Waves

http://folk.uio.no/valeriem/spice/Frame/surfacew/index.html

http://en.wikipedia.org/wiki/Seismic_wave

http://www.colorado.edu/physics/phys2900/homep-ages/Marianne.Hogan/waves.html

More Information on Geophysical Techniques

http://www.cflhd.gov/resources/agm/engApplications/SubsurfaceChartacter/611DeterminDepthStructureFractureBedrock.cfm

http://asstgroup.com/techniques.html

http://www.terradat.co.uk/survey-methods/ground-conductivity-em/

http://terraplus.ca/products/resistivity/ohmmapper.aspx

http://www.negeophysical.com/

http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Meth-ods/Electromagnetic_Methods/Ground-Penetrating_Radar.htm

http://www.epa.gov/esd/cmb/GeophysicsWebsite/pages/reference/methods/Surface_Geophysical_Meth-ods/Seismic_Methods/Surface_Wave_Methods.htm

http://www.cflhd.gov/resources/agm/engApplications/SubsurfaceChartacter/623DeterminingRippabilityRock.cfm

http://www.geonics.com

Other References

Caterpillar Inc., 2010, Caterpillar performance handbook (40th ed.): Caterpillar, Inc., Peoria, Ill., 1, 442 p.

Cakir, R. and Walsh, T.J. (2011) Shallow seismic site char-acterization at 23 strong-motion station sites in and near Washington State. U.S. Geological Survey Award No. G10AP00027. (http://earthquake.usgs.gov/research/external/reports/G10AP00027.pdf)

Powers, M.H. and Burton, B.L., 2012, Measurement of near-surface seismic compression wave velocities using refraction tomography at a proposed construction site on the Presidio of Monterey, California; U.S. Geologic Survey Open-File Report 2012-1991, 17 p. (http://www.cflhd.gov/resources/agm/engApplications/SubsurfaceChartacter/623DeterminingRippabilityRock.cfm)

Vendor Websites

http://www.geometrics.com

http://www.tromino.edu

http://www.northwestgeophysics.com/

http://www.gfinstruments.cz

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In the Woods May 2015