Northeast Geophysical Services

Methods Offered Include: Resistivity Profiling, Borehole Logging, Electromagnetics, Magnetics, Ground Penetrating Radar, Seismic Refraction

Northeast Geophysical Services (NGS) has provided geophysical and geological services to industry and governments since 1978. NGS's experience with a broad range of clients combined with the strong geological background of its professional staff provides clients with a unique combination of skills and experience. The parent company, NGS, Inc., offers geological consulting services in addition to our geophysical expertise.

At NGS we are continuously upgrading our geophysical instrumentation and related computer software. Recently purchased equipment includes new seismic, resistivity and borehole logging systems. The company's two principals have over 45 years of combined experience with a broad range of geophysical and geologic applications. Our strong geological background provides for knowledgeable and realistic interpretation of geophysical data. Our personnel are OSHA certified for working on hazardous sites.

NGS has developed a reputation for providing excellent quality work at very reasonable prices. We operate primarily in the Northeastern U.S. including all of New England, New York, New Jersey and Pennsylvania. We have also recently done work in Antigua, Georgia, Virginia and Ohio. We have shown that we can be cost competitive even with other geophysical firms that may be much closer to the work site.

NGS welcomes inquiries regarding the appropriateness of various geophysical methods to individual problems and site conditions. Below we have included brief descriptions of several geophysical techniques including some of their typical applications and limitations.

NGS offers all of the geophysical techniques discussed below. For more information, please call, write or E-mail.

We look forward to hearing from you.

Northeast Geophysical Services
4 Union Street, Suite 3
Bangor, ME 04401

phone: 207-942-2700
fax:207-942-8798

ngsinc@negeophysical.com

Geophysical Methods & Applications

Introduction
Seismic Refraction
Seismic Reflection
Ground Penetrating Radar
Magnetics
Electromagnetics(EM-61, EM-31, EM-34, VLF and others)
Resistivity
Borehole Logging
Gravity
Induced Polarization
Marine Geophysics

Geology/Geophysics & Other Links

Introduction

The following was prepared by Northeast Geophysical Services (NGS) staff. It is provided to help clients and potential clients understand the range, capabilities, and limitations of geophysical methods in common use for environmental and engineering applications.

Medical doctors often use non-invasive testing such as X-rays, MRI's and electrocardiograms before operating. Why shouldn't a scientist or engineer do the same? Wouldn't it be useful to know the bedrock topography before establishing boring locations at a landfill site, for example? There's a great deal of useful and very cost-effective information that geophysics can provide which will enhance interpretation of the subsurface.

Seismic Refraction

depth to water table
depth to top of indurated till
depth to bedrock
fractures zones in bedrock
bedrock contour mapping
rippability

In engineering and hydrogeology, seismic refraction has many applications. Often, bedrock structure and topography control contaminant migration. Seismic refraction is a valuable tool for mapping bedrock troughs and fractures. It is usually more cost-effective and gives better coverage than drilling alone.

Constraints: Layer velocity (density) must increase with depth. Layers must be of sufficient thickness to be detectable. Data collected directly over loose fill (landfills) or in the presence of excessive cultural noise will result in sub-standard results. Single narrow fractures are too small to be detected.

Method: The seismic refraction method utilizes sound waves. Sound travels at different velocities though different materials and is refracted at layer interfaces.

A seismic wave is usually generated using a sledge hammer, a specially designed seismic gun that employs a blank shotgun shell, or a seismic weight drop tool. The wave's travel time from the sound source to refracting layers, along those layers and back to detectors (called geophones) is precisely measured. From the time-distance relationships, subsurface layer velocities and thicknesses can be calculated.

Fracture zones can often be detected because they usually have a lower seismic velocity than solid bedrock. The velocity of sound through water saturated material is about 5000 feet per second while the velocity though crystalline bedrock generally ranges from 12,000 to 18,000 feet per second. By calculating the velocity of sound along the bedrock surface, low velocity zones, which may represent fractures, can be delineated.

Our system utilizes up to 24 geophones at one time. The geophones can be linearly spaced any distance apart, but most often are spaced 10 to 50 feet apart. In general, the greater the expected bedrock depth, the greater the geophone spacing. Shorter spacings and sometimes radial patterns are used in fracture zone detection studies.

Northeast Geophysical Services has both 12- and 24 channel seismographs made by EG&G Geometrics. The 24-channel instrument has an internal computer. Seismic field data are stored on 3.5 inch disks for later computer analysis.

Final results are provided to the client in a full report which includes tables of subsurface depths and elevations, and cross-section profiles. Results are also available as bedrock contour maps and on computer disks.

Index

Seismic Reflection

graphically depicts subsurface stratigraphy and bedrock profiles
can often differentiate unconsolidated units such as sand, clay, or gravel
marine surveys

Seismic reflection is useful for graphically profiling subsurface stratigraphy. It is used to map clay and sand lenses and bedrock troughs. Applications of this technique include determination of depth to bedrock, aquifer location studies, and mapping of overburden stratigraphy.

Constraints: Reflection surveys are highly site specific. A shallow ground water table is required. Reflection surveys are useful for exploration depths of 50 feet to several hundred feet.

Method: Seismic reflection is a geophysical technique in which acoustic waves, reflected directly from underground surfaces with density contrasts, are used to map soil and bedrock stratigraphy.

Successful application depends upon the ability of the ground to transmit high frequency seismic energy (saturated clays, for example, transmit high frequency energy quite well). The method overcomes some of the potential problems encountered in refraction surveys (such as the assumption that subsurface layer velocities increase with depth and that layers are thick enough to be detectable).

The equipment used is nearly identical to that used in a seismic refraction survey. The field technique, however, differs and ground coverage is usually slower than with a refraction survey.

The client is provided with a full report, including graphic profiles of subsurface structures.

Index

Ground Penetrating Radar

locating underground storage tanks
mapping utilities and re-bar
determining fill thickness
mapping geologic strata, aquifers, aquicludes, voids, shallow bedrock units, and fractures
archaeology
site clearance for drilling

Ground penetrating radar (GPR) provides a graphic image of the subsurface and has a variety of applications. GPR is commonly used as part of Phase II environmental site assessments and other environmental studies to locate underground features. GPR is extremely useful for determining the location, depth of burial and orientation of tanks, pipes, utilities and other objects.

Constraints: Exploration depth can be limited by soil or water with high conductivity. Detectability depends upon a dielectric contrast between the subsurface feature and the surrounding material. Closely spaced survey lines are required to locate small objects. A relatively smooth surface is also necessary.

Method: GPR utilizes high frequency radio waves to probe the subsurface without disturbing the ground surface. GPR data is collected continuously as the instrument is towed over the ground surface. Radar pulses are transmitted downward from an antenna and are reflected from underground surfaces. The reflected signals return to a receiver creating a continuous graphic profile of the subsurface. Reflecting surfaces appear as bands on the profile.

Reflection of radar waves occurs at interfaces having contrasting electrical properties (which are controlled largely by composition and moisture content of the material). Examples of reflecting surfaces are soil horizons, soil-rock or air-rock interfaces, water tables, and solid metallic or non-metallic objects.

Radar penetration and resolution vary with the antenna used, as indicated in this table. They also vary with soil and rock conditions.

     Antenna     Penetration    Resolution

     900-MHz      ~0-5  feet       high

     500-MHz      ~0-15 feet       high

     300-MHz      ~0-30 feet       average

     80-MHz       ~0-80 feet       low

The instrument used by Northeast Geophysical for GPR surveys is the SIR System-3 with 300-MHz and 500-MHz antennas manufactured by Geophysical Survey Systems, Inc. Our system is equipped with a color video monitor and a magnetic tape recorder.

The end product is a continuous graphic profile as shown above. When travel time/depth relations are known, a depth scale can be substituted for the travel-time scale on the profile allowing estimation of absolute depths.

The results of a GPR survey are usually presented to the client in a full report which includes an index map showing locations of traverses, interpreted radar anomalies, and if requested, graphic profiles.

Index

Magnetics

location of buried tanks and drums
geologic mapping
mineral exploration
archaeology

Magnetometer surveys are rapid and efficient. Magnetometers can be used to detect buried ferrous metal objects (tanks or drums) or bedrock features with contrasting magnetite content. Detection depends on the amount of magnetic material present and its distance from the sensor. A single steel drum can be detected at burial depths up to 15 or 20 feet. Burial depth can be estimated from magnetometer data collected using the gradient method.

Constraints: Utilities, power lines, buildings, and metallic debris can cause interference. Solar magnetic storms may cause fluctuations in readings. The size and depth of objects affect detectability.

Method: When a ferrous material is placed within a magnetic field such as the earth's, it develops an induced magnetic field. The induced field is superimposed on the earth's field at that location creating a magnetic anomaly.

A magnetometer survey for hydrogeologic and engineering applications is conducted on foot, by one operator. The survey can be along single lines or along a series of parallel traverses with readings taken every 5 to 50 feet. Spacing of traverses and readings depends on the width of the expected anomaly. For instance, tank searches may be conducted at a 5-foot spacing while geologic mapping may be conducted at a 50-foot spacing.

In the gradient method, the total field is measured simultaneously at two elevations by using two sensors on a staff separated by a fixed distance. The difference in magnetic intensity between the two sensors divided by the distance between them is the vertical gradient. This technique reduces interference from solar magnetic storms and regional magnetic changes. This technique is particularly useful for locating small, shallow objects and is also useful for estimating burial depth of objects.

The results of our magnetometer surveys are presented to the client in a full report that generally includes magnetic profiles, and an index map showing locations of magnetic anomalies. Magnetic contour maps and other graphic presentations are also available.

Index

Electromagnetics

ideal for locating contaminant plumes
locating buried metal objects
lateral and vertical conductivity mapping
periodic landfill perimeter surveys to monitor groundwater quality
baseline monitoring for proposed landfills
locating bedrock fractures

There are a variety of inductive electromagnetic (EM) methods that measure subsurface electrical properties. Three common applications of these methods are terrain conductivity measurements, metal detection, and bedrock fracture detection. EM is also used in borehole geophysical logging.

Terrain conductivity surveys are ideal for locating contaminant plumes from salt piles, landfills, and other sources. They can also provide baseline conductivity data around landfill sites prior to construction, and periodically after landfill establishment, to detect and monitor any contaminant plumes. Metal detector surveys can locate buried or hidden metal objects. They can also complement terrain conductivity or GPR surveys by mapping buried waste. NGS often recommends a combination of EM-61 and GPR techniques for tank, drum and pipe searches. Bedrock fracture detection can be accomplished using very low frequency EM (VLF-EM). Locating fractures is often useful in groundwater supply or contamination studies. EM surveys are non-invasive, rapid and economical, making them well-suited for hazardous waste and hydrogeologic studies.

Constraints: Measurements are affected by power lines, metal fences, metal debris, and utilities. Fracture detection is affected by overburden thickness, soil conductivity, and orientation and dip of the fractures.

Method: Electromagnetic induction surveys (terrain conductivity and metal detector) work by inducing current into the ground from a transmitter coil. The resulting secondary electromagnetic field set up by any ground conductors is then measured at a receiver coil. The presence of metals, ions, or clays increases the ground conductivity. Conductivity readings are reported in milliSiemens per meter (equivalent to millimhos per meter). Metal detector readings are generally reported in parts per thousand of the total field.

Detection depth of EM instruments is a function of the transmitter-to-receiver coil separation and the coil orientation (horizontal or vertical). Small coil separations, as in metal detectors and pipe locators, may "see" 2 to 6 feet into the ground. Larger coil separations can be used to detect conductive materials up to several hundred feet deep.

The EM-61 Metal Detection System is a portable time domain instrument with a coincident transmitter/receiver coil and second parallel receiver coil for depth to target estimation and rejection of surface metal response. The EM-61 was designed specifically to locate medium to large buried metal objects such as drums and tanks while being relatively insensitive to cultural features such as fences, buildings and power lines. The technique can detect a single 55 gallon drum at a depth of over ten feet. The size and burial depth of the metal will determine the strength of the response. Preliminary metal detection maps can usually be produced on-site immediately following data collection. EM-61 can be used as a stand-alone technique for buried metal detection or in conjunction with another technique, such as Ground Penetrating Radar (GPR), for overall site clearance or for drum, tank and pipe location.

Very low frequency EM (VLF-EM) is an inductive technique which measures very low frequency horizontal EM signals from remote military transmitters. Localized conductors, such as water-filled fractures, cause angular disturbances in this signal which are measured with the VLF-EM instrument (in degrees from the horizontal). VLF-EM can best detect linear, steeply dipping conductors oriented in the direction of the transmitter. Detection depth depends largely upon overall ground conductivity, but is commonly over 100 feet.

Northeast Geophysical has several electromagnetic instruments including Geonics EM-31 (with an exploration depth of 18 feet), EM-34 (with a variable exploration depth of 25 to 180 feet), EM-16 (VLF) which is used mainly to locate bedrock fractures, EM-61 (Metal Detection System)and a Fisher M-Scope Metal Detector.

Index

Resistivity

lateral and vertical mapping of contaminant plumes
location of water-filled bedrock fractures
location of gravel aquifers
vertical electrical soundings for stratigraphic information
mineral exploration

The electrical resistivity method is used to characterize vertical and lateral changes in subsurface electrical properties. Vertical changes are measured using the vertical electrical sounding (VES) technique. Lateral changes are measured using the resistivity profiling technique.

Resistivity profiling is used to map spatial changes in subsurface electrical properties. Applications of a resistivity survey are similar to those of electromagnetic (terrain conductivity) surveys. Resistivity profiling is most commonly used to map contaminated groundwater plumes.

Vertical electrical soundings (VES) are used to laterally trace clay layers, and in conjunction with borehole data, to characterize electrically distinct layers.

Constraints: VES: Soundings are affected by changes in surface relief and lateral changes in resistivity. The electrode array length is about 10 times the depth of investigation. Profiling: Resistivity profiling is slower and more expensive than EM surveying.

Method: An electrical current is introduced directly (as opposed to inductively as with electromagnetic surveys) into the ground through a pair of electrodes. The resulting voltage difference is measured between another pair of electrodes. The subsurface apparent resistivity is then calculated. Resistivity is the reciprocal of conductivity. Thus, measuring resistivity provides information on ground conductivity.

The vertical electrical sounding (VES) method measures vertical changes in the resistivity of the geological strata. In the field, a series of resistivity measurements are made at various electrode spacings centered on a common point. Sampling depth is increased by increasing electrode spacing.

Data is interpreted using an appropriate computer program. The client is provided with a full report which includes modelled depths, thicknesses and resistivities of subsurface layers.

In the resistivity profiling method, four electrodes are positioned at a fixed distance from each other. A current is introduced between two of the electrodes and a voltage potential is measured between the other two electrodes. The electrode pairs are moved along a surveyed line and the electrical measurements result in a horizontal profile of apparent resistivity. Different electrode spacings can be used to yield a cross-section of resistivity changes with depth.

The final report includes subsurface apparent resistivity values from profile stations. Results may be plotted as profile lines or contour maps (isopleth resistivity map), or in other presentations according to the clients' needs.

Index

Borehole Logging

vertical mapping of overburden and bedrock stratigraphy
location and orientation of bedrock fractures
monitoring borehole conductivity changes over time
accurate location of well screens
cross-hole studies

Borehole geophysical logging techniques used together with surficial geophysics, well data, and knowledge of the local geology, can be essential in solving problems in groundwater hydrology.

Boreholes are often logged to identify bedrock fractures or fracture zones, for setting packer test intervals, and setting well screens. Another common application is mapping the thickness and continuity of aquifers.

Constraints: Limitations vary with the method used. The type of casing and borehole diameter are factors to consider.

Method: Borehole geophysical logging includes the measurement of various chemical and physical characteristics of materials and fluids in and around a borehole.

The determination of which parameter(s) to measure depends upon the conditions of the borehole and the information sought. Some of the more common borehole logging techniques used in groundwater hydrology are listed and explained in the table on the following page.

Log interpretation includes identification of potential fracture locations and distinctive hydro-geophysical units which can be compared with fractures and rock or sediment layers identified in the descriptive log. Data are collected and stored digitally, and can be presented at any scale requested.

The following table gives brief descriptions of some measurements that are commonly logged in boreholes, and their applications. Our equipment can accommodate all of these and other measurements.

Borehole Geophysical Logging
LOG	PARAMETERS MEASURED	APPLICATIONS
CALIPER	Borehole or casing diameter.	Fracture identification, lithologic changes, and well construction.
NATURAL GAMMA	Natural gamma radioactivity.	Lithology and estimation of clay content in overburden.
FLUID TEMPERATURE	Temperature of borehole fluid.	Indicates geothermal gradient, and water flow in borehole or between borehole and fractures.
FLUID RESISTIVITY	Resistivity of borehole fluid.	Indicates water flow within borehole, or between borehole and fractures; and water quality.
SINGLE POINT RESISTANCE	Resistance of materials between probe and ground surface electrode.	Lithology, fracture identification, and location of well screens.
NORMAL RESISTIVITY	Apparent resistivity of material.	Lithology, and water quality.
SPONTANEOUS POTENTIAL (SELF POTENTIAL or SP)	Electrical potentials between probe and surface electrodes.	Lithology, water quality, and in some cases, fractures in resistive crystalline rock.
EM CONDUCTIVITY (INDUCTION)	Electrical conductivity in medium surrounding borehole.	Location of contaminant plumes, conductive clay units, or bedrock fractures. Monitor water quality changes over time.
FLOWMETER: IMPELLER or HEAT-PULSE	Continuous or point measurements of water flow in borehole.	Identification of permeable zones and apparent vertical hydraulic conductivity and flow direction.
BOREHOLE VIDEO	Provides visual record of lithology, fractures, well construction.	Lithologic logging; identification of fractures; examination of casing or well construction.
ACOUSTIC TELEVIEWER	Provides acoustically-generated image of boring walls.	Structural logging; identification and orientation of fractures and foliation; examination of casing or well construction.
OPTICAL TELEVIEWER	Provides optically-generated image of boring walls.	Lithologic & structural logging; identification and orientation of structure & lithologic changes; examination of casing or well construction.

Index

Gravity

bedrock topography under landfills
mapping large metallic mineral deposits
locating subsurface caverns
locating contacts between geologic units of differing mass and density

Gravity surveys can provide useful information where other methods do not work. For example, gravity may be used to map bedrock topography under a landfill, where seismic refraction is limited. Gravity can also be used to map lateral lithologic changes, and faults.

Constraints: Gravity surveys are relatively slow and expensive. Detectability varies with target size, depth and density contrast. Interpretation of data often requires control data from drilling, outcrops, or other sources. Detailed surface topographic survey data is also required.

Method: Gravity is the attraction between masses. The strength of this force is a result of the mass and distance separating the objects.

A gravimeter is used to measure the earth's gravitational attraction at various points over the area of interest. Gravity anomalies are due to differences in density of underlying materials. Gravity anomalies are extremely small relative to the total field and are usually measured in micro-Gals (one micro-Gal is about 1 billionth of the earth's total gravitational field). The equipment used in a gravity survey is extremely delicate and precise. Data interpretation is time consuming even with the use of sophisticated computer programs.

Results are presented in a full report which includes profiles or a contour map and gravity data tables.

Index

Induced Polarization

detection of disseminated metallic minerals
discrimination of clay from silt or sand where formation resistivities are similar

In nature, the induced polarization (I.P.) effect is seen primarily with metallic sulfides, graphite, and clays. For this reason, I.P. surveys have been used extensively in mineral exploration. Recently, I.P. has been applied to hazardous waste landfill and groundwater investigations to identify clay zones. As with electrical resistivity surveys, vertical or horizontal profiles can be generated using I.P. I.P. can also be used in borehole logging.

Constraints: I.P. cannot be done over frozen ground or asphalt because good contact with the ground is required. I.P. is affected by changes in surface relief and lateral changes in resistivity. The electrode array length is about 10 times greater than investigation depth.

Method: Induced polarization is the capacitance effect, or chargeability, exhibited by electrically conductive materials.

Measurement of I.P. is done by pulsing an electric current into the earth at one or two second intervals through metal electrodes. Disseminated conductive minerals in the ground will discharge the stored electrical energy during the pause cycle. The decay rate of the discharge is measured by the I.P. receiver. The decay voltage will be zero if there are no polarizable materials present.

Generally, both I.P. and resistivity measurements are taken simultaneously during the survey. Survey depth is determined by electrode spacing. The final report products are similar to those of resistivity surveys.

Index

Marine Geophysics

harbor and river bathymetry
bridge footing scour surveys
pre-construction dam, bridge, and dock surveys
marine pipeline and cable routing
pre- and post-dredging surveys
marine geologic studies
downed aircraft and sunken vessel searches
marine archaeology

Geophysical methods routinely used in the coastal and inland marine environment include bathymetry (seafloor elevation mapping), sub-bottom profiling, seismic reflection, ground penetrating radar, side-scan sonar, magnetometer surveys, and underwater video.

Methods: Bathymetric surveys provide a depth contour map of the sea-bottom using a recording echo sounder (~200 kHz) and an integrated navigation system. Digitally stored data can be imported into standard contour programs to provide bathymetric maps at any requested scale and contour interval. These surveys are often done in conjunction with other surveys such as sub-bottom profiling.

Sub-bottom profiling works on the same principal as bathymetry. The main difference is in the frequency of the sound source used (1-20 kHz). The lower frequency allows deeper penetration into the sea floor than high frequency signals, but with poorer resolution. The "Chirp" system uses a range of frequencies and has the advantages of both deep penetration and good resolving power.

Side Scan Sonar This is a system which provides a sonic "photograph" of the sea floor. A sound source is transmitted from a towed "fish" in a manner which results in a chart printout. The printout graphically depicts the sea floor features underneath and to both sides of the towed fish. Such features as bedrock, ripple marks, depressions, rocky bottoms, smooth bottoms, boulders, and sunken ships, are well displayed on the printout.

Generally, side scan frequencies range from 100 kHz to 500 kHz. The high frequency transducer (500 kHz) resolves features as small as lobster traps and anchor drag marks.

Ground Penetrating Radar (GPR) is described elsewhere in this manual for land applications. It can also be used effectively in shallow fresh water situations where sub-bottom information up to 10 feet below the bottom is required.

Marine magnetometer surveys are conducted by towing a marine magnetometer near the bottom of the water body. As with magnetometers used for land surveys, the marine magnetometer will identify ferrous metal objects. Marine magnetometer surveys are often used to locate buried archaeological sites.

Underwater video can be used for inspection of pipelines, cables, and other objects. The towed video camera can be used where sufficient visibility exists. The camera is housed in a towed vehicle, and can be maneuvered remotely from the surface.

Navigation With the recent development and availability of the differential global positioning system (DGPS) and the ability of DGPS receivers to interface directly with most marine geophysical equipment, excellent navigation control and accurate plotting of geophysical data has become fairly routine. Horizontal accuracy of 5 meters or better is often possible with standard DGPS and sub-meter accuracy is possible using more elaborate setups.

Index