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G eophysi c al Meth o ds of
Expl o ration and their Application
to Geological Problems
in N • + e w Jersey
G EO R ¢_. 13. W OO LI A R D " S
CHA S I_S P. W I LBE R , Director an d C h ief o f t h e
- + Divi s ion of Forests a nd Park s
M _m g v rr H E. Jo Hns o N , C h ief of t h e Div is ion of Geology and To p ography
Treuton , N. J.
19 41
Ge o physica l M ethods of
Exploration and their Application
to Geological Problems
in New Jersey
by
GE O RGE P. WOOL L ARD
CHAR L ES P. W t LREa , D ir e cto r and Chief o f t he D ivisi o n o f Fo rests and P arks
ME R E DIT HE , JO H N S ON _ Ch ie f o f th e Div isio n of Geo l o g y a nd Topo g rap h y
Tr ento n , N. J.
1941
May 20 , 1941.
Mr. Charles P. Wilber , Director,
Department of C onservation and Development,
Trenton, N. J.
Sir :
It gives me g reat pleasure to recommend for publicati o n the , ecompanying report on , "Geophysical Methods of Exploration and Their Application t o Geological Problems in New Jersey" by George P. Wooltard. Thoug h busily engaged on research prob- lems financed b y t h e Guggenheim Foundat i on and th e Geological Society of America, Dr. Woollard upon my urgent request con- sented to write this report as a contribution to the State in which he resides, i feel that we are greatly indebted to him , both for the time he has generously donated , and 2 or this enlightening report on the uses and value of modern geophysical methods o
Yours very truly,
MER E DITH E. JOHNSON , Ch{ ef of the Division of Geology a n d Topograph 3 ,
[
At the request of Meredith E. Johnson , State Geologist of
New Jersey , the writer has undertaken to laake a brief report on
the various geo p hysical meth o ds u sed at the present time in s u b.
surface geological investigations , and the extent to whic h they
have been applied in New Jersey.
tn presenting this report a three-fold division has been made ;
Part I defines the term geophysics; describes its applications ,
and its particular use in geological work. In addition , its role i_ the finding and developing of the nation ' s natural resources is
considered , as well as the relative cost of different types of geo-
physical wm-k. Part II is a brief smnma r y of the various methods
of geophysical examination that are now being applied to geologi-
cal problems , and Part III deals with the actual investigations
lhat have been made in New Jersey.
As the description of techniques employed is of necessity ex- treme]y brief , a bibliography of published works describing the
various methods in detail has been appended for the use of those desiring more information.
GEORGE P. WOOLLARD
[ 7 ]
I LLUSTRATIONS
P A G E Fronti s p i ece, Outline m ap o f New Jersey showing location of geophysica l work done in New Jersey .............................. 12 P late I ........................................................... 2 6 Fi g. I. T he ear t h as a magnet A. Magne t i c fi eld su r rounding a ba r magne t , B. Magnet i c fi el d surrou n d i n g the e arth and its rel a t ion t o t he geographic coo rdinates 2. A nomalous distribution o f h o rizontal arzd verti c al comp o nents o f the earth's ma g n etic fi e ld o, : er a magne t i c ally disturbed area
Pla t e V ............................................... : .......... 42
PAGE Fig_._ 17. I sostatic a no maly gravitational m a p of New Jersey based tlpofl U. g. Coas t and Geodetic Sur v ey values ................. 50 t8. Typical s e ismograms f r o m Norm_ and L in co ln on C a p e May seismic tra ve rse ....................................... 6 0
_ tN T R O D UC TI ON Although the State of New Jersey has no oi_ciat program of geophysical investigations of geological problems , considerable work has been done in various parts of the State by other agents and agencies with which the State has cooperated through the Divisio n of Geolog y and Topography of tile Depart m ent of Cou- servation and Development. Tile writer , both individually and in collaboration with Dr. Mauriee Ewing and Mr. A. C. Vine of Lehigh U u iversi t y 1 108 m ade su c h b_vesti g 'a t iotts , as have the Uni t ed States C oast and G eodetic Sur v ey and the A meri c a n Te l e- phone and Telegraph Company. Practically all of the work which has been done to date has been of an exploratory character de- signed to test the latent possibilities of so m e of the various geo- physical methods for solving some of the many geological prob- lems presented in tlle State. Thomas A. Edison was one of the first investigators to use geo- 1, bysical measurements i n New Jersey , a_)d he carried out ex- tensive m agnetic measurements over some of the iron o re-bodies in the New Jersey Highlands. However, no other work was done a l ong these lines, either on the ore- b odies where they had proved highly successful or on other geological problems , until recent years. This lapse of activity was due in part to a lack of per- sonnel and funds by the State Geological Survey. A contribut- ing cause may have bee n the attitude of the public which dis- trusted all things that waxed of the mysterious. The determina- tion of the presence of ore beneath the surface of the ground v , 'ithout diggi n g a shaft was a venture smacking of the mysteri- ous a_ c t tberefore no t o_e f o r which the public funds were t o be spent. This attitude is still prevalent in some parts of the coun- try , but tbanks largely to the oil industry which relies to a large extent upon geophysical methods in exploring for new oil fields , the public is beginning to accept their use as both normal and right. This increasing use of geophysical methods by other than the oil industry is exemplified by their world-wide use in solving
f
mining problems , their adoption by the U. S. Bureau of Public Roads iu laying out new highways; their use by the War Depart- ment in studying dam sites; by the United States Geological Sur- vey in studying everything from water-supply problems to in- volved problems in structural geology; by the United States Coast and Geodetic Survey in mapping the ocean bottom off our shores; by the Navy in locating the proximity of possible enemy submarines; by the Merchant Marine in determining the depth of water beneath a ship; by the A rmy in locating the position of hidden artillery miles away; by the Air Corps in locating planes even when they can ' t be seen; by the meterologicai ser- vice in forecasting the flying time for planes even before they leave the ground; and finally, by our power companies , and the telephone and telegraph companies , in the less glamorous work of determining where t o place r epair crew s t o "shoot tr o uble" even before it happens. It is , therefore , not surpris i ng that through geophysics many mines formerly considered worked out and dead have been re- opened, that unsuspected natural resources worth millions of dollars have been discovered iu areas where the surface geology gave no clue o f what lay beneath , and that other areas formerly desert are now verdant due to the discovery o f buried water sup- plies now used f or irrigation. Since geophysics is obviously a very broad ternl as evidenced by the ab o ve list o f its appli c ations , we mu s t define it so that an understanding can be had as to what is meant when the term is used.
GEOPHYSI C S DEFINED
Although geophysics can be briefly defined as the science which treats of the physics of the earth , it includes many diverse fields faufiliar to us under various na m es. Some of these branches are meteorology , the science devoted to t he study of weather phenom- ena which is so important to those using and working in the airways; hydrology , the science dealing with the flow of under- ground and surface waters; oceanography , the science dealing with ocean phenomena such as variations in salinity_ currents and temperature , and which control not only the temperature of our coasts and the sea-food available , but also serve as a first line of defense for our country. This defense aspect has been brought to the notice of the public only in the last few years by the publica- tion of maps of the ocean bottom off our shores Which, used in conjunction with sonic depth-finders , permit ships to know aeeur-
knowledge a b out the rocks beneath the surface, the "practical" lung , faced _or example with the problem of obtaining a ground- water supply , usually consults a well driller or even a water diviner rather than a geologist. That geology at the present writing is neither as broad in its sco p e as the first definition or as limited as the second concept is the result of two opposite trends. The first , which is primarily respousible for the average layman's concept of the present status of geology , is due to the f_ct that abo_t 1900 or a little later , a good deal of the United States had been mapped geologically--at least on a reeonnsissance basis. That is , maps had been drawn which showed the areal distribution of the various kinds of rock at lhe surface and these had been classified as to their geologic
- _ge and corre l ated with similar rocks in other areas. A s very few people had been particular about keeping an t_ccurate record of the material encountered in drillb_g and digging wells , only a very sketc h y idea was available as to what lay beneath the sur- face. True , the dip of outcrops had been o b served and the thick- hess of stratigraphic beds beneath the surface computed, bat this method could not take into account the presence of buried erosion surfaces or all the changes in dip that occu r bel_eath the surface. Therefore it is not surprising that the citizens of many states felt that t helr Geological Surveys had fulfilled their purpose when the surface geology had beeu m_pped , since obtaining a knowledge of the subsurface formations did not appear to be feasible irom an economic standpoin t. A s a result , the appropriations made to many of the Surveys were drastically reduced and geologic re- search was necessarily curtailed. Th e result o f this p o licy is that , today p e opl e arc haviug to spend th o usands of dollars annuMly in unplanned and often ill-advised private prospecting to get inf o rmation which s n o t macle available to others and which usually is soon l o st or forgo t ten. The following examples serve to illustrate the futility and waste of such a policy. In selecting a factory site it is necessary to know the founda- tion conditions as well as the quantity and quality of the avM- able water suppIy. That information is seldom available md has to be obtained--often at ver" consider_ble cost--by the parties interested. So , also , the foundation data needed for the construe- tion of bridges and dams must be o b tained b y the individu3ls con- cerned. Towels along the coast decide without benefit of geologic advice upon munlcipa] improvemet_ts sneh as deepening their yacht basins and cud by ruining their water supply. People buy charming homesitesNEW JERSEY GEOLOGICAL SURVEY i_1the country without knowiug whether they
can get water for a hundred dollars or whether it will cost them thousands_._ Others sink thousands of dollars in local oil wells with no real knowledge as to whe t her they have a chance of getting their money back or not. States set up heavily financed councils and commissions to entice industry to build plants withi n their b orders without even knowing .what potential mineral wealth they have to o ffer or if they can furnish the newcomer with a water supply. To be sure they tell of the railways and rivers serving various localities, the abundant labor, the cheap fuel, the low taxes; but there arc few areas where the geologic factors which also enter into whether a venture will be a success or not are known with the same d e gree o f certainty. These examples show the present need for a knowledge of what is beneath the surface of t h e ground as well as what is at the surface. Formerly mineral prospects were found on the basis of surface outcrops and their subsurface extension had to be followed by a very expensive program of shafting and drilling. Similarly o il and coal deposits were located by gas leaks and outcrops , al- t ho u gh structural g e o l og ic intelTretations of surface outcrops were later applied iu locating possible oil pools. Occasionally then, as now , promoters persuaded people to drill for oil purely o n a h unch a n d New Jersey has q u ite a f e w suc h "oil" we lls which cost thousands of dollars. Contrast these methods, how- ever, with the prcse_lt ones used by the oil industry; with the methods of development used on mineral prospects in the mining areas of Canada , and on the great gold deposits of South Africa.
except as a final step. The deposits studied are from a few hun- dred feet to thousands of feet beneath th e surface, a nd there are few if an y surfa c e indica t im_s o f their p rese n ce. These investiga- ti o ns are being done cheapl y and economically--in fact they cost less than the few drill holes that are sunk to verify the indica- tions obtained b y instruments which in 1 9 00 or even 1 9 20 were not thought of in connection with geology. These instr u m e n t s have all been borrowed from the various branches of geophysics and have been adapted to the problems of geology. Magnetome- ters are being used to study not the broad relations of the earth's magnetic field as in Terrestrial Magnetism , but to study the many local magnetic disturbances in order to determine whether they 'are due to a sheet of trap rock, an area o_ serpentine, a gabbro mass, or a po t e n t ia l ir on o r e dep o s it. Seism o graphs are used n o t to study earthquakes , but rather the speed of sound through rock from man-made earthquakes caused by the explosion of charges
the surrounding rock, au indirect approa c h is frequently neces- sary. Thus oil is not hunted directly, but iustead the geologic structural conditions are s o nght which will permit oil, if present, to segregate int o wbat are commonly known as pools. That is, a structural trap is bunted which involves a displacement of one or more "key" geologic horizons which are known from well data t o bear a definite relation to oil reservoir sands. This same sort of technique is also used in locating the position of artesian water sands since it is difficult to work with the water directly except in special cases. Of course the information about the ge o logic structure obtained by geophysical methods can als o be obtained from bore hole and well data, but it is generally e conomically impossible to sink a m u ffieient number of deep test wells or bore holes in an mmxplored area to show what is beneath the surface far even a small area--let al o ne a large one--and as a result this method is g e uerally reserved to test geophysical indications or to develop kn o wn mineralized areas.
CHO I CE OF _ , / [ETHODS ]:_E S T ADAPTE D TO EACFI PR OB L E M
In cho o sing the g eoph ys i c al nlethod to be appl i ed to any g eo- logical problem the general nature of the problenl has to be con- sidered first. For instance , if a survey were being carried ou t t o determine the possible iron ore reserves in the New Jersey High- lands , the magnetic method of prospecting would imm e diately suggest itself since these ores are known to b e highly magnetic. They could also be mapped quite st_tisfact o rily by using any of the electrical methods s ince the or e is lar g ely magnetite , a min- eral which is a good electrical c o nductor as compared with the enclosing country rock. Als o , since magnetite has a much higher density thau the e o mltry rock , these deposi t s might be slapped by gravitational methods. Electro-magnetlc methods could prob- ably be used as well. In this area, though , the first choice would be the msgnetie method, since the greatest difference in instru-. ment readings would be expected with this method, For example, the magnetic susceptibility of magnetite is 97 ,3 50 c.g.s, units , while that for the average country rock is only around 1 0 00. In other words a factor of change of about 9 7 may be expected in the readings as one passes from country r o ck over a magnetite ore-body. In comparison, the fact o r o f change for the electrical method would be about 8 t o i and for the other nlett_ods men- tioned it would b e even less. Consider n o w an artesian water supply problem: say that oi findin g the depth to a certain water horizon. Two geophysical
methods of solution suggest themselves. One would be to t ake advantage of the possible change in electrical properties of the water as compared to the country rock , and the other would be n indirect method , such as the seismic , whereby some horizon whose position relative to the artesian stratum is kno_l and could be located. Since the seismic method would require the use of dynamite as well as much more expensive equipment, the electrical method w o uld be the cheaper and would probably be tried first; but if the locality being studied were near a city or industrial area wit h ground current dlsturhances due to power leaks or street railwa y s , the seismic method might be the one finally adopted since bad ground current conditions might make accurat e and reliable electrical measurements impossible. ,Similarly in the case of an operating mine where it is desired to map the extension of some ore-body it mig'ht prove necessary to use a less favorable method than the natural one indicat e d due to the susceptibility of the more favorable method to external influences. For exmnple, the gravitational m e thod might be selected to follow an ore-body that is naturally magnetic. In this case the tracks , motors and underground mining equipment might prove as msgoetic or m o re so than the ore-body and thus give : false picture , or fluctuating magnetic fields might be o b tained due to the use of direct current in the mine. O ther examples might be cited, but the above give an idea of some of the factors controlling the choice of methods other than what might be called the "natural selection".
In brief, the natural selection of geophysical methods involves the use of the magnetic method on problems involving naturally magnetic b o dies, and the use of gravitational methods ou prob- lems involving changes in mass distribution such as the location of salt drones in the G ulf Coastal Plain, etc. However , there are .other factors which have to be considered in making a choice of methods. For instance , in the areal delineation of subsurface bodies the gravitational method has been highly successful , but these data give very little absoh;te information coucerning depth. Similarly the magnetic method is useful in locating magne t ic bodies , gives some informstioa on their depth , b ut the exact areal extent of the bodies can not always be determined because of polarization effects. Electrical methods cm_ be used as can also the seismic methods for determiniug both areal and depth meas- ureme n ts, b ut b oth these methods are handicapped by slowness of operation and the electrical methods are also suscepti b le to
of operation and large areal coverage per unit of time. The mag- netometer and the gravity meter fall into this class. In the oil industry, areas showing favorable structural indications obtained with these instruments are generally explored with the more expensive seismic methods before any drilling is done. However , in most other work an effort has to be made to hold down the number of methods used to a mininmm and this is done _hrough a study of the pro b lem and all the related factors including the c o st per unit of area to be covered. The following figures give an idea of the relative cost-coverage values for the various methods. The values are only approximate and are b ased upon data taken from the oil industry.
M e t h od Co s t per part y per mont h Magnetic $1000. Gravity meter $2000.4000. Electrical $2000.--3000. Seismic $8000.--15000.
M e thod Ar e a cover e d pe r p a rty p e r month Magnetic 50 0 --70 0 squ a re nti l es G ravity meter 500--70 0 square mi]es Electrical 2--1 4 square miles Seismic 2--14 square miles
Although considerable variation in the cost of the different methods is indicated, the surprising thing is that eve n for seismic investigations th e cost is still much ]ess than the cost of getting the same information by drilling. T hat some Ststes feel justified in carrying out geophysical in - vestigations as part of their general geological program is indi. cared by M_ssouri where a complete state-wide magnetic survey has been made with stations on a mile-square grid over the entire State. This work has been in progress for seven years and will be br o ught to completion this year. Many of the localities showing abnormal magnetic conditions have already b e en checked by the use of gravitational methods, and some of the more pronounced anmnaly areas in the mhtera]ized part of the State are to be core- drilled to ascertain the value of the indicated ore-deposits_._ In parts of the State where there is a possibility of finding oil, these maguetic investigations have already led private interests to check them further with both gravitational and seismic measure- ments. Here then is one example of a Sta t e tha t h a s invested heavily in geophysical work in the expectation of expanding its
exploitable m i nera l resources e v en t h o ugh the surface geology was well known. T hat the w o rk has been a good investment is indicated by the interest displayed by oll companies in checking wi t h more cxgetlsive g eo physical methods the likely-looking areas indicated by the State's magnetic investigations; by the adoption of the mag_)etic method by several of the mining companies in the s t a t e f o r explora t ion work , and by the discovery of hitherto un- kno w n and ufisuspected potential ore-bodies which i_ppear to b e ss large as any now known in the State. Similar magnetic investigations have been conducted in recent years by the Alabama Geological Survey , the Sollth Dakota G e o - logical Survey, the Arkansas Geological Survey and the Missis- sippi G eological Survey ; and in the last-nmned State d r il li n g by a private company on a favorable structure outlined by the Survey has resulted in the discovery of the first oil po o l discovered in M ississippi. T his success has l ed t o _ w re prospecting and at the present time deep wells are b cit_g suuk in Florida and_ Georgia to investigate structures ou t lined by geo p hysical work and believed to b e favora b le for th e accumulation of oil. In Texas, Nevada, New Mexico and the Hawaiian Islands, geo- physical methods have been successfully employed by govern- mental agenc i es in the search for ground-water supplies. T h e indirect effect of geophysics and subsurface exploration may b e il l ustrated by Indiana. This State had closed its geologi- cal survey etJrly in this century since it was believed t h at there was nothi n g further to learn about the geology of the State as al l the surface geology had been mapped. The receIlt success of sub- surface exploration iu adjacent Illinois, however, and the dis- covery of the large oil resources there has l ed Indiana to reopen her geological survey in order to discover something ab o ut her potential underground wealth. The State of Illinois can also be used as an example of how a geological survey can serve the peop l e of a State. In Illinois the geological survey has a technical staff--n o t counting office and • laboratory help--of more t h a n fifty specialists. It also maintains chemica l labo r ato r ies , physical laboratories , aud other experi- mental la b orat o ries in which men work on problems concerning the natural rcs o t_rces of the State such as possible uses for their eh_y s , sands, shales , limestones , coal and oil. They also work ou pr o blems relating to production, metallurgical treatment, etc. ; all this being in addition to the work of mapping both the surface and subsurface geology. In Illinois if any industry has a problem in using a natural resource, the State makes that prob l em its own
